Master Thesis, Department of Geosciences
Swelling properties of Alum Shale as a function of its mineralogy
Desta Terefe
Swelling properties of Alum Shale as a function of its mineralogy
Desta Terefe
Master Thesis in Geosciences Discipline: Geology Department of Geosciences
Faculty of Mathematics and Natural Sciences
University of Oslo
01.11.2016
© Desta Terefe, 2016
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i
Abstract
Numerous reports on expansive shales have received wide attention in recent decades.
Damages and greater financial loss associated with expansive shale rocks have been documented in different countries. In Norway, challenges and problems in tunnels and underground excavations associated with the Upper Cambrian-Middle Ordovician black shale, Alum Shale, have been experienced since 1900s.
When evaluating various research reports on causes of concrete degradation through Alum Shale swelling, it is obvious that there is still the need to analyze further which minerals and mineral-related swelling processes are responsible in order to prevent harmful concrete compositions and to ensure safe and long-lasting structural integrity after construction work.
In this pretext, an extensive laboratory work was undertaken to address the extent to which the mineralogy and swelling potential can be correlated, based on the hypothesis that gypsum influences the swelling property of the Alum Shale. In addition, possible effects of clay minerals and other unidentified mineral reactions on the swelling phenomenon were investigated.
Fourteen samples, two of which are from a basement of an old building at Oslo city area (Møllergata) and twelve from the Gran Tunnel project site were studied. For each sample petrography and mineralogy were determined by optical microscopy, secondary electron microscope and X-ray diffraction and the swelling potential was obtained from free swelling and swelling pressure indices.
Swelling potentials were obtained on seven samples with conventional and labour-intensive schemes determined by the free swelling index of rock powder submerged in water and swell measurements in response to consolidation, saturation, and rebound in an oedometer.
The high pH value (low acidification), the low sulphur reactivity from pyrites and the absence of catalytic effect of pyrrhotite in pyrite oxidation showed how low the formation of gypsum could be in the shale. Gypsum was not detected in the 12 samples from Gran. The low amount of gypsum (about 1,5% and 2,8%) in samples from the Oslo city area showed the insignificant effect of swell in the Alum Shale. Rather, a fairly good positive correlation between measured illite/muscovite contents and swelling pressure was observed under these conditions.
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Acknowledgement
The last two years has been an incredible journey and an intensive period. This journey has been bumpy but also I have gained and acquired immense knowledge in this period.
The end result would have been nothing without the help of the following people.
I am grateful to Håkon Olav Austrheim who gave me the opportunity to work on this paper and guided me all the way to the end.
I want also to express my appreciation to PER Hagelia without his help and support this paper would not have been possible. His patience, constant guidance, encouragement, abundantly helpful, and sense of humor helped me pass over the hurdles at times were discouraging.
I would like to extend my appreciation to Beyene Girma Haile who helped me in the laboratory work at UiO, and Tom–Andre Kynbråten and Jan Inge Senneset who work in the central laboratory of the Norwegian Public Roads Administration (NPRA).
I would like also thank my mother, brother and sisters, who encouraged and helped me all the way and above all for their unreserved love. I would like to thank Rolf and Hirut for your guidance and moral support.
My father (Terefe) was the main reason that I pursue my education to this level. He encouraged me that I can do it. Thanks aba.
iv Contents
Abstract………...i
Acknowledgment……….ii
1. INTRODUCTION ... 1
1.1 OUTLINE OF PREVIOUS INVESTIGATIONS... 2
1.2 OBJECTIVE OF THE STUDY ... 3
2. GEOLOGICAL SETTING ... 4
3. METHODOLOGY ... 6
3.1 PETROGRAPHIC, MINERALOGICAL AND GEOCHEMICAL STUDY ... 6
3.1.1 Optical microscopy ... 6
3.1.2 SEM ... 7
3.1.3 XRD ... 7
3.1.4 XRF ... 9
3.2 SWELLING POTENTIAL TEST... 10
3.2.1 Free swelling test ... 10
3.2.2 Swelling pressure index ... 11
3.3 PH VALUE ... 12
4. MINERALS WITH A POTENTIAL SWELLING ABILITY IN SHALE ROCKS ... 13
4.1 GYPSUM (CASO4*2H2O) FORMATION AND SHALE ROCK SWELLING ... 13
4.1.1 Gypsum formation due to supersaturation of calcium in sulfate rich water ... 13
4.1.2 Gypsum formation as an end product of pyrite oxidation ... 14
4.1.3 Gypsum formation due to pyrrhotite oxidation ... 15
4.1.4 Gypsum formation due to hydration of anhydrite ... 15
4.2 CLAY MINERALS HYDRIC EXPANSION ... 16
4.2.1 Common clay minerals in shale rocks and their swelling property ... 17
5. PETROGRAPHY AND GEOCHEMISTRY OF ALUM SHALE ... 21
5.1 PETROGRAPHY ... 21
5.1.1 Petrographic description of hand specimen samples ... 21
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5.1.2 Thin section description using optical microscopy ... 22
5.1.3 SEM analyses ... 24
5.2 XRD BULK AND CLAY MINERALOGICAL ANALYSES ... 32
5.2.1 Bulk (whole rock) analysis ... 32
5.2.2 Clay mineralogy analysis ... 38
5.3 XRF... 41
6. SWELLING POTENTIAL TEST OF ALUM SHALE ... 42
6.1 FREE SWELL INDEX... 42
6.2 SWELLING PRESSURE INDEX ... 45
6.3 PH VALUE ... 47
7. EVALUATION AND COMPARISON ... 48
7.1 GYPSUM FORMATION DUE TO PYRITE OXIDATION ... 49
7.2 GYPSUM FORMATION DUE TO ANHYDRITE HYDRATION ... 50
7.3 CLAY MINERALS ... 50
8. DISCUSSION ... 52
8.1 SWELLING POTENTIAL OF ALUM SHALE ... 52
8.1.1 Gypsum induced swelling... 52
8.1.2 Illite/muscovite and organic matter effect in Alum Shale swelling ... 57
9. CONCLUSIONS AND RECOMMENDATIONS ... 61
9.1 CONCLUSIONS ... 61
9.2 RECOMMENDATIONS ... 62
Appendices...63
References………..73
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List of Tables
Table 4.1 Classification of clay mineral groups according to 1:1 and 2:1 mineral type
(Moore & Reynolds, 1989)... 17 Table 5.1 Minerals detected by SEM ... 27 Table 5.2 Peak positions of d-values utilized for the qualitative analyses of the bulk XRD samples ... 33 Table 5.3 Minerals detected by XRD bulk mineralogical analyses and their SQ values given in %. Only the crystalline phase included in this table ... 34 Table 5.4 Minerals detected by XRD bulk mineralogical analyses and their SQ values given in % including crystalline and amorphous phase. ... 34 Table 5.5 Clay minerals detected in samples from Oslo (Møllergata) and Gran Tunnel project site ... 39 Table 5.6 XRF analyses results of all the samples studied. ... 41 Table 6.1 Classification of free swelling and swelling pressure ... 43 Table 6.2 Classification of the free swelling result for the seven Alum Shale samples from Oslo and Gran ... 43 Table 6.3 XRD SQ values before the samples tested for free swelling test. ... 44 Table 6.4 XRD SQ values after the samples tested for free swelling test. ... 44 Table 6.5 Swelling pressure measured on rock prism of Alum Shale sample from Oslo (Møllergata) and Gran. ... 45 Table 6.6 pH value of the samples studied for free swelling test ... 47 Table 7.1 Compiled results of the content of pyrite and calcite before (BFST) and after (AFST) free swelling test, pH (BFST and AFST) value and swelling potential tests ... 49
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Table 7.2 The amount of the illite/muscovite and the swelling pressure result. ... 51 Table 8.1 Illite/muscovite and the amorphous phase concentration in the samples tested for swelling pressure. ... 58
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1. Introduction
Numerous reports on expansive shales have received wide attention in recent decades.
Damages and greater financial loss associated with expansive shale rocks have been documented in different countries. Considerable research on the mechanism of heave and factors affecting the swelling behavior has been carried out by several researchers. The many speculations and theories so far suggest that the underlying processes and possible treatments are not yet sufficiently clear.
In Norway, challenges and problems in tunnels and underground excavations associated with the Upper Cambrian-Middle Ordovician black shale, Alum Shale, have been experienced since 1900s (Grønhaug, 2000).
In Scandinavia the name Alum Shale is used for black Cambrian shale. The shale is composed of clay and silt-sized sedimentary rock and contains a lot of organic matter.
Because of its high organic matter content, the shale was considered as a potential source rock for petroleum, for example, in Sweden (1925-1961) during the Second World War for the Swedish navy (Ramberg, 2008).
In the 1700s, Alum Shale had been extracted for potassium alum (potassium-aluminium sulfate used, for example, for tanning) in Norway and at several places in Scandinavia Ramberg (2008). The Cambrian Alum Shale, however, does not have alum in its natural state but it received its name because it was used for potassium alum production (Ramberg, 2008).
Although Alum Shale has been used as industrial raw material, the shale represents a growing challenge to construction activities and causes structural damage. The Norwegian Alum Shale is rich in pyrite and contains minor contents of a highly reactive monoclinic pyrrhotite. It is subject to change in volume and swells when it comes into contact with water and air (Hagelia, 2011).
The Norwegian Public Roads Administration (NPRA) has recently investigated the effects of Alum Shale when in contact with concrete. Until recently, the Alum Shale swelling problem has been addressed by engineering routines and simple chemical classification. But the swelling mechanism is still not clearly understood. It is apparent that formation of gypsum is
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involved at many sites (Bastiensen, 1957). New evidence suggests that at some sites Alum Shale swelling is caused by the formation of gypsum due to hydration of anhydrite (Hagelia, 2011).However, the swelling mechanism is not well understood but formation of gypsum is frequently involved.
This study is based on the hypothesis that gypsum influences the swelling property of the Alum Shale. In addition, possible effects of clay minerals and other unidentified mineral reactions on the swelling phenomenon were investigated. Detailed petrographic and mineralogical studies, free swelling and swelling pressure tests were carried out on fourteen Alum Shale samples collected from the Gran Tunnel project site and the Oslo (Møllergata) area in Norway.
1.1 Outline of previous investigations
The problem of Alum Shale swelling began to be investigated after pyrrhotite/pyrite induced heave had been identified. Since 1930, the Norwegian Alum Shale has been noticed causing repeated structural damage to buildings when it comes into contact with concrete (Grønhaug, 2000). Alum Shale problems became so severe that ‘The Alum Shale Committee’ (1947- 1973) was established to investigate the shale problems associated with structural damage (Grønhaug, 2000).
The committee undertook systematic investigations by testing concretes exposed to Alum Shale (Grønhaug, 2000). Concrete degradation problems were considered to be caused by sulfate attacks resulting in the expansion of concretes. The committee noted the damage associated with the presence of structurally disordered sulfide mineral pyrrhotite (Grønhaug, 2000). Pyrite, the most abundant sulfide mineral in Alum Shale, was considered harmful as a small content of pyrrhotite proved to have a catalytic effect on oxidation of pyrite and hence on the expansion of concretes due to secondary mineral growth (Grønhaug, 2000).
Research on black shale swelling problems was conducted, for example, by Sopp (1966), Bastiansen (1957) and Barth (1943). Bastiansen (1957) observed that formation of gypsum could be the cause and Barth (1943) hypothesized that the swelling may be caused by the presence of anhydrite in the Alum Shale, causing gypsum formation by hydration of anhydrite. However, the study by Sopp (1966) was not conclusive. Abreham (2007) did an
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experiment on pyrites originating from the Slemmestad and Konowsgate in the Oslo area and he found that the pyrites were reactive. In the presence of calcite, Abreham (2007) concluded that reaction between the reactive pyrite and calcite may lead to formation of gypsum which may lead to swelling of Alum Shale.
Hagelia (2011) summarized previous and recent relevant research related to heave caused by the presence of pyrite/pyrrhotite in Oslo Alum Shale. In his research, a leaching experiment on samples taken from the Svardal tunnel in Oslo reflected anhydrite – gypsum conversion, proving an old hypothesis of e.g. Barth (1943). This implies that the pyrite oxidation triggered by minor monoclinic pyrrhotite is not the sole reason for Alum Shale reactivity. The author added that the swelling problem can also be due to supersaturation of calcium and sulfate enriched waters and with the possible effect of clay minerals or other as yet unidentified mineral reactions still to be investigated.
Recently Hawkins and Stevens (2013) presented results on the oxidation of pyrite and growth of secondary sulfates in black shales, the effects of the oxidation process, and how its reaction products can lead to both heave and degradation of foundations. Examples from UK, Canada, USA and Ireland and a number of case studies from other countries were also presented by these authors to illustrate the involvement of pyrrhotite in sulfate-related heave and cause of foundation problems.
1.2 Objective of the study
When evaluating various research reports on causes of concrete degradation through Alum Shale swelling, it is obvious that there is still the need to further analyze which minerals and mineral-related swelling processes are responsible in order to prevent harmful concrete compositions and to ensure safe and long-lasting structural integrity after construction work.
The following study therefore aims at:
1. Identification of problematic minerals
2. Quantification of volume change (swelling) and 3. Swelling control by secondary minerals
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2. Geological setting
The study areas and the sampling site are located at Oslo, the capital city of Norway, and at Gran in the Gran municipality, approximately 50km away from Oslo. The samples collected from the Oslo area originate from a basement of an old building located at Møllergata, but at the time of sampling a new building was under construction. The samples from Gran were collected from a tunnel construction site when excavation took place.
Oslo and Gran are part of the Oslo Region (Oslofeltet) (Fig.2.1). The geological structure of the Oslo Region is characterized by rock types from Precambrian to Permian formations where rocks are folded and thrusted due to tectonic disturbances of the Caledonian orogeny and Permian rifting phase (Calner, Ahlberg, Lehnert, & Erlström, 2013). The Region is 40- 70 km in width and extends 115 km north and south of the city of Oslo (Bruton, Gabrielsen,
& Larsen, 2010). The Upper Cambrian-Lower Ordovician Alum Shale formation formed in a large shallow epicontinental sea found in different parts of the Oslo Region and show differences in shale characteristics (NGI internal report). The formation is predominantly made up of shales and shales with limestone horizons in variable proportions (Morley, 1994;
Schovsbo, 2002).
The foliaceous structured Alum Shale is riched in organic matter and sulfide minerals, such as pyrite. Permian dikes and sills developed during the Permian rifting phase and often intruded the Alum Shale (NGI internal report). The Alum Shales found between the Permian sills are a challenge for engineers and constructors due to its high swelling pressure when in contact with water. The dikes or sills are often heavily fractured and may underwent metamorphism affecting the neighboring Alum Shales. The contact metamorphism effect on the Alum Shale may induce the shale to form secondary minerals, like pyrrhotite, a result of a highly metamorphosed pyrite. The effect of metamorphism, however, had little to no effect on the studied samples from both areas (personal communication with Per Hagelia, 21.10.2016).
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Figure 2.1 The geological structure of Oslo Region (Oslofeltet), where Oslo and Gran found in. (taken from www.google.no)
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3. Methodology
The main goal of this thesis was to study the Alum Shale mineralogy and swelling potential relationship. Two different methods were used to study the two properties of the shale:
1. Petrographic, mineralogical and geochemical analysis and 2. testing swelling pressure and free swelling potential of the shale
14 rock samples were provided by NRPR for this study. Two samples from Oslo (Møllergata) and twelve samples from Gran Tunnel project site. These samples were chosen according to their amount of S and Ca concentration from XRF analyses. This was aimed to get enough amount of pyrite/pyrhotite and calcite which are the ingredient minerals in the process of gypsum formation. The samples were packed in plastic bags to inhibit pyrite/pyrhotite oxidation.
3.1 Petrographic, mineralogical and geochemical study
Optical thin-section description as well as scanning electron microscope (SEM) and hand specimen observations were used for the petrographic study. X-ray powder diffraction (XRD) was used for identifying the mineralogy, and geochemical analyses were applied to detect major and trace elements in the samples by using X-ray fluorescence (XRF) spectroscopy.
3.1.1 Optical microscopy
Fourteen thin sections of approximately 30 µm thick slab of rock were glued onto 2,5cm x 4,5cm glass plates produced by Lars Magne Kirksæter at Petrological Section Service (Petro- Sec).
A Nikon Optiphot-Pol petrographic microscope, available at Geoscience department (UiO), was used to identify constituent minerals and textural properties on the fourteen thin sections samples based on their optical properties.
7 3.1.2 SEM
SEM was used to achieve information about surface topography and sample's spatial variations in elemental compositions (Reed, 2005).
Hitachi SU5000 FE-SEM, at the department of Geoscience (UiO), was used to examine thin sections and small sub samples approximately of 3-4mm. The instrument is equipped with a Dual Bruker XFlash30 EDS system, secondary electron and backscattered electron detectors.
The microscope was operated with an accelerating voltage of 15kV while analyzing the specimens (with spot sizes of sub-micrometer) depending on the nature of imaging being carried out.
Sample pre-treatment and preparation, like carbon coating or gold sputtering, were performed prior to the analysis of the samples to create a conductive layer of metal on the sample which inhibits charging, reduces thermal damage and improves the imaging of samples. For thin sections carbon coating, Cressington 208C carbon Coater was used and for the specimen stubs gold sputtering Gold Coater Quorum Q150R S was applied.
Examinations of the samples were conducted with backscatter electron (BSE) images for delineating compositional variation and secondary electron (SE) images for documenting topographic variation (Reed, 2005). The analyses were guided by Prof. Håkon Austrheim and Senior Engineer Berit Løken Berg.
3.1.3 XRD
XRD was used as a method for a quick and reliable mineral identification by determination of mineral structure at atomic level and unit cell dimensions.
Bulk and clay mineralogical analyses were applied using a Philips X’Pert X-ray diffractometer system at Geoscience department (UIO). The instrument was used to gain qualitative and quantitative information over finely ground and homogenized specimen with detection limits of about 1-2%.
8 Bulk (whole) rock mineralogical analysis
With the bulk (whole) rock XRD analyses, rock powder samples were detected either as ordered crystalline compounds (minerals) or as a high degree of disordered material (amorphous phase). The crystalline compound manifested as a set of discrete intensity based on the location and intensity of peaks on the 2θ scale. The amorphous phases, however, consisted of poorly crystalline materials and did not contribute diffraction peaks.
Sample preparation
The author prepared rock powder using a grinding swing mill for about 2 minutes. The mill reduced the grain size to <100-200µm. About 3g of powder sample was mixed with 8ml of ethanol and further powdered using McCrone micronizer to achieve an average particle size of <10 μm. The micronizer reduced the particle size of the minerals without significantly diminishing the crystallinity of the specimen with milling media. The samples were then dried in an oven at 50°C, transferred into sample holders and placed into the XRD machine.
Samples were then X-rayed and diffracted rays collected, processed and counted. The counted data were analyzed using the Diffrac Eva Software which allowed to evaluate X-ray diffraction data. The data were compared with known powder diffraction patterns from the Powder Diffraction File database published by the International Center for Diffraction Data (ICDD). This analysis allowed identification of minerals (crystalline phase) in the samples and gave qualitative and semi-quantitative (SQ) analysis of bulk XRD samples.
Clay mineralogical analysis
The clay fraction (<2µm) separation method was used to produce oriented clay-mineral specimens for clay mineralogical analysis. The samples were prepared in by two steps:
Step 1: Separation by gravity
10-15g, 1-2mm, hand crushed rock samples were prepared and mixed with 250ml distilled water in a cylinder beaker. The distilled water was first mixed with a chemical dispersing agent, sodium carbonate Na2CO3 (0,125g per liter), to prevent flocculation. The samples
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went through disaggregation for 10 minutes in VWR Ultrasonic Bath. Disaggregation was expected to accelerate separation of clay particles. To remove particles larger than 2µm and settle, the cylinder beaker was filled with 600ml distilled water mixed with Na2CO3. The samples were then allowed to remain in the beaker for a duration of 6 hours and 30 minutes.
The settling rate length of time was calculated according to Stokes' law that describes a connection between particle size and their settling rate.
Step 2: Filtering clay suspension
After fractionation, an aliquot of clay suspension was filtered through a Millipore filter of 0,45µm pore size using the Millipore vacuum technique. The filtered clay was inverted carefully onto a round Pyrex glass platform, placed in aluminum holders and was air-dried for 12 hours.
After air-drying, the samples were treated with ethylene glycol for 12 hours at 60°C in order to detect swelling clays, e.g. smectite which expands from 14 Å (air-dry) to ~17 Å in ethylene glycol (Moore & Reynolds, 1989). After saturation with ethylene glycol the samples were heated to 350°C for about 2 hours to remove expansion of clay minerals. Afterwards, the samples were again heated to 550°C to distinguish kaolinite from chlorite which have overlapping d-spacing. At this temperature kaolinite will become amorphous and its diffraction pattern disappears (Moore & Reynolds, 1989).
Qualitative Analysis of Clay Fraction
The identification of five main clay mineral groups, illite, smectite, kaolinite, chlorite, vermiculite and mixed-layer clays was based on X-ray diffraction peaks. The identification of the mineral groups followed the methods described in Moore and Reynolds (1989) by the aid of Bruker software EVA.
3.1.4 XRF
X-Ray fluorescence (XRF) was selected to determine the concentrations of major and trace elements in Alum Shale samples. This technique allows for rapid screening of the target
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samples (Groover & Izbicki, 2016) of this study, using the resulting semi-quantitative elemental compositions data.
The XRF analysis was carried out during the excavation period at the Gran Tunnel construction site using a NITON XL3t GOLDD+ handheld analyzer. The analysis was conducted by choosing pinpoint areas on a surface of Alum Shale rock. The instrument generates X-rays via a non-radioactive source of Ag X-ray tube with a maximum voltage of 50kV, a maximum power of 2W and a maximum current of 200µA (Frahm, Doonan, &
Kilikoglou, 2014; Simandl et al., 2014). The resulting data registered with faster scan time while the analysis went on. The instrument provides information on Mg to U with lowest detection limits.
To significantly improve the quality and value of XRF data, calibration corrections were derived from linear-regression fit trend results between the measured and expected values and based on previous qualitative analyses of reference samples of Alum Shale and other sedimentary rocks originating from the tunnel project at Gran. Accuracy was verified by this method for the samples used for this study. Twelve of the samples studied were collected from the same area where the analyses were performed.
Examples of calibration curves for eight elements and lower detection limit (LOD) of the analyzer given in mg/kg (ppm) can be found in Appendix B.
3.2 Swelling potential test
Two laboratory test methods were used to study the swelling aptitude of Alum Shale. The tests were carried out under unconfined (free swelling tests) and oedometric (swelling pressure test) conditions.
3.2.1 Free swelling test
Free swelling tests (free swell index) aimed to determine the increase in volume (swelling) of the shale when submerged in water.
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10ml of oven dried powder samples with particle sizes of <20µm were used for the test. The
<20µm powder samples separated were prepared following the same procedure used for clay fraction separation (<2µm) as discussed in section 3.1.3. By applying Stokes' law, the length of time to keep the <20µm in suspension was 3 hours and 45 minutes.
The 10ml powder samples drizzled into 50ml sized cylinders filled with 45ml distilled water.
In case of limited sample material, 5ml samples were used using 25ml sized cylinders filled with 22.5ml distilled water.
The volume (V1) occupied by sample powder was recorded after a 48 hours hydration period.
The free swell index number is the relation between the final volume V1 and the initial volume V0. The free swell index (FS) was calculated with the standard formula used by NPRA.
FS = V1/V0*100%
When FS is the free swell index (%), V1 is the final volume (ml) and V0 is the initial volume (ml).
3.2.2 Swelling pressure index
The swelling pressure test aimed to study the one-dimensional relative swelling potential of Alum Shale to quantify the maximal swelling strain caused by swelling. The test was carried out with a GDSAOS oedometer measuring system used by NPRA.
The test was performed on oven-dried rock prisms (Fig. 3.1) trimmed to fit into a 50 mm diameter oedometer ring in order to avoid friction. The rock prisms area, approximately 6- 8cm2, were placed such that the schistosity was normal to the pressure axis and first pre- consolidated at 4MPa for 24 hours by loading weights. Since the range of the dimensions in the rock prisms varies, re-scaling (normalization) was used to ensure that the dimensions will be approximately in the same range (see Appendix A).
After the consolidation, the specimen was released until it no longer showed any height (volume) change. The oedometer ring was then filled with distilled water and allowed the rock to swell for 24 hours under the applied water. The rock prisms swelling pressures were recorded on a computer-controlled one-axial swelling test apparatus.
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The swelling pressure index is then calculated from the formula:
p = F/A
When F is the maximum axial swelling force recorded and A is the cross sectional area of the specimen
Figure 3.1 An illustration of a rock prism sample from the Gran Tunnel project site placed in an oedometer ring.
The test was carried out by Tom–Andre Kynbråten and Jan Inge Senneset at Central Laboratory of NPRA.
3.3 pH value
The aim of the pH value test was to measure the acidity or alkalinity level of the Alum Shale after it is mixed with distilled water. This was done to study the production of sulfate in the processes of pyrite oxidation.
The pH value of the samples was measured using pH testing kit which indicates level of acid-alkaline state of any liquid. The test was carried out in the process of swelling test to evaluate the acidity or alkalinity change after the sample powder was mixed with distilled water. Readings at the low end of the scale indicate an acidic state, and those on the higher end a more alkaline state.
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4. Minerals with a potential swelling ability in shale rocks
Depending on the mineralogical composition, shale rock can activate its swelling potential if water and/or air are allowed to infiltrate. Shale rocks containing gypsum and expansive clay minerals (sheet silicates like smectite) are most prone to swelling (Azam, Abduljauwad, Al- Shayea, & Al-Amoudi, 1998; Foster, 1954; Hoover, Wang, & Dempsey, 2004) ). Repeated mineralogical problems in shale rocks swelling are due to:
1. Gypsum formation
due to supersaturation of calcium in sulfate rich water
as an end product of pyrite and pyrrhotite oxidation and
due to hydration of anhydrite
2. Clay minerals expansion
4.1 Gypsum (CaSo
4*2H
2O) formation and shale rock swelling
Gypsum is one of the most common minerals in sedimentary environments and formed where sulfate and calcium are common ions in solution. Depending on surrounding temperature and pressure gypsum can keep its specific volume (74,5cm3/mol) (Comodi, Kurnosov, Nazzareni, & Dubrovinsky, 2012) before it starts dehydration. The effect of the high volumetric nature of gypsum causes a big swelling effect in shale rocks.
4.1.1 Gypsum formation due to supersaturation of calcium in sulfate rich water
Gypsum is generally formed after precipitation in sulfate rich water and when there is a supersaturation of calcium ions (1).
Ca2+ + SO42- +2H2O→ CaSO4*2H2O (1)
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4.1.2 Gypsum formation as an end product of pyrite oxidation
Pyrite is the most common mineral of the sulfide mineral group in black shale rocks. Pyrite oxidation and its role in forming acid-sulfate is one of the driving forces leading to shale swelling (Hoover et al., 2004). The acidification in turn reacts with calcium carbonate (calcite), resulting in the growth of gypsum (Hoover et al., 2004).
Different researchers suggest that different mechanisms are involved in the oxidation of pyrite in the presence of oxidizing agents such as O2, Fe3+, MnO2, NO3- and others that can be involved in pyrite oxidation (Ma & Lin, 2013; Schippers & Jørgensen, 2002; van der Perk, 2013). The most naturally occurring process in the oxidation of pyrite is the redox reaction in the presence of O2 and Fe+.
The oxidation of pyrite is described by Young, Taylor, and Anderson (2008) as a result of cathodic and anodic reactions. In these reactions electrons on the pyrite surface area transferred to O2 and Fe3+ by cathodic reaction and then ferrous and sulfate ions are produced by an anodic reaction as described in reaction 2 and reaction 3.
In reaction 2, Fe3+ acts as an effective oxidant for pyrite oxidation when pH falls below 4 to prevent reaction of Fe3+ with water (Hoover et al., 2004). Ma and Lin (2013) described the oxidation reaction as:
FeS2 + 3.5O2 + H2O → Fe2+ + 2SO42- + 2H+ (2) FeS2 + 14Fe3+ + 8H2O → 15Fe2+ + 2SO42- + 16 H+ (3)
Shale rocks contain variable amounts of calcite depending on their formation. The sulfuric acid produced during the oxidation of pyrite reacts with the calcites present in the shale rocks and forms gypsum as shown by reaction 4 (Hawkins & Stevens, 2013). Calcite dissolution takes place when pH is <5 (Brown, Compton, & Narramore, 1993). Gypsum has approximately twice (74,5cm3/mol) (Comodi et al., 2012) the molar volume of calcite (36,3cm3/mol) (Lee & Morse, 1999) which is involved in the expansion or swelling of the shale.
H2SO4 + CaCO3 + H2O → CaSO4.2H2O + CO2 (4)
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4.1.3 Gypsum formation due to pyrrhotite oxidation
Pyrrhotite (Fe1-xS) is a sulfide mineral formed by a pyrite-pyrrhotite ratio change during metamorphism as prograde heating (VOKES, 1993). The mineral formed with variable composition of Fe1-xS where x varies from 0 to 0.125. Similar to pyrite, the two important factors affecting pyrrhotite oxidation are oxygen and ferric iron at pH<4 (Belzile, Maki, Chen, & Goldsack, 1997; Nordstrom & Alpers, 1999). According to Nicholson and Scharer (1994), pyrrhotite oxidizes 20-200 times faster than pyrite when O2 is the primary oxidation agent. The oxidation reaction (5) takes place as follows:
Fe1-xS + (2-(1/2)x) O2 + H2O → (1-x)Fe2++ SO42- + 2xH+ (5)
Pyrrhotite dissolution can also be promoted under acidic conditions and generates Fe2+ and H2S (Belzile, Chen, Cai, & Li, 2004; Bhatti, Bigham, Carlson, & Tuovinen, 1993). The H2S in turn generates sulfuric acid as shown in the reactions (6) and (7):
Fe1-xS + 2H+ → (1-x) Fe2+ + H2S (6)
H2S + 2O2 → 2H+ + SO42- (7)
After the sulfate formation, gypsum formation takes place as shown in reaction 4.
4.1.4 Gypsum formation due to hydration of anhydrite
Anhydrite is one of the problematic minerals in shale rock swelling when it reacts with water (Rauh, Spaun, & Thuro, 2006). Anhydrite, depending on pressure, temperature and presence of foreign ions, dissolves into calcium and sulfate ions and subsequently gypsum is formed when the equilibrium concentration of gypsum is reached (Rauh et al., 2006; Serafeimidis &
Anagnostou, 2012). When gypsum crystallizes, the volume increases by 61% which drives the shale to swell (Rauh et al., 2006). The simple chemical reaction of anhydrite alternation to gypsum (Natau, Fecker, & Pimentel, 2003) can be expressed as:
CaSO4 + 2H2O → CaSO4*2H2O (8)
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4.2 Clay minerals expansion
Clay minerals exist primarily in sedimentary rocks, for example in shale rocks. Clay minerals are very small (approximately <3µm) hydrous aluminum silicate particles classified as phyllosilicates or layer silicates (Houdry, Burt, Pew, & Peters, 1938). The minerals’
structural features are strongly related to the degree of swelling on hydration though all clays are not expandable.
Two distinct structural features - tetrahedral and octahedral - are associated with the formation of the phyllosilicates/sheet silicate and give rise to the different clay mineral structures (Weaver & Pollard, 2011). In all phyllosilicates, the SiO4 tetrahedral planar array arrangement is the dominant structure. The tetrahedral (Fig. 4.1) structure forms when cations e.g. Si4+, Al3+, Fe3+ surrounded by four O2 and the octahedral structure (on the right side of the figure) develops when cations e.g. Al3+, Fe3+, Fe2+, Mg2+ are surrounded by six O2
(Moore & Reynolds, 1989; Weaver & Pollard, 2011).
Figure 4.1 Spheres closely packed to form a tetrahedron and an octahedron structure (From Bjorlykke (2010))
Clay minerals follow the layer type classifications 1:1 or 2:1 based on the number of tetrahedral and octahedral structures that form units of phyllosilicate sheet layers (Moore &
Reynolds, 1989). The 1:1 layer structure (Fig.4.2A) consists of one octahedral and one tetrahedral sheet while the 2:1 layer structure (Fig. 4.2B) consists of two tetrahedral sheets, with the octahedral sheet sandwiched between these two sheets. These sheets are again bonded to form a number of sheet layers (Moore & Reynolds, 1989; Weaver & Pollard, 2011;
Wilson, 2013). Table 4.1 shows the classification of the clay mineral groups according to the 1:1 and 2:1 types. This table is modified from Moore and Reynolds (1989), but some additional classifications (subgroup of the minerals) from the book are not included.
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Figure 4.2 Octahedral (O) and tetrahedral (T) sheet layers (modified from Bergaya and Lagaly (2013))
Table 4.1 Classification of clay mineral groups according to 1:1 and 2:1 mineral type (Moore & Reynolds, 1989)
Layer Type Mineral Groups Species
1:1 Serpentine-Kaolin chrysotile, antigorite, lizardite, berthierine, odinite kaolinite, dickite, nacrite, halloysite
2:1 Talc-Pyrophyllite
Smectite aponite, hectorite, montmorillonite, beidellite, nontronite Vermiculite
Illite illite, glauconite
Mica biotite, phlogophite, lepidolite, muscovite, paragonite, margarite
Chlorite donbassite, sudoite, cookeite, Sepiolite-Palygorskite
4.2.1 Common clay minerals in shale rocks and their swelling property Kaolinite (Al2Si2O5(OH)4)
Kaolinite is one of the most common clay mineral in shale rocks with a 1:1 layer type structure (Moore & Reynolds, 1989). Kaolinite contains the cations Al3+ and Si4+ in its octahedral and tetrahedral sheet structure respectively (Fig. 4.3). Kaolinite belongs to the non-expandable clay mineral group because it is generally resistant to substitution due to its tight hydrogen bonds (Moore & Reynolds, 1989). There is a small number of exchangeable cations associated with the clay structure and this leads to the mineral not to absorb water and not to expand (Mukasa-Tebandeke et al., 2015).
18 Vermiculite (Mg,Fe2+,Al)3 (Al,Si)4)O10(OH)2.4(H2O)
Vermiculite has a structure of the 2:1 type separated by layers of water molecules (Moore &
Reynolds, 1989). When Al3+ cations substitute Si4+ cations, layer charge deficiency occurs, but in vermiculite this deficiency is partly balanced by Mg2+ (Bergaya & Lagaly, 2013). On hydration, the cations (depending on the kind of interlayer cation) located at the interlayer sites of vermiculite are hydrated and its basal spacing changes from about 10.5 to 15.7Å, leading to expansion (Moore & Reynolds, 1989).
Smectite x(Al2-x, Mgx)Si4O100(OH)2
The structural unit of smectite (Fig 4.3) is like the structure of vermiculite. The 2:1 silicate layers of smectite, however, have a slight negative charge, normally smaller than that of vermiculite, which is offset by cations, principally Ca2+ and Na+. The difference in the swelling degree of different smectites is due to the nature and extent of octahedral substitution. Hydration of the interlayer cations (between the unit layers ) of smectite causes the structure of the mineral to expand with a dimension varying from about 9.6 Å to 18Å, resulting in nearly complete separation of the individual layers (Moore & Reynolds, 1989).
Mica ((K)(Al, Mg, Fe)2-3AlSi3O10 (OH)2)
Mica is a non-expanding lattice clay with the same structure as smectite except some Si4+
replaced by Al3+. K+ occupies the interlayer sites between two 2:1 layers (Fig 4.3) and the charge deficiency caused by Al3+ is replaced by K+. Typical examples of mica are muscovite (KAl2(Si3Al)O10(OH)2), phlogopite (KMg3(Si3Al)O10(OH)2) and biotite (K(Mg, Fe)3(Si3Al)O10(OH)2). This replacement neutralized by non-exchangeable potassium, resulting in mica having a very low degree of swelling. Clay-sized micas similar to muscovite are called illite.
19 Illite ((K1-x)(Al, Mg, Fe)2-3AlSi3O10 (OH)2)
The main difference of illite and muscovite are that the amount of aluminum replaced with silicon is less in illite (Bergaya & Lagaly, 2013) . Consequently, illite has a lower potassium content than muscovite. Illite is also characterized by a greater degree of replacements by potassium (as in mica), resulting in a very low degree of swelling (Bergaya & Lagaly, 2013;
Moore & Reynolds, 1989).
Chlorite ((Mg, Fe,Al)6(Si,Al)4(OH)8)
The structure of chlorite resembles mica with a 2:1 negatively charged layer connected by an interlayer of a positively charged octahedral hydroxide sheet (Bergaya & Lagaly, 2013;
Moore & Reynolds, 1989). Chlorite contains cations Mg2+, Al3+,Si4+ and Fe2+ in its octahedral and tetrahedral sheet structure (Bergaya & Lagaly, 2013). Chlorite is a stable mineral and does exhibit inter-lamellar swelling (De Boodt, Hayes, & Herbillon, 2013).
Mixed layer clay minerals
According to (Moore & Reynolds, 1989), mixed layer clay minerals are explained as mixtures of more than one clay mineral with individual components stacked in various ways and form a new structure different from the individual components. This structure forms due to the similarity that exists between the layers of the different clay minerals. Examples of known structures of this type are illite-smectite, mica-montmorillonite, chlorite-smectite, vermiculite-chlorite, illite-chlorite, and illite-chlorite-montmorillonite, etc. In contact with water, for example, the exchangeable interlayer in illite and smectite detected at diffraction peak between 10 and 14Å while in illite/smectite mixed layer is characterized by the d spacing changes (from illite (10Å) and smectite (17 Å)) to 27Å (Moore & Reynolds, 1989).
20
Figure 4.3 Illustration of different clay minerals structures together with interlayers (modified from Bjorlykke (2010))
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5. Petrography and Geochemistry of Alum Shale
The objective of this chapter is to characterize Alum Shale samples collected from two different areas. Textural, mineralogical, and elemental composition of the samples were studied using hand specimen description, optical microscopy petrographic examination, SEM, XRD and XRF.
Fourteen Alum Shale samples from two locations were provided by the Norwegian Public Roads Administration (NPRA) for this study. Twelve samples were collected from a recent tunnel construction site, Gran Tunnel (located in Gran municipality, Norway) and two samples from Oslo city (located at Møllergata area, Norway). Of the fourteen samples, nine tunnel face samples (9178-3, 9178-4, 9178-5, 9354-2, 9354-4, 9354-5, 9199-5, 9327-4 and 9215-2) and three tunnel core samples (9214, 9224-5 and 9260) were collected from Gran Tunnel project site respectively. The other two samples (Oslo-M1 and Oslo-M2) were collected from Oslo city area (Møllergata) from a basement of an old building where a new building was under construction at the time of sampling.
5.1 Petrography
5.1.1 Petrographic description of hand specimen samples
In hand specimen the Alum Shale samples vary in color from gray to black (Fig. 5.1). The shale appears homogeneous and is characterized by a predominance of fine-grained materials. Few inclusions of scattered yellowish and whitish cemented materials were observed in the matrix of the shales. These minerals are possibly pyrite and calcite. The Alum Shale samples are composed of thinly laminated material or fissile where the rock splits into thin pieces along the laminations. Fissility is a common feature of shale rocks that distinguishes them from other fine grained rock. The fissility was more developed in the darker samples than the greyish ones and the latter one was more massive.
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A B
Figure 5.1 An illustration of Alum Shale samples from the Gran Tunnel project site A. Tunnel core samples, note the color variation ( greyish and darker). B. Tunnel face samples.
5.1.2 Thin section description using optical microscopy
Light optical examination was done in plane- and cross- polarized light. Two different types of samples were observed, black (Fig.5.2A and B) and light colored (Fig.5.2C or D). The blackish colored Alum Shale samples were dominated by very fine grained, colorless and grey minerals with a varying size of calcite veins. Band like structure with alternating light and black color were typical of the blackish samples. As they were opaque colored it was difficult to identify most of the minerals.
The other type of samples was calcite rich and show greyish-brown color in plain polarized light (Fig.5.2C) and high order interference colors like pink, blue and yellow in crossed polarized light (Fig. 5.2 D). The minerals also showed a distinct twinning and very high relief.
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A B
C D
Figure 5.2 Thin sections photographs (30x) of Alum Shale samples from the Gran Tunnel project site. A. sample 9354-5, the blackish Alum Shale with light and dark band like structure of the shale. B. sample 9178-4, blackish relatively with large calcite veins in cross polarized light. C and D, sample 9224-5, a calcite rich sample with plain and crossed polarized light respectively. The bright interference color and the distinct twinning feature of the calcite minerals are also shown in 5.2D.
24 5.1.3 SEM analyses
Fourteen thin sections and six small cut samples were studied under SEM. Two different types of Alum Shales were recognized.
Type-1 Samples
Alum Shale composed predominantly of white mica together with considerable amount of quartz, pyrite and K-feldspar and accessory minerals scattered throughout the shale (Fig.5.3).
Of the 14 samples, eleven samples (9178-3, 9178-4, 9178-5, 9199-5, 9327-4, 9354-4, 9354- 5, 9215, 9214, 9260 and Oslo-M1) belong to this type of Alum Shale.
Type-2 Samples
Alum Shale composed predominantly of calcite (mostly containing low to negligible amount of magnesium) where quartz and pyrite are randomly distributed in the calcite matrix. In the fractured and porous area of these samples pyrite, quartz, white mica, carbonaceous material and some accessory minerals observed filling the opening space (Fig 5.4). Three samples, Oslo-M2, 9224-5, and 9354-2, belong to this type of Alum Shale. The calcite content in the Oslo-M2 sample was lower than in the other two samples.
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Figure 5.3 Backscattered electron images (x300) the mica rich tunnel face sample 9178-3 The tiny fibrous greyish crystals are white micas and the darkest background is the organic matter. The others marked grains are quartz (2), pyrite (3) and K-feldspar (4). The brighter grains are minerals with a relative high average atomic number as compared to the darker phases (most of the grains are very small in size and it was difficult to mark them on the figure).
Figure 5.4 Backscattered electron images (x400) the Calcite (Cal) rich tunnel face sample 9354-2.
The other minerals are quartz (2), pyrite (3) and apatite (5).
Petrography (and mineral chemistry) of the two different types of samples are described and presented below. Identification of the main constituent minerals and minerals related to swelling mechanisms was the main focus of SEM analyses. Particular emphasis was put on a careful examination of minerals that are assumed to control the swelling property of Alum Shale. In addition to thin sections, six small pieces of the Alum Shale were studied by SEM to identify the presence of clay minerals like smectite.
26 Type-1 samples description
Petrography
All Type-1 samples were composed predominantly of tiny flakes of white mica with a considerable amount of quartz, pyrite and K-feldspar (Fig 5.3). The tiny elongated crystals of (<1-5µm (mostly <2µm) thick and 1-15µm long) the white mica were micro-laminated and exhibited different orientation oblique to the bedding plain as they bend around other minerals (Fig. 5.4A and B). The SEM image on Figure 5.3, Figure 5.5A and B show planar a fabric structure (foliation) of the white mica caused by compaction around resistant minerals like quartz, pyrite and some accessory minerals.
An extensive use of SEM and EDS analyses revealed that the samples contained grains with a high carbon content (>70 wt%) assumed to represent organic matter (OM). The organic matter (OM) residues were observed mingled with the white mica and formed band like structure (Fig 5.3 and Fig 5.5A), and in some parts of the shale filling intergranular fractures (Fig 5.5C).
Grains of quartz, K-feldspar and pyrite were scattered throughout the samples except some pyrite grains which were accumulated together in certain parts of the shale. Quartz was the major inorganic component recognized as subhedral to anhedral ranges in size mostly 2- 14µm and rarely occurred as long prismatic crystals (5–50 μm). Finely dispersed mineral particles within the organic matter were observed as well. K-feldspar ranged from 6-16µm and was mostly anhedral shaped. Pyrites were formed mainly as clusters of microcrystal of 0,1- 1µm held together by a weak bonding and formed framboidal pyrites ranging in grain sizes from 2µm–10µm. Some framboidal pyrites were observed with a size of >10µm. It was also formed as loose aggregates of framboids and lumps. Furthermore, some pyrites were detected as cubes.
Apatite and sphalerite were the two common accessory minerals detected in Type-1 samples.
The minor accessory components were calcite (low magnesium), dolomite, ankerite,
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anhydrite and/or gypsum, barite, rutile, chalcopyrite, pentlandite, chlorite, monazite, iron oxide, zircon, xenotime, and siderite.
Anhydrite and/or gypsum grains were acicular and fibrous shaped with a grain size of ~2- 5µm long and ~0,5-1µm thick. Though these minerals displayed the same EDS in SEM, the two different grains were discriminated by looking at the nature of their crystal structure. It was suggested that the orthorhombic crystals were possibly anhydrite while the monoclinic shaped grains were gypsum (Fig. 5.8).
No swelling clay minerals were detected in Type-1 sample. Table 5.1 shows all the minerals detected by SEM.
Table 5.1 Minerals detected by SEM
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Figure 5.5. Type-1 Alum Shale samples from Gran (tunnel face) and Oslo city area. A. sample 9327- 4: Type-1 from Gran Tunnel showing white micas (1) exhibiting different orientation and bending around other minerals, Framboidal pyrite (3) and subhedral to anhedral shaped quartz (2). B.
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sample 9178-3 from Gran: aggregates of pyrite (3) grains (near the center of the figure). C. Type-1 sample from Oslo area (Oslo-M1) showing organic matter (OM) filling an opening space of the shale and framboidal pyrite (3).
Type-2 samples description
Type-2 samples were composed predominantly of calcite where quartz and pyrite are randomly distributed in the calcite matrix. These minerals are also found in the fracture of the calcite together with K-feldspar, white mica and some accessory minerals. Accessory minerals found in these samples were dolomite, barite, sphalerite, monazite, rutile, xenotime, and zircon. The minerals quartz, pyrite, white mica and K-feldspar have the same characteristics as described in Type-1 samples. No swelling clay minerals were detected in these samples.
More pictures from both types of samples (from SEM analyses) can be found in Appendix C.
Minerals alteration
Pyrite alteration
According to several studies done by researchers, alteration of pyrites due to oxidation occurred commonly in framboidal pyrite such as shown in research done by Hawkins and Stevens (2013) on pyrite-related heaves as a result of oxidation of pyrite. The authors highlighted that the alteration can sometimes be observed on cubic surfaces of pyrite.
Dissolution on the edges of framboidal pyrite and microcracking on cubic surfaces of pyrite were found to be indicators of an alteration process in pyrites (Hawkins & Stevens, 2013).
According to the research, when pyrite framboids altered, they commonly show an inner core of unaltered pyrite and alteration rims surrounding the core. The rims contain high iron and low sulfur content that indicate migration of sulfur away from the rims to form sulfate ion SO4-2 which leads to gypsum formation as discussed in section 4.1.
Unaltered pyrite framboids, as shown in Figure 5.6., are surrounded by flakes of illite/white mica or other minerals that are typical of Type-1 samples. An example of a secondary electron image (taken from Hawkins and Stevens (2013) of altered framboidal pyrites from Dublin area, Ireland, is shown in Fig 5.6 for comparison.
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Figure 5.6 Backscattered electron image showing unaltered framboidal pyrites. A. Unaltered pyrite from Gran Tunnel (sample 9354-4) and B. Unaltered pyrite from Oslo area (sample Oslo-M1)
Figure 5.7: Altered framboidal pyrites from Dublin, Ireland: A. Altered pyrite framboids in mudstone, B. Altered pyrite framboids from the same area with X-ray maps showing relative abundance of sulfur and iron
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As it is illustrated in Fig. 5.8A, the surface of cubic pyrites from the Oslo area (sample Oslo- M1) which are homogeneous and do not show any cracking that indicate an alteration. An example of secondary electron image showing an altered pyrite cube from a black shale sample from Dublin, Ireland is shown together for comparison.
Figure 5.8 Backscattered electron image showing unaltered and altered cubes of pyrites. A.
Unaltered fresh cubes of pyrites from the Oslo area (sample Oslo-M1) and altered cube of pyrite in mudstone from Dublin, Ireland
Though the above alteration signs were observed in pyrite oxidation processes, in case of small pyrite grains like in Alum Shale, it may be difficult to detect the very small tiny grains (~0,1µm) of altered pyrites.
Anhydrite alteration and Gypsum formation
Tabular grains of anhydrite and/or gypsum are observed near the surface of pyrite grains in Oslo-M1 sample (Fig. 5.9). In the middle of the image, marked with black arrow, the orthorhombic shaped crystal grains are possibly anhydrite while the monoclinic shaped marked with black arrows are gypsums.
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Figure 5.9 SE image of Oslo-M1 sample. The possible anhydrite grains (pointed with black arrow) and gypsum (pointed with blue arrow)
5.2 XRD bulk and clay mineralogical analyses
In addition to SEM, XRD bulk mineralogical analyses were performed on all the 14 samples for more accurate and detailed description of mineralogy. Clay mineralogical analyses were performed by re-examining the samples after they were treated chemically by air drying, ethylene glycolation and heating to test the presence and relative amount of clay minerals.
5.2.1 Bulk (whole rock) analysis
The results of the bulk mineralogical analyses, compared with respect to known “standard”
patterns, are given in Table 5.3. Peak positions of d-values utilized for the qualitative analyses of the main minerals in the samples are shown in Table 5.2 Apart from the minerals (crystalline materials), non-crystalline materials (amorphous phase) were also detected by XRD. The abundance of the minerals and the amorphous phases presented in Table 5.4 are semi-quantitative values which are estimations of relative proportions of different phases in the specimen.
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Table 5.2 Peak positions of d-values utilized for the qualitative analyses of the bulk XRD samples
Mineral d-value (Å)
Illite/mica 10.1
Quartz 4.25
K-feldspar 3.24
Pyrite 2.71
Plagioclase 3.19
Calcite 3.04
Dolomite 2.89
Gypsum 7.56
X-Ray Diffraction analyses of bulk mineralogy revealed the constituent minerals of the Alum Shale from the two areas as clay minerals (in the form of illite/mica and chlorite/kaolinite), silica (in the form of quartz), sulfide (in the form of pyrite, sphalerite and pentlandite), feldspar (in the form of K-feldspar and plagioclase feldspar), carbonate (in the form calcite (with very low magnisium) and dolomite), phosphate (in the form of apatite), and oxide (in the form of rutile). Some minerals detected by SEM may not be seen in the XRD analyses due to the detection limit of XRD0 which is in the order of 1-2%.
Illite/mica, quartz, pyrite and K-feldspar are the primary minerals in most of the samples.
Illite/mica was the most abundant mineral with an average value of about 24%. Following are quartz, pyrite, K-feldspar with an average value of 17,5%, 13,7%, 11% respectively.
Pyrite in one of the samples (9260) was about 45%. This can be due to the minerals’
concentration in specific areas when the sample was taken for the XRD test.
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Calcite, dolomite, plagioclase, sphalerite and apatite were the most common minerals found in lower amounts (<5% in average) with an exception of three samples, one from Oslo area (Oslo-M1) and two from the Gran Tunnel (9214 and 9354-2). In these samples, Oslo-M1, 9214 and 9354-2, calcite was the main constituent of the shale with 35,9%, 57,6% and 74,8% respectively. Other minerals detected in few samples in very small quantities were chlorite/kaolinite, rutile and pentlandite.
Table 5.3 Minerals detected by XRD bulk mineralogical analyses and their SQ values given in %. Only the crystalline phase included in this table
Table 5.4 Minerals detected by XRD bulk mineralogical analyses and their SQ values given in % including crystalline and amorphous phase.
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Mineral distribution in the samples
In this section each sample is described generally according to the main constituent mineral type and their abundance. Minerals related to the shale swelling property (pyrite and calcite) are also included to show their presence and abundance in each sample. All values given are derived from the main peaks in the X-ray diffractograms. The amorphous phase SQ value is not included in the calculation and the values given below are the proportional values only from the crystalline phase.
Tunnel core samples (9214, 9224-5 and 9260)
Sample 9214 is the most illite/mica (36,9%) rich specimen compared to the other two core samples. Quartz (15%) was the second most abundant mineral after illite/mica. K-feldspar was observed in considerable amounts (14,8%) while plagioclase was found to be of 7%.
The sample contained 9,4% pyrite and very low quantities of calcite of about 1%.
Based on the XRD analyses, sample 9224-5 was dominated by calcite (57,6%) of the low magnesium calcite type. In this sample illite/mica was around 11,8% while K-feldspar and plagioclase were around 7,9% and 2,8% respectively. Pyrite was found in very low amounts (3,4%).
Sample 9260 was predominantly composed of pyrite of 44%. This sample contained considerable amount of illite/mica (18%) and quartz (12,8%). 5,8% and 1,8% of the sample is K-feldspar and plagioclase respectively. The sample contained nearly 1% of low magnesium calcite.
Tunnel face samples (9178-3, 9178-4, 9178-5, 9199-5, 9215-2, 9327-4, 9354- 2, 9354-4 and 9354-5)
Sample 9178-3 and 9178-4
Samples 9178-3 and 9178-4 contained the same type of minerals. The significant difference in these samples are sample 9178-4 is predominantly composed of quartz (41,7%) while
