Reliance of physico-mechanical properties on petrographic characteristics: consequences from the study of Utla granites, north-west Pakistan

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Bulletin of Engineering Geology andthe EnvironmentThe official journal of the IAEG ISSN 1435-9529 Bull Eng Geol EnvironDOI 10.1007/s10064-014-0690-9

Reliance of physico-mechanicalproperties on petrographic characteristics:consequences from the study of Utlagranites, north-west Pakistan

Muhammad Sajid & Mohammad Arif

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ORIGINAL PAPER

Reliance of physico-mechanical properties on petrographiccharacteristics: consequences from the study of Utla granites,north-west Pakistan

Muhammad Sajid • Mohammad Arif

Received: 29 May 2014 / Accepted: 7 October 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract The relationship between the mechanical

properties and petrographic features of granites from the

Utla area, north-west Pakistan, was studied. Three different

textural varieties of granites (megacrystic coarse-grained,

fine-grained and foliated coarse-grained) were collected.

Following a comprehensive petrographic analysis, their

uniaxial compressive and tensile strengths as well their

physical properties (porosity, water absorption and specific

gravity) were determined. The modal concentrations, sizes

and shapes of minerals as well as their distribution and

alignment were analyzed during the petrographic obser-

vations. A detailed investigation utilizing statistical ana-

lysis revealed that megacrystic coarse-grained granites are

stronger than the other two varieties because of greater

variance in the grain sizes of their constituent minerals,

their unaligned nature, lower abundances of micaceous

minerals, and lower porosity and water absorption values.

Keywords Petrography � Physico-mechanical properties �Granites � Statistical analysis � Pakistan

Introduction

Variations in the mechanical properties of rocks with

analogous chemical signatures are largely associated with

differences in their textures and modal mineralogical

compositions. ‘‘Texture’’ refers to the relationships

among the mineral grains in a rock, as well as their sizes

and shapes (McPhie et al. 1993; Bucher and Frey 1994).

The engineering properties of rocks can be assessed

through their intrinsic properties, including texture and

mineralogy (Lindqvist et al. 2007). The relationships

between the petrographic features and the mechanical

properties of various rocks type have been studied by a

number of research workers (Shakoor and Bonelli 1991;

Tugrul and Zarif 1999; Pomonis et al. 2007). Diamantis

et al. (2011) correlated wave velocities with the petro-

graphic and physico-mechanical properties of peridotites

from central Greece. Grain-to-grain contact is a textural

feature that is an important influence on rock strength

(Lundquist and Goransson 2001). Computerized image

analysis using a scanning electron microscope (SEM) and

a polarizing microscope for the determination of rock

texture, which is then correlated with the various

mechanical properties of rocks, has been described by

several authors (Helibronner 2000; Herwegh 2000;

Akesson et al. 2003). Rigopoulos et al. (2010) investi-

gated the effect of weathering and alteration on the

engineering properties of basic rocks from Greece.

Raisanan (2004) reported that mechanical properties are

significantly influenced by the abundance of fine-grained

minerals, grain-size distribution, and degree of inter-

locking of grain boundaries of minerals.

Rocks of granitic composition occur in numerous areas

of north-west Pakistan. The mechanical characteristics of

granites from Malakand and Ambela (Fig. 1) have been

studied in detail (Din et al. 1993; Din and Rafiq 1997). Arif

et al. (1999) investigated the mechanics of Mansehra

granitoids (Fig. 1) and concluded that the degree of

weathering, old age, and coarser grain size of these rocks

are factors that are responsible for their low strength

M. Sajid (&)

Camborne School of Mines, University of Exeter,

Penryn Campus, Cornwall TR10 9EZ, UK

e-mail: [email protected]

M. Arif

Department of Geology, University of Peshawar,

Peshawar 25120, Pakistan

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Bull Eng Geol Environ

DOI 10.1007/s10064-014-0690-9

Author's personal copy

values. The mechanical properties of the huge, exposed and

easily accessible Utla granitoids (Fig. 1) need to be

explored. Utla granitoids of three different textural types—

megacrystic coarse-grained (CG), fine-grained (FG) and

foliated coarse grained (CGF)—were selected for analysis

in the study described in the present paper (Fig. 2).

There is a general impression that fine-grained rocks are

stronger than their coarse-grained counterparts (Tugrul and

Zarif 1999; Bell 2007). However, it is not always the case

as the present study attained opposite results. The prime

goal of this paper is to discuss the mentioned gap of grain

size control on the mechanical properties between previous

Fig. 1 Geological map of northwestern Pakistan showing location of the studied area. The bold stars show the locations at which samples were

collected

Fig. 2 Core samples of the

different textural varieties of

Utla granite (scale is in inches)

M. Sajid, M. Arif

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studies and current results. The paper also describes the

differences in the mechanical characteristics of chemically

similar and texturally contrasting rocks. Petrographic fea-

tures, including the modal mineralogy, alignment of min-

erals, grain size, and grain size distribution, are correlated

with the physical and mechanical properties through

regression analysis.

Regional geology

Utla granites constitute the northern portion of the Indian

plate, separated from the Kohistan Island Arc (KIA) to the

north by the major thrust zone known as the Main Mantle

Thrust (MMT) (Searle et al. 1999). They intrude Pre-

Cambrian meta-sedimentary rocks (Tanawal quartzite;

Fig. 1). Alkaline magmatic episodes were interpreted in

Permian towards south of MMT which are collectively

known as Peshawar Plain Alkaline Igneous Province

(PPAIP) (Kempe 1973; Kemps and Jan 1980). The Ambela

Granitic Complex (AGC) (Fig. 1) constitutes the largest

portion of the PPAIP. Rafiq and Jan (1988) stated that the

Utla granites are in spatial continuity with the AGC, sug-

gesting an eastward extension. However, Hussain et al.

(2004) mapped these rocks along with the granitic rocks of

Swat and Mansehra. The Mansehra granites are a typical

example of S-type granites. They lie to the east of the Utla

granites, have a whole-rock Rb–Sr age of 516 ± 16 Ma

(Le Fort et al. 1980) and are considered to be synchronous

with the Swat granites and granitic gneisses (Le Fort et al.

1983). Petrographic and geochemical similarities further

directed Sajid et al. (2014) to link the Utla granites with the

granitic rocks from Mansehra.

Methodology

Bulk samples of three different textural varieties of Utla

granite were collected. Cylindrical core samples from each

variety were obtained using a core drilling machine. A

sample of CG granite and another of FG granite had visible

fractures and were therefore discarded. Petrographic

information was acquired from a thin section obtained from

each core sample. The lack of any intergranular and con-

tinuous fracturing in the thin section excluded the possi-

bility of any nonvisible crack/fracture. The ISRM

specifications (2007) were followed in the preparation of

seven core samples of each type, which were then

Table 1 Modal compositions

of the megacrystic coarse-

grained granite, foliated coarse-

grained granite and fine-grained

granite

Qtz quartz, Alkf alkali feldspar,

including orthoclase and

microcline, Plg plagioclase, Bt

biotite, Mus muscovite, Tour

tourmaline, Sph sphene, Apt

apatite, And andalusite, Mzt

monazite, OM opaque minerals

Sample no. Qtz Alkf Plg Bt Mus Tour Apt And Mzt Sph OM

Megacrystic granite

CG-1 24.0 41.0 16.0 6.0 8.0 4.4 – 0.2 – – 0.4

CG-2 23.0 40.0 19.0 6.0 6.3 5.4 0.1 – – – 0.3

CG-3 27.0 43.0 13.0 6.0 5.7 5.0 – 0.1 0.2 – –

CG-4 25.0 42.0 15.5 5.4 6.3 5.1 0.1 – 0.1 – 0.5

CG-5 26.5 39.0 15.8 6.0 8.0 4.5 – – – – 0.2

CG-6 27.0 42.0 14.0 4.0 7.0 5.7 – 0.2 – – 0.1

CG-7 25.8 40.0 11.5 6.5 10.8 4.7 0.4 – – – 0.4

Foliated granite

CGF-1 27.5 20.0 20.0 15.0 16.0 – 0.2 – – 0.8 0.5

CGF-2 29.0 20.0 20.0 17.0 13.0 0.5 0.1 – 0.1 – 0.3

CGF-3 30.0 20.0 20.0 13.0 16.0 – – – 0.1 0.6 0.3

CGF-4 28.0 24.0 19.0 15.0 13.0 0.3 – – – 0.5 0.2

CGF-5 28.0 24.0 18.0 15.0 14.0 – – – – 0.6 0.4

CGF-6 30.0 20.0 20.0 16.0 13.0 – 0.2 – – 0.2 0.6

CGF-7 31.0 19.0 18.0 16.0 15.0 – – – – 0.2 0.8

Fine-grained granite

FG-1 32.5 36.3 13.4 5.5 9.0 2.7 – 0.2 – – 0.4

FG-2 26.5 38.7 17.5 5.2 9.0 2.6 0.5 – – – –

FG-3 25.7 41.2 14.5 4.8 10.6 2.3 – 0.6 – – 0.3

FG-4 28.0 40.3 13.2 5.3 9.5 2.9 – – 0.2 – 0.6

FG-5 31.0 43.2 10.6 4.2 7.6 2.6 – 0.8 – – –

FG-6 26.6 40.2 15.0 5.5 10.2 2.2 0.1 0.3 – – –

FG-7 27.3 36.6 16.5 5.8 9.7 2.5 – 0.9 – – 0.8

Reliance of physico-mechanical properties on petrographic characteristics

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processed with a universal testing machine to obtain their

strength values, including uniaxial compressive strength

(UCS) and uniaxial tensile strength (UTS) values. Core

samples with 44 mm diameter were used and a length-to-

diameter ratio of 2–2.5:1 was maintained during UCS

testing. The Brazillian test was used for the UTS value, as

suggested by the ISRM (2007), using disc-shaped samples

with a thickness-to-diameter ratio of 1:2. The water satu-

ration method (ISRM 2007) was employed to measure the

porosity, water absorption (WA) and specific gravity (SG).

The calculated values of porosity did not provide signifi-

cant information on pore size and the distribution of pores,

but they did afford some insight into the volume of pores in

the whole rock.

Petrographic description

Although they contained different relative proportions of

both essential and accessory minerals, all of the studied

samples were granitic in terms of their modal mineralogical

compositions (Table 1; Fig. 3) (Le Maitre 2002). However,

detailed examination leads to their distinction into three

types on the basis of texture. All of the samples were

slightly weathered with minor discolouration along the

grain contacts, corresponding to grade II according to the

weathering classification system (Table 2) (Brown 1981;

Hencher and Martin 1982). Slightly more discolouration

was observed for the megacrystic coarse-grained (CG)

granite, but this discolouration was not strong enough to

change the grade of weathering. The presence of a similar

grade of weathering in all of the samples also minimized its

effect on the correlation between petrographic features and

mechanical properties, as discussed later. The petrographic

details of each of these varieties are described in detail

below.

Fig. 3 International Union of

Geological Sciences (IUGS)

classification (triangular

diagram) for the Utla granites

Table 2 Weathering classification system (Brown 1981)

Class Term Description

I Fresh rock No visible sign of rock material

weathering

II Slightly

weathered rock

Discolouration indicates weathering of

rock materials and discontinuity

surfaces

III Moderately

weathered rock

Less than half of the rock material is

decomposed and/or disintegrated to soil

IV Highly

weathered rock

More than half of the rock material is

discomposed and/or disintegrated to soil

V Completely

weathered rock

All rock material is decomposed and/or

disintegrated to soil. The original mass

structure is still largely intact

VI Residual soil All rock material is converted to soil. The

mass structure and material fabric are

destroyed

M. Sajid, M. Arif

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Megacrystic coarse-grained granite (CG)

The quartz-to-feldspar ratios and concentrations of mica-

ceous minerals (averaging *13 %, including biotite and

muscovite) of these granites were lower (Table 1) relative

to the other two varieties. The megacrysts mainly com-

prised subhedral to euhedral grains of plagioclase and

alkali feldspar and ranged up to 2 cm in size in hand

specimens. Partial to complete sericitization was observed

in these megacrysts (Fig. 4a). The ground mass lacked

any kind of mineral alignment (Fig. 4b) and the constit-

uents were mostly medium-grained, so substantial grain-

size variation was attributed to these rocks. It consisted

mainly of quartz, feldspar, muscovite, biotite and tour-

maline along with minor amounts of monazite, andalusite

and apatite.

Fig. 4 Micrographs of a sericitized feldspar megacrysts, b the

uniform groundmass (with a relatively wide grain-size distribution),

c the alignment of micaceous minerals, d micas and recrystallized

quartz wrapped around a feldspar megacryst, e the uniform size of

major minerals, and f the recrystallization of quartz

Reliance of physico-mechanical properties on petrographic characteristics

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Foliated coarse-grained granite (CGF)

These rocks were also megacrystic, but the ground-mass

constituents showed well-developed gneissose character

(Fig. 4c). The quartz-to-feldspar ratios and concentrations

of micaceous minerals (averaging *29) were much greater

than seen for the other two varieties. The shape and altered

nature of the megacrysts as well as the grain-size distri-

bution in these rocks were same as for the CG granite

(Fig. 4d), but the intensity of recrystallization of quartz

grains in the ground mass was greater.

Fine-grained granite (FG)

These rock samples had a homogeneous grain size

(Fig. 4e), making them very distinct from the other two

granites (Fig. 3). Quartz grains were extensively recrys-

tallized, providing evidence of deformation in these rocks.

Clusters of fresh fine-grained recrystallized quartz grains

occurred along strained alkali feldspar and quartz grains

showing undulose extinction (Fig. 4f). Feldspars also dis-

played alterations to clay minerals(s) and sericite. The

modal concentration of quartz and micaceous minerals

(Table 1) was greater than the CG granite.

Physico-mechanical properties and relationship

to petrographic features

The results of mechanical and physical tests are presented

in Table 3. In general, UCS values of all the rock samples,

regardless of texture, categorize those as moderately

strong from an engineering perspective (Table 4). How-

ever, two of the samples of CG granite (CG-6 and CG-7)

had UCS values of [50 MPa and were designated as

strong rocks.

The strength values, including both UCS and UTS, were

higher for CG granite than for the other two varieties

(Table 3). This result contrasts with the results of earlier

studies, most of which indicated that fine-grained rocks are

stronger than coarse-grained ones. Despite the similar grain

size dissemination to CG, the CGF samples had lowest

strength values (Table 3). The values of water absorption

and porosity for FG granite were distinctly greater than

those for CG and CGF granites (Table 3).

Simple linear regression analysis with a 95 % confi-

dence limit was performed with Grapher 8.2.460 to

Table 3 UCS, UTS, water absorption, specific gravity and porosity

of each rock sample studied

Sample

no.

UCS

(MPa)

UTS

(MPa)

Water

absorption

(%)

Specific

gravity

Porosity

(%)

CG-1 40.257 5.222 0.903 2.686 2.368

CG-2 29.242 4.698 0.904 2.690 2.374

CG-3 48.308 5.519 0.849 2.673 2.219

CG-4 40.772 5.226 0.855 2.723 2.275

CG-5 47.535 5.332 0.858 2.669 2.239

CG-6 52.366 6.289 0.760 2.690 2.002

CG-7 63.315 6.387 0.745 2.727 1.990

CGF-1 28.598 2.638 0.890 2.698 2.345

CGF-2 36.263 4.237 0.826 2.710 2.190

CGF-3 37.036 4.764 0.795 2.693 2.096

CGF-4 17.262 1.162 1.146 2.711 3.013

CGF-5 23.059 2.575 0.919 2.701 2.423

CGF-6 40.063 4.787 0.767 2.698 2.028

CGF-7 42.511 5.250 0.738 2.696 1.952

FG-1 46.053 2.958 1.507 2.632 3.815

FG-2 40.965 2.085 1.574 2.622 3.964

FG-3 38.260 2.083 1.626 2.628 4.098

FG-4 43.413 2.631 1.541 2.628 3.892

FG-5 44.636 2.713 1.514 2.635 3.835

FG-6 41.352 2.534 1.556 2.632 3.933

FG-7 42.446 2.586 1.544 2.635 3.911

Table 4 Grades of unconfined compressive strength (Geological Society Engineering Group Working Party 1977; Commission of Engineering

Geological Mapping of the IAEG 1979; ISRM Commission on the Classification of Rocks and Rock Masses 1981)

Geological Society IAEG ISRM

Description UCS (MPa) Description UCS (MPa) Description UCS (MPa)

Very weak \1.25 Weak \15 Very low \6

Weak 1.25–5.00 Moderately weak 15–50 Low 6–10

Moderately weak 5.00–15.50 Strong 50–120 Moderate 20–60

Moderately strong 12.50–50 Very strong 120–130 High 60–200

Strong 50–100 Extremely Strong Over 230 Very high Over 200

Very strong 100–200

Extremely strong Over 200

IAEG International Association of Engineering Geologists, ISRM International Society for Rock Mechanics

M. Sajid, M. Arif

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investigate the significant correlation between the observed

petrographic features of rocks and their physical and

mechanical properties. The derived equations, coefficients

of determination (R2), and confidence limits are presented

in subsequent figures. The value of R2 was calculated using

the least squares method. Multiple regression techniques

(logarithmic, exponential and power) were also investi-

gated but the results are not presented here because they

yielded lower R2 values than obtained with a single linear

input. The validity of the derived equations was verified by

performing t tests via normal distribution. A critical t test

value of 2.18 was obtained with 12 degrees of freedom and

a 95 % confidence limit (2.78 for equation in Fig. 10 with

4 degrees of freedom). The calculated t values for given

data were significantly higher than the critical value, sug-

gesting their validity.

Plots (Figs. 5, 6) of strength values (UCS and UTS)

against the quartz-to-feldspar ratio pointed to positive

relationships between the parameters. A similar relation-

ship was also observed by previous researchers (Tugrul and

Zarif 1999). Grain-to-grain contact has more important

influence on the strength of rock than the concentrations of

minerals (Lindqvist et al. 2007). Quartz content showed an

inverse relationship (Fig. 7) to porosity, indicating that

empty spaces were filled by anhedral quartz grains during

the process of recrystallization. Similar behavior was also

observed for granitic rocks from Turkey (Tugrul and Zarif

1999).

The strength of rock is curtailed to greatest extent by

increase in volume of vacant spaces inside them. Such

behaviour is anticipated from the inverse relationship

between UCS and respective porosity and water absorption

values (Figs. 8, 9).

A significant parallel affiliation is perceived when

average mica contents of studied rocks are plotted against

their respective average UCS values (Fig. 10).

Fig. 5 Relationship between UCS and quartz-to-feldspar ratio.

Symbols are defined in Fig. 3Fig. 6 Relationship between UTS and the quartz-to-feldspar ratio.

Symbols are defined in Fig. 3

Fig. 7 Relationship between total porosity and the quartz content.

Symbols are defined in Fig. 3

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A strong positive relationship is apparent in the plot of

UCS versus UTS for these rocks (Fig. 11). According to

Brady and Brown (2004), UCS of rocks is eight times their

UTS, however, Farmer (1983) proposed the ratio between

UCS to UTS to be 10:1. Combining the mentioned out-

comes, UCS to UTS ratios of rocks fall in the range of 8 to

10. The average UCS to UTS ratio is 8.2, 9.6 and 17.02 for

the CG, CGF and FG granites respectively. The ratios for

CG and CGF granites fall within the permissible range, but

the ratio is beyond the upper limit of this range for FG

granites. This higher than ‘‘normal’’ UCS-to-UTS ratio of

FG granite could occur because of an overestimated UCS

or because of a decreased UTS value for some (unidenti-

fied) reason. Alternatively, this high ratio could be real, in

which case a good argument for an extension to the upper

limit of the recommended ratio is needed. However, such

Fig. 8 Relationship between UCS and porosity. Symbols are defined

in Fig. 3

Fig. 9 Relationship between UCS and water absorption. Symbols are

defined in Fig. 3

Fig. 10 Relationship between UCS and mica content. Symbols are

defined in Fig. 3

Fig. 11 Relationship between UCS and UTS. Symbols are defined in

Fig. 3

M. Sajid, M. Arif

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an extension cannot be suggested based on the limited

amount of data presented here.

Discussion and conclusions

The physical and mechanical properties of rocks determine

their strength and durability and hence their utility for

different engineering purposes. These properties depend on

petrographic characteristics, including both modal com-

position and texture. Table 5 summarizes the results of

mechanical tests and important petrographic characteristics

for the studied rocks. As mentioned earlier, rocks with fine-

grained texture are generally stronger than their coarser

equivalent (Bell 2007). However, the opposite conclusion

was drawn for the rocks in this investigation. The fine-

grained varieties of Utla granites have noticeably lower

strengths than the coarser granites (Table 5). This dimi-

nution of strength can be explained by the water absorption

and porosity values for fine-grained granites, which are

much higher (Table 5). The increases in water absorption

and porosity are due to extensive recrystallization, as is

evident from the deformational features observed during

petrographic analysis.

A high concentration of physically strong minerals (e.g.

quartz) adds strength to rocks, as observed in Figs. 4 and 5.

In comparison, CG granite has greater strength than CGF,

although the concentrations of quartz are greater in the

latter (average = 29 %) than the former (aver-

age = 25 %). This decreased strength of CGF granites is

attributed to their higher concentrations (average *15 %)

and alignment of micaceous minerals. Such alignment

increases rock foliation which act as discontinuity planes,

allowing cracks to propagate.

There are also significant amounts of micaceous min-

erals in CG and FG granites (averaging *6.5 % and

*7 %, respectively), but these minerals do not adversely

affect granite strength because of their unaligned nature.

The alignment of flaky minerals have more important

influence on strength of rock than their modal concentra-

tion (Lindqvist et al. 2007).

Surprisingly, the concentration of quartz is also greater

in FG granites (averaging *28 %) than in CG granites,

even though the FG granites are less strong. In addition to

the reason explained above, this lower strength of FG

granites can be attributed to the uniform grain size

observed in FG granites but not in CG granites. According

to Raisanen (2004), equigranular rocks have lower strength

than inequigranular rocks, as variations in rock grain size

lead to greater resistance to crumbling and fragmentation.

This thorough comparison and analysis has revealed that

the following petrographic parameters have important

influences on the strength of the studied rocks:

1. Mineralogical concentrations of essential minerals.

2. Sizes and shapes of constituent minerals and their

degrees of variation.

3. Preferred orientation of mineral grains.

4. Volume of empty spaces.

Although there is a broad impression that fine-grained

rocks are stronger than coarse-grained rocks, the degree of

recrystallization is an important factor that must also be con-

sidered. It greatly influences the water absorption capacity/

porosity of rock, which has a negative effect on its strength.

From the above discussion, we can conclude that coarse-

grained megacrystic (CG) Utla granites show greater

strength than FG and CGF granites because of (a) the

heterogeneous distribution of grain sizes, (b) the nonfoli-

ated nature and lower concentrations of micaceous miner-

als, and (c) lower porosity and water-absorption values.

Although the degree of variation in grain size for CGF

granites is rather similar to CG granites, CGF granites are

weaker due to strong alignment of flaky mineral grains.

Acknowledgments The National Center of Excellence in Geology,

University of Peshawar, Pakistan provided facilities for carrying out

fieldwork and determining porosity, specific gravity and water

absorption. The petrographic studies were conducted in the Depart-

ment of Geology, University of Peshawar, Pakistan. The instruments

in the Material Testing Laboratory of the Civil Department, KP

University of Engineering and Technology, Peshawar (Pakistan) were

utilized to determine the mechanical properties. The support of Dr.

Muhammad Ashraf (UET Peshawar) when conducting the strength

tests is also greatly acknowledged.

Table 5 Summary of the mechanical/physical test results and petrographic observations for the studied samples

Sample no. UCS (MPa) UTS (MPa) Porosity (%) WA (%) SG Petrographic features

CG 45.97 ± 10.71 5.52 ± 0.61 2.21 ± 0.15 0.84 ± 0.06 2.69 ± 0.02 Nonfoliated

Greater grain-size variation

CGF 32.11 ± 9.38 3.63 ± 1.52 2.29 ± 0.36 0.87 ± 0.14 2.7 ± 0.006 Strongly foliated

Greater grain-size variation

FG 42.45 ± 2.57 2.51 ± 0.32 3.92 ± 0.09 1.55 ± 0.04 2.63 ± 0.004 Nonfoliated

Uniform grain size

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