Assessment of mineralogical and petrographic factors affecting petro-physical properties, strength and cracking processes of volcanic rocks

    Assessment of mineralogical and petrographic factors affecting petro-physical properties, strength and cracking processes of volcanic rocks ¨ ¨ ul Omer Und¨ PII: DOI: Reference:

S0013-7952(16)30168-5 doi: 10.1016/j.enggeo.2016.06.001 ENGEO 4311

To appear in:

Engineering Geology

Received date: Revised date: Accepted date:

9 December 2015 26 May 2016 1 June 2016

¨ ul, Omer, ¨ Please cite this article as: Und¨ Assessment of mineralogical and petrographic factors affecting petro-physical properties, strength and cracking processes of volcanic rocks, Engineering Geology (2016), doi: 10.1016/j.enggeo.2016.06.001

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ACCEPTED MANUSCRIPT Assessment of Mineralogical and Petrographic Factors Affecting Petro-physical Properties, Strength and Cracking Processes of Volcanic Rocks

Istanbul University

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Engineering Faculty, Geological Engineering Department Avcilar, 34320 Istanbul, Turkey +90 212 4737070 / 17833

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[email protected]

Swiss Federal Institute of Technology, Zurich

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Institute of Geology, Engineering Geology

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Sonneggstrasse 5, 8092 Zurich, Switzerland

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Ömer Ündül

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Abstract Microtextural features influence the petrophysical and mechanical properties of rocks, including the elastic properties and cracking processes under axial loads. Volcanic rocks with

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varying microtextural characteristics and compositions were evaluated to assess the effect of

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the main constituents’ mineralogical, petrographic and micro-structural features on the petrophysical properties, elastic properties and strength. Furthermore, detailed analyses were also used to understand the cracking processes under axial loads. The analyses of the cracking

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processes were completed on thin sections obtained from the mechanically tested specimens.

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The results from the quantitative mineralogical and petrographic studies and observations from thin sections revealed that the mineral mass fractions have an effect on the specific

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gravity and loss-on-ignition (LOI) values. On the other hand, unconfined compressive

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strength (UCS) and elastic properties are mostly affected by petrographic variables (e.g., mineral content). The UCS values tend to decrease with a relative increase in the groundmass

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with respect to the phenocryst content and vice versa. Biotite is the only mineral that influences the UCS individually. Geometric features (e.g., Feret’s diameter and perimeter) of

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opaque minerals and biotite are determined to be important constituents influencing the Young’s modulus.

Under axial loads, crack propagation is strongly dependent on the proportional distribution of the phenocryst and groundmass. It was observed that increasing groundmass content leads to predominantly axial cracks. The cracks tend to bend or propagate as a boundary crack when they reach the boundary of an unaltered phenocryst. Thus, axial and shear cracks co-exist with increasing amounts of phenocrysts, which hinders crack propagation. Occasionally, cracks can penetrate the altered or opaque phenocrysts, depending on the degrees of orientation (with respect to applied load) of the same minerals. Furthermore, the synthesis of the measurements obtained from UCS tests and thin section observations reveals that an increase in the amount of phenocrysts, which promote the formation of more shear cracks in addition to axial cracks,

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causes an increase in the radial strain and Poisson’s ratio. The other factors investigated had minor effects on physical and mechanical properties of studied rocks.

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Keywords: Cracking process, elastic properties, petro-physical properties, quantitative

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mineralogy and petrography, volcanic rocks

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INTRODUCTION

Rock behavior under stress is an important phenomenon that should be conducted when completing engineering studies related to the earth (e.g., host rock for a nuclear repository or

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tunnels, building material, slopes, foundations, etc.). Petrophysical (e.g., unit weights, ultrasonic P-wave velocity (Vp), etc.), mechanical (unconfined compressive strength (UCS))

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and elastic properties (e.g., Young’s modulus, Poisson’s ratio) define the rock behavior. These parameters are influenced by the mineralogical, petrographic and micro-structural

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constituents, which are frequently termed as the microtexture of the rock. Furthermore, slight differences in the microtexture reveal considerable variations in the physical and mechanical

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properties (Tapponnier and Brace, 1976).

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Studies based on modal analysis and defining petrographic variations have been conducted to

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illuminate the effects of the above-mentioned microtextural variations on the petrophysical and mechanical properties (Shakoor and Bonelli, 1991; Ulusay et al., 1994; Bell and Lindsay,

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1999; Tugrul and Zarif, 1999; Tamrakar et al., 2007; Yılmaz et al., 2011; Gupta and Sharma, 2012; Yeşiloğlu-Gültekin et al., 2013 and others). However, due to the heterogeneities in

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rocks, these studies often obtained conflicting results for the same petrophysical and mechanical properties when correlating similar petrographic and mineralogical variables (Prikryl, 2006). Thus, decades of studies on quantitative petrographic variables (Howarth and Rowlands, 1986; Prikryl, 2001, 2006; Nishiyama et al., 2002; Åkesson et al., 2004; Öztürk et al., 2004; Jeng et al., 2004; Räisänen, 2004; Zorlu et al., 2004; Tandon and Gupta 2013) and quantitative mineralogical methods (Coggan et al., 2013; Amann et al., 2014; Ündül et al., 2015 and others) are being adapted to rock mechanical studies to understand the relationships expressed above and to overcome any bias. Furthermore, two important properties influencing the rock behavior are dependent on the microtexture (Eberhardt et al., 1999; Nicksiar and Martin 2013b; Amann et al., 2014 and many others). These properties are 1) crack initiation stresses (CI), which can provide useful

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estimations for spalling (Nicksiar and Martin 2013a) and can be considered as a lower bound estimate for the long-term strength threshold of crystalline rocks (Damjanac and Fairhurst 2010), and 2) crack propagation, which determines the failure processes of rocks and enables

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an understanding of the cracking processes near underground rock structures (Bossart et al.,

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2002; Cai et al., 2004; Kaiser and Kim 2008).

Hatzor and Palchik (1997) and Eberhardt et al. (1998 and 1999) discussed the variations of crack initiation stress levels and peak strengths related to grain size. Nicksiar and Martin

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(2013a and b) introduced the impact of the geometrical heterogeneity of grains on the CI and

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UCS. Wasantha and Ranjith (2004) discussed the effect of water on cracking under uniaxial and triaxial conditions. Additionally, the influence and importance of microtextural variations (Nicksiar and Martin, 2013b), Amann et al., 2014; Ündül et al., 2015) and strength properties

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of each constituent mineral on the CI and UCS were introduced (Lan et al., 2010; Mahabadi et al., 2012; Ündül et al., 2015). The influencing factors of specific minerals (e.g., biotite

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grains) were also mentioned and highlighted (Tapponnier and Brace, 1976; Nishiyama et al., 2002; Räisänen, 2004; Mahabadi et al., 2012; Ündül et al., 2015).

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Furthermore, Gerçek (2007) briefly reviewed the importance of Poisson’s ratio in engineering applications and its effect on the elastic deformation of materials, intact rocks, and rock masses. Schöpfer et al. (2009) announced discrete element modeling results on the changes in some mechanical properties including Young’s modulus with porosity and crack density. The above-mentioned studies reveal that petrophysical, mechanical and elastic properties, including crack initiation stress levels and crack propagation, are responsible for the variations in the geomechanical properties of rocks and rock behavior in different engineering facilities (e.g., tunnels, deep excavations, geological material, etc.). For this reason, understanding the interaction between microtextural variations, petrophysical properties, mechanical properties and cracking processes to assess the immediate and long-term behavior of rocks is important and interesting for various geoengineering facilities and research.

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This research is conducted on volcanic rocks collected from Gökçeada island (NW Turkey, Figure 1) and initiated by a series of unconfined compressive strength tests. Petrographic studies were conducted on thin sections obtained from some volcanic rock specimens before

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and after mechanical tests. Mineralogical studies were also carried out on the same specimens

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by quantitatively evaluating the X-ray powder diffraction analysis. The study investigates the relationships between quantified mineralogical and petrographic variables and petrophysical, mechanical and elastic properties. In addition, the study examines the cracking processes of

GEOLOGICAL SETTING

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volcanic rocks with various microtextural properties under axial loads.

The study area is composed of Tertiary sediments underlain by metamorphic rocks. The ages

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of the rocks in the sedimentary sequence descend from west to east across the study area. The

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sedimentary units can generally be characterized as Eocene turbidities and overlying fossiliferous limestone, Oligo-Miocene sandstones, conglomerates and volcanics. Mudstones

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and conglomerates characterize the Upper Miocene-Pliocene strata. Volcanic rocks in the study area are found as lava flows, small dykes and sills or volcanoclastics interlayered with

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the sedimentary strata (Figure 2) (Akartuna, 1950; Temel and Çiftçi, 2002; Koral et al., 2009). The study area is cut by a set of faults. The major faults are right lateral strike-slip faults that trend in a NE-SW direction almost parallel to the seismically active North Anatolian Fault. Oblique to these major faults, antithetic and synthetic faults exist. The NW-SE trending antithetic faults have dip-slip components. The synthetic faults have a normal fault character with a component of left lateral strike-slip (Koral et al., 2009). 3.

SAMPLING AND TESTING METHODOLOGY

During field studies, block samples (minimum size of 40 cm x 40 cm x 40 cm) were collected from the lava flows of the Miocene aged Hisarlıdağ volcanics. The samples were carefully selected to avoid any signs of weathering and macroscopic heterogeneity (e.g., veins,

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fractures etc.). All of the block samples were collected from fresh and/or slightly weathered profiles, as described by ANON (1995).

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The studied rocks were mainly composed of various amounts of feldspar, amphibole, biotite

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and groundmass, with uncommon quartz phenocrysts (Figure 3). The groundmass was mostly

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fine grained, but in some specimens, coarser grained groundmass was present. Macroscopically, the samples were andesite and rhyodacite.

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3.1. Quantitative Mineralogical and Petrographic Analyses

X-ray powder diffraction (XRD) analyses were conducted to determine the mineralogy of the

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tested samples. 33 specimen were homogenized after crushing to a grain size below 0.4mm. A representatively 2g specimen was milled in ethanol to a grain size below 20 µm with a

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McCrone micronizing mill. X-ray diffraction measurements were accomplished utilizing a

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Bragg-Brentano diffractometer (Bruker AXS D8) using Co Kalpha radiation. The powder samples were step-scanned at room temperature from 2.5 to 80° 2alpha (step width 0.02°

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2alpha, counting time 4 s per step). The software DIFFRACplus (BRUKER AXS) and the Rietveld program AutoQuan (GE SEIFERT) were used to determine the qualitative phase

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composition and the quantitative mineral composition of the samples respectively. The data obtained after this evaluation and calculation step will be termed as mass fraction. Furthermore Loss-on-ignition (LOI) values, were determined gravimetrically by weighing the samples in ceramic crucibles before and after ignition at 1050 °C for 2 hours. Microscopic studies were conducted on more than 70 thin sections using an optical microscope. The thin sections were prepared from each mechanically tested core specimen either before or after the mechanical testing. To analyze the cracks formed due to loading, the thin sections were orientated parallel to the loading direction, and the loading directions are all marked with arrows in any related figures. After mechanical loading, the macroscopically fractured zones were selected for thin section preparation. The central part of the core, where observable macroscopic fractures did not exist, was selected for thin section preparation. For a

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better identification of cracks under the optical microscope, prior to thin section preparation blue stained epoxy resin which penetrate newly formed cracks was injected into the samples. Digital images of the thin sections were obtained by Nikon Digital Sight DS_U3 system

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utilizing NIS Elements Imaging software 4.00. These images were analyzed by the ImageJ

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v1.46r image processing software developed by National Institute of Health, USA (Rasband, 2012).

To conduct a quantitative petrographic analysis, the mosaics of the photomicrographs were

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generated by stitching together the photomicrographs. At this stage, some photomicrographs

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were only composed of a limited number of minerals. To maintain the same scale for every photomicrograph of each thin section, 180 µm was selected as a threshold for an optimum

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view of the mineral grain sizes. The groundmass below this grain size was nearly

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homogeneous in size. Thus, this was the optimum threshold for the rocks in this study for the best view and reliable measurements.

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During analyses, the Feret’s diameter (i.e., maximum caliper), perimeter (i.e., the length of the outside boundary of the selected grain) and area of each mineral grain were measured.

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Throughout these studies, each mineral grain having a major axis (e.g., Feret’s diameter) greater than 180 µm was traced within a defined area. Grains with a major axis length (Feret’s diameter) smaller than 180 µm were considered to be groundmass (see previous paragraph). The measurements were used to calculate the areal distribution of the minerals, the normalized area and proportional relationships of the constituent minerals and groundmass. The calculated areal distribution of the minerals will hereafter be referred to as the mineral content. The total area of the primary minerals was normalized with respect to the total measurement area. Furthermore, the angularity (Eq.1), which defines whether a grain is a circle (i.e., when the value approaches one) or has an elongated shape (i.e., when the value approaches zero) (Ferreira and Rasband, 2012), was also calculated for each mineral grain.

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Area Perimeter 2

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Eq.1

3.2. Petro-physical and Mechanical tests

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The petrophysical properties including the specific gravity (Gs), unit weights (), total

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porosity (nt), and P-wave velocity (Vp) of the studied volcanic rocks were determined according to the procedures proposed by the ISRM (1981 and 2007). Prior to mechanical tests, the P-wave velocities of each specimen were measured along the axial axis using a

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Proceq Pundit Lab device under dry conditions. During the measurements, the length of each specimen was entered into the Proceq device and the P-wave velocities were obtained by

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dividing the length of the specimen by the arrival time of the signal. The petrophysical properties of all of the mechanically tested 35 specimens were identified.

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For the mechanical tests, 35 cylindrical specimens with a diameter of 43 mm (length – diameter ratio of 2) were extracted from the blocks collected in the field. After cutting the

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edges of the cores, the end faces were parallel and met the requirements of the ISRM (1979) suggested methods. The cores were collected in such a way as to maintain an orientation

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perpendicular to the flow direction of the lava. The unconfined compressive strength tests were performed using a stiff 2000 kN servo-hydraulic rock testing device with a digital feedback control at the rock mechanical laboratory at the Chair of Engineering Geology of the Swiss Federal Institute of Technology in Zurich (Figure 4). To avoid the influence of edge effects on the strain measurements, radial and axial strain gauges were placed on the specimen at half of the specimen height. Axial strain gauges (Type BD 25/50, DD1) were firmly attached to opposite sides of the specimens. The displacement measured by a single gauge (Type 3544-150M-120m-ST) attached to a chain wrapped tightly around the specimen was used to calculate the radial strain. To obtain a constant circumferential displacement, which is used as a feedback signal for controlling the load, the axial load was increased by a rate of 0.03 mm/min.

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3.3. Analyzing Cracking Processes In this study, the cracking processes are defined by crack initiation stress levels and crack propagation under axial stresses. Among the different methodologies proposed (Brace et al.,

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1966; Lajtai, 1974; Martin and Chandler, 1994; Eberhardt et al., 1998; Přikryl et al., 2003;

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Nicksiar and Martin, 2012), two strain-based methods were used to determine the CI. These methods are based on defining the stress point where the volumetric strain curve (Brace et al., 1966) and the radial strain curve (Lajtai, 1974) deviate from linearity in the early linear

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segment at low axial strain. The validity of these methods in determining the CI stress levels was discussed in previous studies (Amann et al., 2011; Nicksiar and Martin, 2012; Amann et

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al., 2014, Nicksiar and Martin, 2013a and others).

Following the unconfined compression tests, the defining stress-induced cracks, including

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crack propagation paths and directions, were based on a careful examination of the thin

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sections under a polarizing microscope. The thin section preparation including the injection of

RESULTS AND DISCUSSION 4.1.

Mineralogical and Petrographic data

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a blue-stained epoxy resin for this stage is given in detail in section 3.1.

The results from the quantitative analyses of the mineralogical data are given in Table 1. Feldspar minerals (orthoclase and plagioclase), followed by amphibole and quartz are dominant in each specimen, according to the data from the quantitative XRD studies. In Table 1, the areal distributions of the major components defined during petrographic studies are also given. The groundmass constitutes the major areal distribution of each specimen, ranging from 46.6 to 79%. In contrast, almost all of the feldspars detected under the microscope were plagioclase feldspars (Table 1, Figure 5). Thus, the orthoclase feldspar, identified by the XRD studies, primarily exists within the groundmass. On the other hand, some constituent minerals (e.g., amphibole) are occasionally opacified. The quantitative mineralogical analysis suggests that opaque minerals (e.g., magnetite) rarely exist (Table 1). Thus, the opacified and opaque

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minerals together are called opaque minerals unless otherwise indicated. The petrographic analyses suggest that increasing the phenocryst content leads to a decrease in the groundmass content. This observation can be considered as a substitute for heterogeneity. The

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heterogeneity related to the content of the groundmass and the phenocrysts of such rocks were

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discussed by Ündül et al. (2015). Additionally, the measured Feret’s diameter and the perimeters collected during the microscopic studies of the main constituent minerals are given in Table 2. Variations in Petro-physical Properties

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4.2.

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Some basic petrophysical properties (specific gravity (Gs), total porosity (nt), unit weight () and P-Wave Velocity (Vp)) and LOI values for the studied rocks are given in Table 3.

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Empirical equations showing the relationships between the various petrophysical properties

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and microtextural variables are given in Table 4. In Table 4, the effects of the phenocryst content and mass fractions on the physical and mechanical properties are listed. Furthermore

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student t-tests have been performed to determine the relationship between the investigated parameters. The significance of r values were investigated by the t-test assuming that both

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variables are normally distributed. Critical tabulated t-values taken for the 95% confidence level are given in Table 4. Values obtained from t-test conclude that there is a correlation between the variables given in Table 4 with a confidence level of 95%. Among all of the minerals detected in the quantitative XRD analyses, the mass fractions of quartz, magnetite and biotite exhibit a relationship with specific gravity. Increasing the mass fraction of quartz causes the specific gravity of the specimen to decrease (Figure 6 and Table 4). Increasing the mass fraction of magnetite (r=0.68) and biotite (r=0.63) cause an increase in the specific gravity (Table 4). This situation is related to the specific gravity of each major mineral (Table 5). Magnetite and biotite have relatively higher specific gravities, whereas quartz has a relatively lower specific gravity. Moreover, the very limited quartz content (e.g., quartz phenocrysts) reveals that most of the quartz exists within the groundmass. Thus, the

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mass fraction of quartz, which ranges between 8.9 and 25.5%, has the highest impact on the specific gravity for the studied rocks.

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During the synthesis of quantitative petrographic data and physical properties, the effect of

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opaque minerals and biotite on the P-wave velocity was detected. The analyses revealed that

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increasing the Feret’s diameter of opaque minerals and biotite leads to a decrease in the Pwave velocity (r=-0.66 and r=-0.60, respectively) (Table 4). A similar effect on the P-wave velocity was also determined for the perimeter of opaque minerals (r=-0.65, Table 4).

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Furthermore, quantitative mineralogical data confirm that the mass fraction of feldspar

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minerals influences the P-wave velocity (Table 4). Schon (1996) and Barton (2007) declared a decrease in Vp with increasing porosity but in this study no relation was obtained between

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these two parameters. The lack of a relationship between the porosity and Vp, for low porosity

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crystalline rocks was also suggested by Tandon and Gupta (2013). The LOI values are commonly used to clarify the petrophysical properties of rocks. The LOI

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value is also known to be an indicator of the weathering stage of rock (Aristizabal et al., 2005; Arıkan et al., 2007; Ündül and Tuğrul, 2012). For the studied rocks, low LOI values (Table 3)

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indicate that the samples have unweathered or slightly weathered grades. The effect of the mass fractions of feldspar (orthoclase + plagioclase) (Figure 6 and Table 4) and magnetite on the LOI values were determined using quantitative mineralogical data (Table 4). In addition, slight changes in LOI occurred due to the mass fraction of biotite (Table 4). On the other hand, microscopic studies concluded that the plagioclase content is the only petrographic parameter that influences the LOI values (Table 4). Although the samples were collected from fresh and slightly weathered zones, petrographic analysis revealed that unaltered and lesser amounts of altered minerals coexist. Among the major constituent minerals, plagioclase has a higher tendency to be altered than other minerals. Thus, the feldspar minerals have the strongest relationship with LOI (Figure 6). Randomly distributed feldspar minerals with various alteration grades (Figure 7) in the same

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specimen mask a higher correlation coefficient for the relationship of feldspar minerals and LOI. Variations in Strength and Cracking Processes Under Uniaxial Loading

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4.3.

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The unconfined compressive strength of the studied rocks varies between 102 and 289 MPa

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(Table 3). Microtextural parameters such as the main constituent mineral content, groundmass content and porosity are the basic factors affecting the UCS. Because porosity is one of the

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important factors that determines the characteristics of fluid flow in rocks (e.g., migration, distribution), the relationship between the porosity and UCS was widely investigated. Data in

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this study imply that UCS decreases with increasing porosity (Table 4). Ulusay et al. (1994), Jeng et al. (2004), Sousa et al. (2005), Cantisani et al. (2013) and data provided by the models

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of Baud et al. (2014) demonstrated a similar trend with varying correlation coefficients.

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The data provided in Table 3 and 4 illustrate the weak influence of the ratio of the total

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plagioclase and amphibole content to the groundmass content on the UCS for both rock compositions (e.g., samples with andesite and rhyodacite compositions evaluated together) (r = 0.37). In contrast, a stronger relationship for the same ratio was found to influence the UCS

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(r = 0.76) for rocks that were only composed of andesite (Table 4, Figure 8). The variation in the correlation coefficients in the above-mentioned situations (e.g., samples with andesite and rhyodacite compositions that were evaluated together and andesitic composition only) is probably related to the large difference in the ratio between the phenocrysts and groundmass of the two rock types (e.g., andesite and rhyodacite) (Figure 1 and Table 1). The dominant parameter affecting the strength is dependent on the heterogeneity, which is defined by the phenocryst and groundmass ratio. At considerably lower phenocryst (e.g., plagioclase and amphibole) and groundmass ratios, the groundmass has a larger effect on the strength. On the other hand, when there is a higher phenocryst (e.g., plagioclase and amphibole) and groundmass ratio, the phenocryst content has a larger effect. Even the heterogeneity threshold level is not clear enough to differentiate whether the groundmass or phenocryst content is the

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dominant factor controlling the UCS. The relationship shown in Figure 8 Figureconcludes that, for the andesitic rocks in this study, the relative increase in phenocrysts (e.g., plagioclase and amphibole) compared to the groundmass positively influences the UCS. For considerably

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lower phenocryst (e.g., plagioclase and amphibole) and groundmass ratios (e.g., lower

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heterogeneity), the effect of the phenocrysts on the UCS could not be identified. The increase in the UCS with increasing plagioclase and amphibole phenocrysts might be related to the overall contribution of the various minerals found in the groundmass that have relatively

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lower elastic moduli than the phenocrysts (e.g., orthoclase, clay minerals) (Ündül et al.,

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2015).

Furthermore, Zorlu et al. (2004) and Tandon and Gupta (2013) announced that the UCS

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increases with increasing quartz content. In our study, a weak relationship between the mass

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fraction of quartz and the UCS is obtained (r=0.46, Table 4). And no reliable relationship was found between the quartz content (e.g., quartz phenocrysts) and the UCS. The weak

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relationship between the mass fraction of quartz and the UCS found in this study is probably related to the distribution of quartz minerals mostly within the groundmass. Ulusay et al.

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(1994) suggested similar weak relationships between quartz and UCS. In this study, the highest correlation coefficient between the UCS and discrete mineral contents was established between biotite and the UCS. The biotite content was found to be the only mineral individually affecting the UCS. As the biotite content increases, a decrease in the peak strength was observed (r=-0.56, Table 4). Following the initiation of cracks, two microtextural components (e.g., phenocrysts and groundmass content) are considered to be major parameters that influence the crack propagation under axial loads. After the cracks emanate, they tend to propagate parallel and sub-parallel to the applied load, according to the Griffith (1924) theory of rupture. However, once they reach a boundary of a mineral, the crack aligns as a boundary crack, following the stiffness contrast generated by the different elastic properties. Or, if the constituent mineral

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contains a preferential pathway that was generated during the tectonic history of the rock, the crack may propagate along that preferential pathway (Amann et al., 2014, Ündül et al., 2015). This preferential pathway can be a plane of weakness where the crack can propagate (e.g.,

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cleavage planes of biotite grains, previously formed cracks within plagioclase) (Nishiyama et

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al., 2002; Mahabadi et al., 2012; Nicksiar and Martin, 2013a; Ündül et al., 2015). With increasing stress, the cracks can penetrate any component, even those with higher elastic moduli (Amann et al., 2014).

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For the rhyodacite specimen (e.g., higher groundmass to phenocryst ratio), the stress-induced

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loading parallel cracks are more evident than the cracks observed in the andesitic specimen (e.g., relatively lower groundmass to phenocryst ratio). In such a situation of higher

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groundmass content, the cracks initiated under the axial loads propagate parallel (e.g., 0-5

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degrees) to the applied load (Figure 9). In andesitic composition phenocrysts have larger areal distribution and the parallelism of the propagating cracks can be interrupted more densely

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than in the situation of rhyodacite composition. The increase in phenocrysts with relatively higher elastic moduli than the overall elastic modulus of the groundmass hinders the

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propagation of loading parallel cracks, leading to sub-parallel and/or boundary cracks (Figure 9). Similar findings for andesitic rocks were reported by Ündül et al. (2015). In such cases of cracking (e.g., cracking interrupted by a stiffer phenocryst), a delay in the coalescence of cracks can occur, and higher stress levels are required to break the stiffer phenocrysts and/or to coalesce the stress-induced cracks. As a consequence, the failure of the specimen begins at higher stress levels. Thus, the peak strength increases with increasing phenocrysts that have a higher stiffness with respect to the groundmass. Recently, Amann et al. (2014) reported such increases in the peak strength for heterogeneous rocks (e.g., components that have a stiffness contrast) on the crack propagation under uniaxial loads. These findings validate the interpretations regarding the increase in the UCS with increasing phenocryst to groundmass ratios in this study.

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Another important rock parameter is the stress level for crack initiation. Crack initiation stress levels were determined using two methods that were based on strain. According to the data obtained from analyses of uniaxial compressive strength tests, the cracks are initiated at stress

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levels that are approximately 0.44-0.46 of the UCS (Figure 10). These stress levels

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corroborate the findings of Ündül et al. (2015), which used andesitic rocks with less data. In this study, although different volcanic rocks are used and there is a larger data set, the relationship between the σCI and UCS is almost constant. Nicksiar and Martin (2013)

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concluded similar proportional relationships between crack initiation and peak stress for

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crystalline rocks.

4.3.1. Effect of alteration and opaque minerals on cracking

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The preferential paths for crack growth in plagioclase due to previously formed cracks

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generated during the tectonic history of the rock were discussed by Ündül et al. (2015). Additionally, the results from this study suggest that the alteration and opacification of

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minerals may also influence the cracking process. Despite the collection of the samples from unweathered rock masses, alterations and opacifications of minerals were observed during

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petrographic studies. The importance of the alteration of plagioclase minerals and their effect on cracking was also concluded by Åkesson et al. (2004). The authors indicated that the alteration of plagioclase minerals and their effect on cracking is more relevant than the effect of twinning planes. The alteration and opacification of the main constituent minerals (e.g., plagioclase and amphibole) is shown in Figure 11. The plagioclase minerals exhibit different types of alteration related to their composition (e.g., anorthic and albite composition) (Figure 7). In this study, the plagioclase grains are mostly altered to a clay zone in the center of the grain (Figure 7) or an alteration rim on the outer part of the grain (Figure 11). These alteration zones influence the cracking processes. The boundaries of two components with different stiffness properties (e.g., unaltered and altered plagioclase) generate a plane of weakness.

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Along these boundaries between the alteration rim and the unaltered plagioclase, cracks can emanate and/or propagate as intra-granular cracks (Figure 7 and Figure 11).

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The effect of unaltered amphibole phenocrysts on the σCI, UCS and crack propagation was

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discussed by Ündül et al. (2015). Under a uniaxial load, amphibole grains that are stiffer than

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the surrounding groundmass hamper crack propagation, and the crack propagates as a boundary crack or is bent by the amphibole grain and propagates into a relatively softer (e.g., lower Young’s modulus) groundmass. The crack may also propagate along a previously

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formed crack within the amphibole phenocryst (Ündül et al., 2015). However, due to the

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opacification of the amphibole grains, the stress-induced cracks can emanate and/or propagate within the amphibole mineral independent from the previously formed cracks (Figure 11b). In

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addition, the orientation of the altered amphibole grains with respect to the applied load

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dominantly influences the crack propagation (Figure 9 and Figure 11b). In Figure 9, loading parallel cracks are bent under the influence of the altered amphibole instead of propagating

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into the mineral. However, in Figure 11b, the cracks are formed within the grain independent from a plane of weakness. These findings reveal that the orientation is important for crack

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propagation even in altered amphibole grains. According to the previous statements and discussions, alteration and opacification processes of the main constituent minerals affect the cracking of each mineral individually. Thus, the impact of minerals on the σCI, UCS and cracking processes is hindered (e.g., varying correlation coefficients between petrographic variables) due to the co-existence of altered and unaltered minerals in the same specimen. This evidence suggests that the mechanical parameters are only partially dependent on textural parameter Similar findings on the relationship between cracking and opaque minerals based on the stiffness contrast were reported by Åkesson et al. (2004). Additionally, this study supports the importance of the orientation of the opacified minerals with respect to the loading direction.

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The increased cracking due to the alteration of minerals is validated by the relationship between the Young’s modulus and LOI values (Table 4).The LOI values increase as the overall Young’s modulus of the specimen decreases. Although the samples were collected

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from fresh and slightly weathered rock masses, the co-existence of minerals with various degrees of alteration masks the relationship between the Young’s modulus and LOI, yet the

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influence (e.g., slight decrease in Young’s modulus values) of the LOI is still apparent (Table

4.4.

Variations in Elastic Properties

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4).

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The Young’s modulus and Poisson’s ratio are important mechanical properties used to predict the geomechanical behavior (e.g., deformation) of rocks during construction (e.g., deep

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excavations, drilling of wells, assessment of changes in nuclear waste repositories near fields

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etc.) (Gerçek, 2007; Schöpfer et al., 2009; Hudson et al., 2011, etc.). The analyses of mineralogical, petrographic and mechanical data suggest that phenocrysts and

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groundmass are the primary microtextural factors that influence the Poisson’s ratio. A

observed.

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convincing relationship between the mass fractions of minerals and Poisson’s ratio was not

Samples with higher groundmass content exhibit a more homogenous texture (e.g., rhyodacite composition). Under uniaxial loading, the cracks align dominantly parallel to the applied load unless they reach a phenocryst boundary that generates the heterogeneity (Figure 9). As described in the above sections (sections 3.2-3.3), the existence of phenocrysts changes the crack propagation. Heterogeneity increases with the relative increase in the phenocryst content against groundmass. This situation is primarily observed in samples of andesitic composition. Although the cracks propagate parallel to the applied load in a homogeneous texture, with increasing heterogeneity (e.g., increase in phenocryst content), axial and shear cracks are generated together (see section 3.3). Thus, the Poisson’s ratio decreases with increasing groundmass content and increases with an increase in the phenocryst content

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(Figure 12) due to the combined effect of axial splitting and shearing, which generates an increase in the radial strain (Ündül et al., 2015).

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The relationships of the same parameters (e.g., ratio of the total content of plagioclase and

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amphibole to the groundmass content in relation to Poisson’s ratio) of the two different

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compositions of rocks in this study (e.g., andesitic and rhyodacite compositions) are given in Table 4, , separately. The increase in Poisson’s ratio and the relative increase in the total plagioclase and amphibole content with respect to the groundmass content has a stronger

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relationship for andesitic specimens (r=0.66), whereas there is a weaker relationship when the

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specimens have both andesitic and rhyodacite composition (r=0.56) (Table 4). The effect of the same parameter on Poisson’s ratio has the highest correlation coefficient (r=0.87) for the

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specimen of rhyodacite composition.

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The discrete effect of the groundmass content on Poisson’s ratio has a stronger relationship than the combined effect of the plagioclase, amphibole and groundmass content (Table 4).

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The Poisson’s ratio decreases due to an increase in the groundmass content. According to the previously mentioned microtextural parameters and findings influencing Poisson’s ratio, it is

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evident that the change in groundmass content of the rhyodacite specimens has the highest impact. The influencing factor of the groundmass on Poisson’s ratio also validates the previously discussed combined effect of the plagioclase, amphibole and groundmass content on Poisson’s ratio. The synthesis of the quantitative petrographic variables and elastic properties suggests that the Young’s modulus decreases with increasing grain size and the perimeter of opaque minerals (Table 4). This finding is supported by the observations of crack propagation in opacified minerals (Figure 11), where the stress-induced cracks may propagate within opacified minerals independent of any planes of weakness. As a consequence, the overall stiffness is affected by the existence of the opacified minerals. On the other hand, increasing angularity (e.g., opaque minerals with a sphere shape) of the opaque minerals increases the Young’s

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modulus (Table 4). This observation can be related to the stress concentration accumulated around the relatively weak opacified mineral grains that have a spherical shape and transform

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the stress to the neighboring constituents (e.g., groundmass).

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According to the data obtained in this study, the overall Young’s moduli of the specimens are

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dependent on the geometric properties of opaque minerals. The Feret’s diameter and perimeter of opaque minerals has the highest influence on the Young’s modulus. These results are compatible with previous observations on the cracking processes. The increase in the size

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of opaque minerals decreases the strength and increases the deformation of the specimen.

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Furthermore, biotite is the only individual mineral affecting the UCS, which also influences the Young’s modulus (Table 4). As the size of the biotite grains increases, there is a clear

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and opacified members of biotite.

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decrease in the Young’s modulus. This observation might be related to the cleavage planes

On the other hand the interactions of multiple inputs were investigated applying a multiple

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regression analysis. The best linear multiple models for uniaxial compressive strength and

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Young’ modulus are given in Table 6.

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CONCLUSION

The petrophysical, mechanical and elastic properties of volcanic rocks were investigated through quantitative mineralogical and petrographic variables. This research leads to the

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following concluding remarks:

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1. The mass fractions of the minerals influence the specific gravity, whereas none of the quantitative petrographic variables (e.g., mineral content) has an effect. 2. The geometrical properties of opaque minerals (e.g., Feret’s diameter and perimeter),

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biotite (e.g., Feret’s diameter) and the mass fraction of feldspar minerals influence the

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Vp. An increase in the grain size of opaque minerals and biotite leads to a decrease in Vp. However, an increase in the mass fraction of feldspar increases the Vp.

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3. The mass fraction of the feldspar minerals has the strongest effect on the LOI. An

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increase in the mass fraction of feldspar minerals generates an increase in LOI. 4. The mineral content, groundmass content and porosity are the main microtextural

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parameters influencing the UCS. The mass fractions of the minerals did not have any clear relationship with σCI and UCS.

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The UCS decreases with increasing total porosity. The increase in the groundmass with respect to the main constituent phenocrysts causes the UCS to decrease; however, the increase in the phenocryst content (e.g., amphibole and plagioclase) with respect to groundmass causes an increase in the UCS. The biotite content is the only phenocryst that individually affects the UCS. 5. The phenocrysts and groundmass content are considered to be the major factors that influence the crack propagation. Increasing the phenocryst content promotes an increase in radial strain, resulting in an increase in Poisson’s ratio. Increasing the groundmass content generates cracks that primarily align parallel to the applied load. Compared to the samples with higher phenocryst contents, the Poisson’s ratio decreases with increasing groundmass.

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6. Opaque and altered minerals influence the UCS and crack propagation. With an increasing amount of opaque minerals, there is a decrease in the UCS. The stressinduced cracks can penetrate opacified or altered grains. These grains were originally

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unaltered and stiffer, and the cracks were previously unable to propagate. However, in

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the case of opacified minerals the orientation of the mineral with respect to the loading direction is clear. The orientation of opaque minerals should be considered when studying crack propagation and elastic properties of similar rocks.

biotite and increasing LOI values.

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7. The Young’s modulus of the studied rocks decreases with increasing grain size of

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8. Changes in some petrophysical properties (e.g., specific gravity, LOI) can be defined by the data obtained from quantitative mineralogical analyses. On the other hand,

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petrographic investigations (e.g., mineral content, grain geometries including grain

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size and orientation, previously formed cracks, etc.) provide essential data to

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understand the behavior of the studied rocks under unconfined axial loads. 9. The synthesis of both mineralogical and petrographic data is crucial for better approximations and analysis of rock behavior.

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10. Due to the co-existence of altered and unaltered minerals with various orientations with respect to the applied load in the same specimen, the impact of individual minerals on the σCI, UCS, and crack propagation remains unclear. Thus, the results of this study had low correlation coefficients.

Acknowledgments The author is grateful to Dr. Florian Amann (ETH – Institute of Geology, Engineering Geology) for his suggestions and would like to thank to Prof. Dr. Simon Löw (ETH – Institute of Geology, Engineering Geology) for providing laboratory facilities. Author also wishes to thank Dr. Namık Aysal (İstanbul University – Geological Engineering Dept.) and Dr. Michael

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Plötze (ETH – Institute for Geotechnical Engineering, Environmental Engineering and Clay Mineralogy) for their help on quantitative mineralogical analysis and petrographic analysis respectively. The author thanks to Prof. Tamrakar and other anonymous reviewers for their

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comprehensive review and comments.

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Figure 1: Location map of the study area.

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Figure 2: Sampling locations and the geological map of the study area (Geological map modified from Koral et al., 2009).

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Figure 3: Typical macroscopic views of samples with varying textures. Figure (a), (b), and (c) represent an overview of specimens with an andesitic composition and (d) represents a rhyodacite specimen. (a), (b), and (c) have relatively coarser phenocrysts than in (d). The groundmass content is relatively higher in (d) than the other specimen.

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Figure 4: A view of the experimental setup

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Figure 5: Typical microscopic views of studied rocks under parallel nicols prior to mechanical testing a) a view from an andesite b) a view from rhyodacite.

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Figure 6: The relations between a) specific gravity and mass fraction of quartz b) LOI and mass fraction of feldspar minerals

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Figure 7: The core of the plagioclase phenocrysts is more anorthic or andesine in composition. Outer part is in albite composition. In such composition the alteration begins from inner parts and the outer part is more resistant to alteration.

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Figure 8: The influence of main constitutive minerals on UCS *Note that only the data points for andesitic composition are plotted

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Figure 9: Crack propagation in specimens of different compositions under axial load a) specimen with rhyodacite composition b) specimen with andesitic composition. For the specimen of rhyodacite composition the ratio of total plagioclase and amphibole content to groundmass content is 0.28. * Even some amphibole grains are altered, due to the orientation of altered grain (with respect to do loading direction) the cracks propagate as boundary crack or penetrate into the groundmass instead of propagating within the altered mineral.

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Figure 10: The relation between crack initiation stress levels and peak stresses Note that data points represented by circle are obtained utilizing the method proposed by Lajtai (1974) and lower trend-line was drawn for these data points. Data points represented by square are obtained utilizing the method proposed by Brace et al. (1966) and the upper trend-line was drawn for these data points.

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Figure 11: Views of a) opacified amphibole phenocrysts and the alteration rim of a plagioclase phenocryst. The crack propagating along the boundary of altered and unaltered section is indicated by blue epoxy b) stress induced cracks within an opacified amphibole phenocryst

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Figure 12: Stress–strain response for specimen with varying normalized area of phenocrysts and groundmass

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Table 1: Results obtained from quantitative mineralogical and petrographic analyses Normalized area of measured components

Quantitative XRD analyses results SN Plag. (%)

Q. (%)

Mag. (%)

GrM. Amp. (%) (%)

Bio. (%)

1

4.2

2.3

1.0

21.9

48.3

13.1

2.7

59.4

15.7

9.0

13.5

2.8

2

3.1

2.1

1.5

23.5

47.8

13.0

2.6

54.8

2.6

8.4

32.7

1.8

3

4.0

2.1

2.0

22.8

46.4

13.5

2.9

51.1

4

4.9

2.5

1.1

23.5

45.6

13.3

2.8

53.5

5

18.9

1.3

1.2

25.1

26.6

11.9

0.7

6

18.6

1.4

1.6

24.1

26.9

11.2

7

19.4

1.8

1.2

24.4

25

8

17.6

0.5

1.5

24.0

9

17.1

1.3

1.2

10

16.9

0.8

11

15.2

12

T

Orth. (%)

IP

CM (%)

Plag. Fel. OpM. (%) (%)

33.1

0.5

1.6

2.2

33.8

1.1

55.0

16.7

5.3

21.4

0.7

0.5

53.9

21.2

4.5

18.0

1.5

12.1

0.7

55.2

17.0

6.3

19.1

1.8

28.9

10.2

0.2

56.5

4.9

13.4

23.6

1.6

21.9

35.9

14.5

1.8

50.4

13.2

0.3

34.3

1.0

1.4

21.5

36.4

14.3

1.9

58.8

12.2

3.3

23.3

0.7

0.5

1.5

21.6

39.4

14.6

1.2

63.0

11.4

12.7

11.9

1.6

16.1

1.2

1.5

21.2

37.5

14.0

1.7

46.6

26.0

1.0

25.0

0.9

13

2.5

0.4

6.50

20.0

51.1

8.9

2.8

69.0

4.0

2.0

29.9

8.4

14

2.6

3.1

8.30

19.6

47.4

9.6

2.9

61.4

3.1

1.9

25.0

6.8

15

2.0

2.9

8.20

19.6

48.1

9.9

2.9

73.8

2.9

0.1

23.1

3.7

16

1.30

0.40

11.1

22.0

37.3

22.8

1.5

69.0

7.4

0.1

17.8

0.5

17

0.9

0.20

11.9

21.9

37.3

22.5

1.3

58.1

7.6

0.5

22.9

0.4

18

1.0

19

1.2

20

6.9

21

6.7

22

TE

NU

SC R

2.0

D

10.8

MA

Bio. (%)

CE P

Amp. (%)

0.30

10.7

23.3

36.6

22.6

0.9

70.1

6.9

0.6

17.8

0.3

0.50

10.2

23.2

36.9

23.1

0.9

79.0

5.2

0.6

17.1

0,4

1.6

5.4

21.0

40.9

12.0

2.7

60.3

20.1

3.1

15.8

6.6

1.8

3.6

22.7

42.3

12.0

2.0

52.7

3.6

1.2

40.3

1.9

AC

L6

L5

L4

L3

L2

L1

SL

6.7

1.7

4.7

21.6

41.2

11.8

2.6

59.7

6.5

4.6

27.6

4.0

5.7

1.9

4.0

22.1

41.8

12.4

2.3

58.1

12.9

9.1

19.1

1.9

6.3

1.9

4.2

21.6

42.7

12.0

2.8

60.8

16.3

4.1

17.5

3.8

25

10.1

1.1

4.7

20.7

43.5

14.0

3.0

56.2

6.8

0.2

34.6

1.0

26

9.7

3.0

4.6

20.3

43.2

13.8

2.9

60.5

8.8

0.1

28.6

0.9

27

8.9

1.6

6.3

20.3

45.9

9.9

3.0

61.3

9.6

0.1

26.1

1.3

28

9.6

2.9

4.6

20.7

42.9

13.9

2.9

64.8

8.9

0.5

24.9

1.1

29

1.1

0.5

2.0

25.1

42.3

23.3

0.9

67.0

4.8

1.3

18.2

1.5

30

1.0

0.9

2.1

26.3

38.2

25.5

0.8

66.1

5.6

0.2

20.0

2.5

31

-

-

-

-

-

-

-

69.4

4.4

0.7

17.9

2.5

32

-

-

-

-

-

-

-

69.5

5.1

0.7

19.8

5.0

33

6.2

2.5

6.5

21.0

37.0

13.8

2.7

65.6

3.7

3.5

21.8

4.6

34

14.7

1.7

5.7

17.2

41.0

16.1

1.4

64.7

10.9

3.8

19.3

1.1

35 19.5 1.5 7.9 16.3 36.5 For sample locations please refer to Figure 2

12.6

2.2

61.9

10.0

4.0

19.0

1.4

23

L9

L8

L7

24

ACCEPTED MANUSCRIPT Specimen between 1-15, 20-28 and 33-35 are in andesite composition, 16-19 and 29-30 are in rhyodacite composition Specimen 31 and 32 are also in rhyodacite composition detected under microscope CM: Clay Minerals (sum of mass fractions of chlorite, vermiculite and smectite) Note that only the mass fractions of major mineralogical components are given.

AC

CE P

TE

D

MA

NU

SC R

IP

T

SL: Sampling Location, SN: Sample Number, Amp.: Amphibole, Bio.: Biotite, Orth.: Orthoclase, Plag.: Plagioclase, Q.: Quartz, M.: Magnetite, GrM.:Groundmass, Fel.: Feldspar, OpM.: Opaque Minerals

45

ACCEPTED MANUSCRIPT

46

Table 2: Average Feret’s diameter, average perimeters and angularity values of major constituent minerals

0.55

0.21

0.66

0.39

1.45

0.74

1.74

1.08

0.78

2

0.34

0.33

0.48

0.44

0.86

1.20

1.30

0.75

3

0.45

0.20

0.51

0.25

1.13

1.20

T

1.15

1.37

0.65

0.75

4

0.32

0.37

0.62

0.23

0.83

1.36

1.67

0.56

0.77

5

0.43

0.70

0.57

0.51

1.05

1.58

1.65

1.44

0.72

6

0.50

0.56

0.96

0.36

1.26

1.33

2.61

0.82

0.67

7

0.38

0.58

0.59

0.55

0.95

1.40

1.70

1.57

0.73

8

0.39

0.61

0.72

0.44

1.01

1.18

1.94

1.17

0.72

9

0.36

0.25

0.52

0.18

0.89

1.29

1.42

0.49

0.79

10

0.28

0.22

0.62

0.22

0.71

0.95

1.67

0.42

0.79

11

0.32

0.21

0.49

0.22

0.80

1.31

1.33

0.64

0.78

12

0.34

0.20

0.57

0.19

0.85

0.74

1.51

0.53

0.80

13

0.43

0.39

0.68

0.45

1.04

1.06

1.90

1.13

0.71

14

0.39

0.72

0.63

0.34

0.94

1.83

1.68

0.85

0.69

15

0.39

0.28

0.79

0.37

0.97

0.57

2.00

0.94

0.72

16

0.37

0.41

0.56

0.27

0.95

0.82

1.50

0.75

0.81

17

0.49

0.40

0.60

0.24

1.17

0.5

1.67

0.67

0.78

18

0.45

0.48

0.64

0.30

1.11

1.24

1.77

0.80

0.74

19

0.35

20

0.43

21

0.36

22

0.45

SC R

NU

MA

D

TE

IP

1

CE P

Average Feret’s diameter (mm) Average perimeters (mm) Angularity Sample No Amphibole Biotite Plagioclase Opaque Amphibole Biotite Plagioclase Opaque Opaque feldspar Minerals feldspar Minerals Minerals

0.54

0.26

0.86

0.92

1.44

0.67

0.79

0.37

0.46

0.37

1.03

0.91

1.25

0.94

0.69

0.25

0.58

0.28

0.89

1.08

1.56

0.71

0.70

0.48

0.38

0.35

1.16

1.11

1.18

0.87

0.71

0.35

0.24

0.49

0.24

0.88

1.26

1.35

0.59

0.68

0.31

0.46

0.64

0.23

0.77

1.51

1.71

0.59

0.73

25

0.30

0.28

0.46

0.21

0.75

0.80

1.21

0.39

0.79

26

0.29

0.19

0.37

0.19

0.75

0.40

0.98

0.35

0.79

27

0.31

0.18

0.32

0.20

0.78

0.35

0.85

0.44

0.79

28

0.28

0.19

0.32

0.18

0.73

0.80

0.86

0.36

0.81

29

0.58

0.67

0.54

0.22

1.47

2.24

1.44

0.58

0.68

30

0.46

0.75

0.51

0.28

1.17

2.33

1.35

0.76

0.69

31

0.63

0.52

0.60

0.35

1.57

1.94

1.61

0.95

0.65

32

0.67

0.71

0.66

0.29

1.60

2.08

1.73

0.79

0.67

33

0.34

0.49

0.65

0.36

0.85

2.48

1.83

0.91

0.72

34

0.29

0.24

0.66

0.23

0.73

2.17

1.70

0.62

0.74

35

0.37

0.81

0.54

0.32

0.91

1.64

1.65

0.88

0.70

23 24

AC

0.48

ACCEPTED MANUSCRIPT

47

peak

CI, vol

CI, rad

(MPa)

(MPa)

(MPa)

-

108

-

52

-

-

158

-

-

4.83

57.0

0.25

T

Table 3: Petro-physical and mechanical test results of the samples

126

127

25.7

5.12

53.5

0.24

217

107

101

5.51

25.1

3.83

33.9

0.22

200

78

79

2.64

5.97

24.8

3.64

2.57

2.62

5.12

24.8

3.34

8

2.70

2.67

7.61

24.7

3.60

9

1.40

2.62

1.77

25.7

4.74

10

1.49

2.68

4.29

25.6

11

1.35

2.67

3.56

12

1.57

2.63

13

2.02

14

LOI

Gs

nt

(kN/m3)

Vp (km/s)

E (GPa)



1

0.44

2.70

5.57

25.5

4.99

-

2

0.49

2.70

5.42

25.5

4.59

3

0.53

2.70

4.72

25.7

4

0.62

2.72

5.43

5

2.35

2.66

6

2.45

7

272

SC R

IP

Sample No

0.21

135

74

71

30.8

0.20

154

71

71

27.0

0.20

102

47

46

53.4

0.17

238

109

101

4.45

51.2

0.15

154

76

73

25.7

4.45

48.7

0.15

148

62

63

2.44

25.6

4.89

56.2

0.25

289

130

129

2.68

5.80

25.2

4.53

38.6

0.14

169

91

68

1.63

2.72

7.56

25.1

4.52

39.4

0.15

123

54

53

15

1.88

2.72

7.55

25.1

4.41

40.3

0.16

124

55

54

16

2.11

2.57

0.84

25.5

4.59

42.8

0.18

233

107

109

17

2.07

2.56

0.29

25.5

4.96

43.5

0.22

231

112

106

18

2.13

2.59

1.88

25.4

4.66

42.0

0.18

229

110

100

19

2.15

2.57

0.66

25.5

4.72

44.8

0.14

242

117

112

20

0.72

2.71

6.62

25.3

4.42

37.6

0.17

151

74

70

21

0.76

2.75

7.63

25.4

4.56

40.3

0.2

162

74

73

22

0.77

2.75

8.52

25.2

4.29

38.3

0.18

153

69

69

23

0.79

2.76

8.28

25.3

4.34

39.3

0.17

144

71

68

24

0.90

2.74

7.36

25.4

4.34

41.3

0.18

160

70

65

25

1.30

2.69

5.37

25.5

4.74

49.0

0.16

247

101

100

26

1.17

2.70

6.17

25.3

4.93

44.5

0.21

237

107

102

27

1.44

2.74

7.48

25.4

4.83

43.8

0.17

153

65

62

28

1.14

2.69

5.14

25.5

4.85

45.0

0.16

189

84

83

29

2.06

2.58

2.81

25.1

4.35

41.6

0.18

195

84

84

30

1.11

2.54

3.67

24.5

4.33

38.0

0.20

180

79

81

31

1.54

2.56

2.93

24.9

4.16

39.8

0.16

167

77

72

32

2.09

2.54

1.93

24.9

4.58

41.7

0.20

205

90

91

33

1.51

2.67

5.51

25.2

3.80

31.8

0.17

137

61

62

34

1.77

2.72

6.81

25.4

4.60

42.6

0.15

151

67

57

35

2.92

2.67

6.15

25.1

3.97

27.4

0.16

102

49

48

MA

D

TE

CE P

AC

NU

29.4

Gs:Specific gravity,nt:total porosity,: Unit weight; E: Young’s Modulus; : Poisson’s value; peak: peak strength; CI, vol: Axial stress at crack initiation obtained from volumetric strain response according to Brace et al. (1966); CI, lat: Axial stress at crack initiation obtained from lateral strain response according to Lajtai (1974)

ACCEPTED MANUSCRIPT

48

Table 4: Empirical equations for predicting varying physico-mechanical properties of the studied rocks.

Equation

r

Significance test of ―r‖ value5

Specific Gravity

Gs = -0.0107Mq.+ 2.8255

-0.79

tc=7.174>t0.9500, n=33 = 1.960 = tt

Specific Gravity

Gs = 0.0458Mmag.+ 2.5820

0.68

tc=5.164>t0.9500, n=33 = 1.960 = tt

Specific Gravity

Gs = 0.0449Mbio.+ 2.6021

0.63

tc=4.517>t0.9500, n=33 = 1.960 = tt

P-wave Velocity

Vp = 0.0462Mplg. + 2.6147

0.69

P-wave Velocity

Vp = 0.0484Mfelds. +1.4664

0.67

tc=5.025>t0.9500, n=33 = 1.960 = tt

P-wave Velocity

Vp = -2.8861Fopa. + 5.3214

-0.66

tc=5.047>t0.9500, n=33 = 1.960 = tt

P-wave Velocity

Vp = -0.9441Popa. + 5.1855

-0.65

tc=4.914>t0.9500, n=35 = 1.960 = tt

P-wave Velocity

Vp = -1.3182Fbio. + 4.9994

-0.60

tc=4.308>t0.9500, n=35 = 1.960 = tt

Loss on ignition

LOI = -0.0814MFelds. + 6.5564

-0.70

tc=5.458>t0.9500, n=33 = 1.960 = tt

Loss on ignition

LOI = -0.0677Mplg. + 4.2314

-0.63

tc=4.660>t0.9500, n=33 = 1.960 = tt

Loss on ignition

LOI = -0.4856Mmag. + 2.483

-0.63

tc=4.660>t0.9500, n=33 = 1.960 = tt

Loss on ignition

LOI = -0.3724Mbio. + 2.0915

-0.46

tc=2.976>t0.9500, n=33 = 1.960 = tt

Loss on ignition

LOI = -0.0671Cplg. + 3.0915

-0.64

tc=4.785>t0.9500, n=35 = 1.960 = tt

Unconf. Comp. Strength

UCS = 5.2455Mq. + 102.36

0.47

tc=2.965>t0.9500, n=33 = 1.960 = tt

Unconf. Comp. Strength

UCS = -13.531nt + 246.23

-0.64

tc=4.785>t0.9500, n=35 = 1.960 = tt

Unconf. Comp. Strength

UCS = 97.058(CPlg+CAmf)/CGrM + 125.12

0.37

tc=2.288>t0.9500, n=35 = 1.960 = tt

Unconf. Comp. Strength

UCS = 219.75(CPlg+CAmf)/CGrM1 + 35.758

0.76

tc=5.847>t0.9500, n=27 = 2.052 = tt

Unconf. Comp. Strength

UCS = -7.5966Cb. + 202.93

-0.56

tc=3.883>t0.9500, n=35 = 1.960 = tt

Poisson’s Ratio

 = 0.0882(CPlg + CAmf)/CGrM + 0.134

0.56

tc=3.883>t0.9500, n=35 = 1.960 = tt

+ 0.1099

0.66

tc=4.393>t0.9500, n=27 = 2.052 = tt

 = 0.3047 (CPlg+CAmf)/CGrM2 + 0.0709

0.87

tc=4.322>t0.9500, n=8 = 2.262 = tt

 = -0.0026CGrM + 0.3406

-0.62

tc=4.539>t0.9500, n=35 = 1.960 = tt

 = -0.0038CGrM1 + 0.4078  = -0.0038CGrM2 + 0.444

-0.72

tc=5.188>t0.9500, n=27 = 2.052 = tt

-0.88

tc=4.538>t0.9500, n=8 = 2.262 = tt

Young’s Modulus

E=-6.6561LOI+52.326

-0.55

tc=3.783>t0.9500, n=35 = 1.960 = tt

Young’s Modulus

E = -57.69Fopa. + 58.573

-0.72

tc=5.960>t0.9500, n=35 = 1.960 = tt

Young’s Modulus

E = -18.502Popa. + 55.578

-0.69

tc=5.476>t0.9500, n=35 = 1.960 = tt

Young’s Modulus

E = -26.325Fbio. + 52.743

-0.66

tc=5.047>t0.9500, n=35 = 1.960 = tt

Young’s Modulus

Poisson’s Ratio

AC

Poisson’s Ratio

tc=5.308>t0.9500, n=33 = 1.960 = tt

IP

SC R

NU

MA

D

CE P

Poisson’s Ratio

Poisson’s Ratio

TE

 = 0.1191(CPlg +

Poisson’s Ratio

CAmf)/CGrM1

T

Parameters

E = 106.78Angopa. + 36.92

0.63

tc=4.660>t0.9500, n=35 = 1.960 = tt

Crack initiation

3

CI, vol = 0.46UCS

0.97

tc=22.921>t0.9500, n=35 = 1.960 = tt

Crack initiation

4

CI, rad = 0.44UCS

0.97

tc=22.921>t0.9500, n=35 = 1.960 = tt

Note that significance tests were made at 95% confidence interval. 1 Data obtained from specimen with an andesitic composition 2 Data obtained from specimen with a rhyodacite composition 3 Data obtained using the method proposed by Brace et al (1966) 4 Data obtained using the method proposed by Lajtai (1974) 5 Since 95% confidence (two tailed) is required, the total shaded area α unders student’s distribution is 0.05; then the shaded area on the right side is (0.05/2=0.025) by symmetry. Hence the area to the left of ―t‖ is (10.025=0.975) and t stands for 97.5, the percentile t0.975 (Spiegel and Boxer 1972; Mendenhall and Beaver 1991; Arıoğlu and Tokgöz 2011; Tokgöz et al.,2015) r = the coefficient of correlation Camf. = Amphibole content

ACCEPTED MANUSCRIPT

49

tc 

r

, n = number of data

MA

1 r 2     n2  tt = tabulated critical values of t

NU

Calculated student number

SC R

IP

T

CGrM = Groundmass content for all types of compositions 1 CGrM = Groundmass content values obtained from specimen only of andesitic composition 2 CGrM = Groundmass content values obtained from specimen only of rhyodacite composition Cplg. = Plagioclase content Angopa. = Angularity of opaque minerals Fopa. = Feret’s diameter of opaque minerals Fbio. = Feret’s diameter of biotite Mbio. = Mass fraction of biotite MFelds = Mass fraction of total feldspar minerals (i.e sum of mass fraction of orthclase and plagioclase given in Table 1) Mmag.= Mass fraction of magnetite Mplg. = Mass fraction of plagioclase Mq. = Mass fraction of quartz nt:total porosity Popa. = Perimeter of opaque minerals CI, vol: See explanations in Table 4 CI, lat: See explanations in Table 4

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In this table primarily the effects of major constituents by means of content and mass fractions are highlighted.

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Table 5: Specific gravities, elastic properties and hardness values of major constituent minerals Biotite

Magnetite

Plagioclase

Orthoclase

Quartz

2.9-3.4

2.7-3.1

5.15-5.2

2.62-2.76

2.55-2.63

2.59-2.65

183±10** 5.5-6.5

81-106 6-6.5

73-74 6

91-105 7

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E (GPa) * 118-128 63-64 Mohs hardness 5-6 2.5-3 * Data provided from Prodaivoda et al. (2004) ** Data provided from Chicot et al. (2011)

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Gs

Amphibole

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Table 6: Some significant multiple regression equations

Equations

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T

UCS = 191.887(CPlg+CAmf)/CGrM + 155.341 MFelds.+ 836.322 Mq – 147.441 UCS = 108.151 (CPlg+CAmf)/CGrM – 13.448LOI – 16.017nt + 219.914 E = 10.268 (CPlg+CAmf)/CGrM – 7.641LOI – 1.953nt + 58.069 E = 7.690 (CPlg+CAmf)/CGrM – 42.543Fopa. – 11.618Fbio. + 54.776

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For abbreviations please refer to Table 4.

0.811 0.810 0.826 0.806

ACCEPTED MANUSCRIPT Highlights Micro-textural parameter were analyzed quantitatively

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Effect of mineral alteration on cracking was investigated

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Discrete and combined effects of micro textural parameters were studied

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Combined effect of phenocrysts and groundmass on cracking is significant

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