Optical anomaly in near-end-member grossular garnet from the

Transcription

Int. j. econ. environ. geol. Vol:6(1) 1-7, 2015
Available online at www.econ-environ-geol.org
Optical anomaly in near-end-member grossular garnet from the Jeffrey mine,
Asbestos, Quebec, Canada
Manzoor Ahmad Badar, Safdar Hussain, Shanawer Niaz*and Ata-ur-Rehman Makhdoom
Department of Physics, University of Sargodha, Sargodha, Pakistan, PK-40100
*Email: [email protected]
Abstract. The optical anomalies, surface and lamellar textures of a birefringent grossular garnet crystal from the
Jeffrey mine Canada have been investigated by optical polarizing microscope, electron-probe micro-analyzer (EPMA)
and infrared spectrometer from the stand point of crystal growth. This grossular shows one-to-one correlation between
surface features and its internal textures. The average chemical composition measured by EPMA is
Grs97.4Alm1.2Sps1.2Pyr0.1 as a near-end-member grossular. The surface features correspond to the internal textures
observed under crossed polarizers. Two growth hillocks with regular growth steps elongated parallel to [001] vertical
direction of the (110) face, produce sectoral twins. Such growth steps indicate the orthorhombic symmetry of the
crystal correlated with the growth direction. Some irregular or curved growth steps appearing as lamellae in (110) thin
section are parallel to the sides of (110) face but inclined to the [001] growth direction; this suggests the monoclinic
symmetry. The crystal is optically biaxial (+) and its 2V x angle being close to 90° could not be measured. Infrared
spectroscopy data revealed the presence of [(OH) 4] group substitution at [SiO4], the tetrahedral site. Optical
birefringence of the grossular is about 0.002. Back-scattered electron imaging could not detect any zone and
compositional differences of the lamellar texture.
Keywords: garnet, grossular, lamellar texture, cation order, optical anomaly, crystal growth.
them optically anisotropic (Rossman and Aines, 1991;
Deer et al., 1992; McAloon and Hofmeister, 1993). In
common silicate garnets, there is no agreement among
the above mineralogists on the true cause of
birefringence. Among several features proposed for the
inciting factor of the optical anomalies in garnet, main
seems as, cation ordering at X and Y sites, non-cubic
distribution of [(OH)4] groups at [SiO4] and strains.
Introduction
Natural and synthetic calcic garnets of intermediate
chemical composition exhibit birefringence. Such
garnets deviate slightly from cubic symmetry.
Birefringence in garnet was reported over a century
ago (Brewster, 1853; Mallard, 1876; Brauns, 1891), its
origin still remains questionable until now and is
addressed in study. The lack of progress on the
birefringence problem is the unreliable information on
the slight distortions to the garnet structure that are
difficult to detect from different conventional
diffraction methods (Baur and Fischer, 2003).
The common silicate garnet–group minerals occur
in metamorphic, igneous and some sedimentary rocks.
They are divided into two series ugrandite [uvarovite
(Ca3Cr2Si3O12), grossular (Ca3Al2Si3O12), and andradite
(Ca3Fe2Si3O12)] and pyralspite [pyrope (Mg3Al2Si3O12),
almandine (Fe3Al2Si3O12), and spessartine (Mn3Al2Si3
O12)]. Solid solution between the end-members within
each series is common, not outside the series. In general
formula of garnet, [8]X3[6]Y2[4]Z3[4]O12, the eightcoordinated dodecahedral X site contains Ca, Fe2+, Mg,
or Mn2+cations, the six-coordinated octahedral Y site
contains Fe3+, Al, Cr3+, Zr4+ or Ti4+cations, and the fourcoordinated tetrahedral z site contains Si, Fe3+,Al3+, or
[(OH)4] groups (Novak and Gibbs, 1971; Armbruster et
al., 1998). The structure of garnet crystal consists of
alternating ZO4 tetrahedra and YO6 octahedra with X
cations filling spaces to make XO8 dodecahedra. The
eight O atoms in the XO8 polyhedra occur at the corners
as a distorted cubic shape.
Many reasons were given for the cause of
birefringence in the garnet:(1) strain arising from
plastic deformation(Allen and Buseck, 1988), (2)
magneto-optic effects due to incorporation of rareearth cations in place of Ca (Blanc and Maisonneuve,
1973), (3) presence of OH groups inconsistent with
cubic symmetry (Rossman and Aines, 1986), (4) sector
twinning and polysynthetic twinning (Ingerson and
Barksdale, 1943), (5) residual strain arising from
lattice mismatch at composition, subgrain or twin
boundaries (Chase and Lefever, 1960; Lessing and
Standish, 1973; Kitamura and Komatsu, 1978; Antao,
2013a, b) and (6) cation order in X and Y sites that
leads to lower symmetry than cubic (Takéuchi et al.,
1982; Akizuki, 1984, 1989; Allen and Buseck, 1988;
Angel et al., 1989; Kingma and Downs, 1989; Griffen
et al., 1992; Akizuki et al., 1998; Wildner and Andrut,
2001; Shtukenberg et al., 2001, 2005; FrankKamenetskaya et al., 2007). Some garnets may show
birefringence under crossed polarized light indicating
In the present study of near-end-member grossular
garnet of the Jeffrey mine, we investigate (1) the
optical anomalies and optical indicatrix of different
growth sectors using a polarizing microscope mounted
with universal stage, (2) the variation in chemical
1
©SEGMITE
composition across the {110} growth sectors with
EPMA and (3) non-cubic distribution of [(OH)4]
groups at [SiO4] site with infrared spectroscopy.
marked central lines of growth hillocks on the (110)
face appear as sector twin boundaries (indicated by
black arrows) with different contrast in the (110) thin
section and depressions like growth steps as lamellae
in the (110) thin section. These lamellae originate
from, sector boundaries, the edges of the face and
never cross the sector boundaries. The orientations of
most of the lamellae are parallel to the edges of the
(110) face. The presence of irregularities like cracks
and spotty extinction preclude accurate determination
of the orientation of optic axes, 2V angles and true
value of birefringence. The optical birefringence of the
grossular is estimated visually by comparison with a
Michel Levy interference colour chart up to 0.002. One
to one correlation between surface features and the
internal textures is not perfect as some parts seem
strained during the growth of the crystal.
1. Specimen characterization
This near end-member grossular crystal occurs in
the rodingties associated with serpentintes in the
Jeffrey mine, Asbestos, Quebec, Canada. The Jeffrey
mine is located in Asbestos city on the Nicollet River
at latitude 45°47' N and longitude 71°58' W(Ross,
1967, its Fig. 3). These rocks were formed by
metasomatism below 300ºC (Evans et al. 1976). This
locality is famous for different colour, beautiful
crystals of grossular; vesuvianite and well-terminated
crystals of prehnite and pectolite. The locality is now
closed for further mining. The grossular crystals
studied are of light brown and pale orange colour. The
crystals with well developed and flat dodecahedral
(110) faces, measure up to 8 mm in largest dimension.
In some crystals, small (112) faces in form of borders
of the (110) faces are also found. The (110) faces
posses regular and irregular growth steps, and
sometimes natural etch pits and striations as well. In
the studied specimen, neither (112) faces nor naturally
etched (110) faces are found.
The optical orientations of the optical indicatrix
and the 2V values of the Jeffrey mine grossular are
measured on the (110) thin section by the use of
universal stage (Fig 1c). The principal vibration
directions and optic axes of {110} sectors are
represented by X, Y, Z, OA (optically uniform part
indicated by star in Fig 1b) and X΄, Y΄, Z΄, OA΄ (the
regions with lamellae having inclined extinction in Fig
1b).
2. Experimental Procedures
2.1 Optical microscope observations and universal
stage measurements
The surface features of (110) faces of grossular
crystal from Jeffrey mine were studied with an
interference-contrast
reflected-light
polarizing
microscope. Large (110) faces are covered with fine
striations elongated along [001]. Small (110) growth
faces show regular and irregular growth steps with a
few growth hillocks having vicinal faces. Figure 1a
explores the surface features of a small (110) face
taken with a reflecting light microscope. The
crystallographic direction [001] is indicated vertically
in the (110) growth sector. There are two growth
hillocks in the central part of the (110) face nearly
parallel to the vertical [001]. The central lines of the
hillocks are marked by dotted lines. On the right side
of the (110) face, the growth steps (shown by black
arrows) are simple, regular and parallel to [001]
direction. While in some places, the growth steps are
irregular and modify into grooves or depressions
(indicated by white arrows). Near the top of the face,
some natural lines divide the surface into rhombic
blocks. The sides of the blocks are parallel to the edges
of (110) face of the natural grossular crystal. The
symmetry of the crystal can be determined from the
orientations of the growth steps with respect to [001]
on the (110) face.
To inspect the internal texture of the crystal, a thin
section parallel to the (110) face was prepared,
polished and examined with a petrographic polarizing
microscope under crossed polarizers (Fig 1b). The
2
Fig. 1. Optical micrographs of a (110) rhombohedral
growth face of a grossular from the Jeffrey mine,
Quebec, Canada. (a)Surface features with reflectedlight polarizing microscope; the crystallographic
direction [001] is indicated. Central lines of elongated
hillocks along the [001] direction are marked. Regular
and irregular growth steps are arrowed black and white
respectively. (b) The corresponding cross-polarized
photomicrograph of the (110) thin section of (110)
growth face (a); the [001] direction is vertical.
Centrally marked lines of hillocks appear as sector
twinned boundaries indicated by black arrows.
Depressions in the irregular steps change into lamellae
(inclined to [001] vertical direction showing
monoclinic symmetry) are pointed out by white
arrows. Uniformly dark contrasted areas like indicated
by star, exhibit orthorhombic symmetry. (c)
Stereographic projection of the optical properties of the
Jeffrey mine grossular measured on a (110) thin
section are represented by X, Y, Z and X΄, Y΄, Z΄. X
and X΄ are along the growth direction [110]. Optic
axes OA and OA΄ are indicated. The 2Vx (+) value is
about 90°.
4. Results and discussions
The growth features and the internal textures of a (110)
face of a natural, birefringent grossular crystal from the
Jeffrey mine Canada are closely correlated (Fig 1a, b).
Such correlations between surface features and the
internal texture of natural garnet crystals describe the
history of growth process. The symmetry of a
birefringent crystal is determined by the direction of
growth
Table 1. Chemical composition of the {110} birefringent
sector from a thin section parallel to the (110) face in
grossular garnet from the Jeffrey mine, Canada.
Analysis
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
CaO
Total
H2O*
Cation Ratio
2.90
0.01
2.00
0.00
0.03
0.03
3.01
7.98
Cations Ratio
Coordination
Cations
Sum (on 12
Oxy.)
4
Si
2.90
Al
0.01
∑Z
2.91
6
Al
1.99
Ti
0.01
∑Y
2.00
8
Fe
0.04
Mn
0.03
Ca
3.01
∑X
3.08
Mol% of end-members
Alm
1.21
Grs
97.4
Sps
1.20
Pyr
0.10
3.2 Electron-probe micro-analyses
The same (110) thin section of the Jeffrey
grossular was polished and carbon-coated for chemical
composition determination. Electron-probe microanalyses were performed with a HITACHI X-569S
microprobe operating at an accelerating voltage of 15
kV and a beam current of 15 nA at the Institute of
Mineralogy, Petrology and Economic Geology,
Tohoku University. The chemical composition is
nearly uniform at different positions in the {110}
growth sectors across the sample. The composition in
the (110) section is Grs98Alm1Sps1 (Table 1). In this
sample, 0.1 wt% Ti, 0.6 wt % Mn and traces of Mg
(0.02 wt %) and Cr (0.01 wt %) are detected. The
concentration of these minor and trace elements is little
higher in the centre compared to the edges of the
crystal face. Back-scattered electron compositional
imaging could not detect any zonation in the {110}
thin sections parallel and to (110) face, at the place of
lamellae near the sector boundaries and growth bands
parallel to the (110) growth surfaces. The chemical
composition is quite homogeneous and the birefringent
lamellae
exhibits
structural
behavior
not
compositional.
Wt%
38.09(0.31)
00.10 (0.01)
22.40(0.20)
00.00
00.63(0.24)
00.60(0.04)
37.07(0.25)
98.8
00.10
* Wt% H2O from Manning and Tricker (1977), determined by wetchemical analysis.
steps on the (110) faces and the orientation of the
lamellae in (110) thin section with respect to the
morphological growth direction [110]. On the (110)
face of the Jeffrey mine grossular, some growth steps
are simple, regular and oriented parallel to the
crystallographic direction [001] indicated in the figure
(Fig 1a). Others are irregular growth steps at some
places and modify into grooves or depressions. In the
optical indicatrix of {110} sector of uniform extinction
of (110) thin section, the principal vibration direction
Y is normal to (110) and coincides with growth
direction [110]. The X and Z vibration directions are
nearly parallel to [11̅0] and [001] directions
respectively. Such vibrational directions and the
orientations of the regular growth steps, predict the
orthorhombic symmetry of the sample. The grooves or
depressions of irregular growth steps are inclined to
the [001] crystallographic direction. Also, the sector
twin boundaries and the lamellae in the {110} sector in
the thin section are inclined, X΄ and Z΄ vibration
directions rotate up to 2° from [11̅0] and [001]
directions respectively; Y΄ vibration is along the
growth direction [110].
This situation infers
3.3 Infrared spectroscopy
The infrared spectrum of the anisotropic grossular was
recorded with a JEOL DIAMOND-20 micro FTIR
spectrometer at Tohoku University Sendai in the range
of wave number from 5000 cm-1 to 1000 cm-1. Spectra
were obtained on doubly polished single crystal slabs
having thickness about 0.2 mm – 0.5 mm. Data was
collected at room temperature with 1 mm aperture to
select the area of interest from different samples.
3
monoclinic symmetry of the grossular crystal. Due to
the limitations of Universal stage, the 2VX angle, being
close to 90°, could not be measured. Thus optically,
this grossular is positively biaxial and has lower
symmetry than cubic.
Average chemical composition of the Jeffrey mine
near end-member grossular birefringent sample
measured by EPMA in this study is about
Gros97Alm1.6Sps1.4{Ca3.01Mn0.03Fe0.04}∑3.08[Al1.99
Ti0.01]∑2.00(Si2.90 Al 0.01OH0.09)∑ 3.00. In this sample, 0.1
wt% Ti, 0.6 wt % Mn and traces of Mg (0.02 wt %)
and Cr (0.01 wt %) are detected. Compositional
images with back-scattered electron imaging from
(110) and (001) thin sections could not be obtained
because of homogeneous chemical composition and
absence of zonation at the place of lamellae, sector
boundaries and the growth bands parallel to (110)
surface. The chemical composition is thoroughly
uniform and birefringent lamella does not exhibit any
compositional behaviour. The total Fe was measured
as Fe2O3 (0.69 wt %) and then converted into FeO and
Fe2O3 by using the equations of Manning and Tricker
(1977). All of the iron was found to be ferrous in the
form of almandine.
Fig. 2.Unpolarized infrared absorption spectra for the
[110] section of the Jeffrey mine grossular(a)
Absorption bands between 5000 cm-1 to 1000 cm-1;
generally 2000- 400 cm-1 show combination mode of
symmetric/asymmetric Si-O stretching vibration, 37003500 cm-1 the O-H stretching vibrations and in the
vicinity of 5200 cm-1, the molecular water in form of
inclusion if present (b) Scanned normal to the growth
bands from rim to core between 3700- 3500 cm-1 the
O-H stretching vibrations at different points with 4
broad peaks 3560, 3621, 3655, 3665 cm-1 and 3
shoulders 3600, 3610 and 3631 cm-1. The absorbance
intensity of the peaks decreases from core to rim, in
addition to the changes in peak position and shape.
Fourier transform infrared spectra of light brown
grossular from Jeffrey mine in this study are shown in
Figure 2. Unpolarized data was collected from oriented
section (001) of thickness less than 1 mm, from {110}
growth face. Figure 2a shows numerous absorption
bands from 5000 cm-1 to 1000 cm-1. Generally, 2000400 cm-1 show stretching vibration and bending motion
of the [Sio4] tetrahedral, 3500- 3700 cm-1the stretching
absorption bands of [(OH)4] as structural water
substituting the [Sio4] tetrahedral and in the vicinity of
5000 cm-1, the molecular water in form inclusion if
present. The absorption peaks of [(OH) 4] the structural
water, normal to the growth bands from rim to core (a0
to a6) of the crystal are shown in Figure 2b. The OH
absorption spectrum consists of 4 broad peaks 3560,
3621, 3655, 3665 cm-1 and 3 shoulders 3600, 3610,
3631 cm-1. A great diversity is noticed from the
behavior of different peaks. The absorbance intensity
of the peaks decreases from core to rim, in addition to
the changes in peak position and shape.
Rossman and Aines (1986) studied the Jeffrey
mine grossular by infrared spectroscopy with H 2O
content of 0.035 wt% and measured variable
birefringence up to 0.001. The amount of the structural
water in the Jeffrey mine grossular garnet reported by
various authors (Manning and Tricker, 1977; Allen and
Buseck, 1988; Rossman and Aines, 1991) using
different methods estimated in the range 0.10 – 0.38
wt%. Their EPMA results of the grossular indicated
chemical composition of 0.71 wt% MnO and 1.46 wt%
FeO with a little zonation. They suggested that a non
cubic orientation of the OH absorption groups is the
cause of birefringence, as the anisotropic absorptions
bands are not observed in the spectra of the optically
isotropic garnets. As the composition of the garnet was
near end-member grossular and the Fe2+ spectra
anisotropy was absent, they rejected the idea of Fe3+Al3+cation ordering at the octahedral site as the cause
of birefringence in this particular garnet. It was
difficult for them to conclude the [(OH) 4] orientation
as a primary cause or its response to another
component with preferential orientation during crystal
growth process. Allen and Buseck (1988) studied the
Jeffrey mine grossular of chemical composition Gr 99
And with different crystallographic techniques and
pointed out a number of causes of birefringence. Their
optical measurements did not determine the cause of
anisotropy while XRD data suggested Fe3+-Al3+cation
ordering at the octahedral site. Their FTIR study
predicted non-cubic orientation of [(OH)4] groups and
HRTEM images showed sector-boundary defects
4
arising from strain. They suggested that presence of
structural water with its non-cubic orientation of OH
groups is the primary cause of birefringence in this
grossular rather than Fe3+-Al3+ ordering, that is the
same result arrived at, in this study.
crystals in cavities and there is no evidence that the
crystals are plastically deformed. . In an ideal endmember grossular, Ca3Al2Si3O12, the X and Y sites are
completely filled with Ca and Al, so the question of
cation order does not exist. If the composition deviates
clearly from the end-member, then cation order may be
a possibility, but slight variations are unlikely to cause
cation ordering.
In garnets specially, cation ordering is explained
by a crystal-growth mechanism suggested by Akizuki
(1981). This general growth mechanism is involved in
the formation of order–disorder growth sectors in
different minerals. In this mechanism, growth steps
nucleate either at the sector boundary or at the edge of
a {110} face. This growth mechanism is consistent
with the presence of spirals or pyramidal shape of
growth hillocks on natural rhombohedral {110} faces
of this grossular sample. On such a face, the growth
steps are parallel to the edges along four directions and
are inclined to the mirror planes. Ordered structures are
produced in case of grossular–andradite garnets of
intermediate chemical composition that lowers the
higher symmetry (Badar et al., 2010, 2013). The
hypothesis of this growth mechanism was supported by
Gali (1983) and Shtukenberg et al. (2001). When a
crystal grows from a low super saturation solution, a
three-dimensional lattice is formed by the stacking of
two-dimensional lattices. The two-dimensional atomic
arrangements exposed on the side faces of the growth
step and their symmetry modifies the degree of
ordering produced during growth. The crystal
symmetry is the result of ordered arrangement of
alkali, Al3+, Fe3+ and Si4+in an effort to satisfy electric
charge balance. Hence, the two-dimensionally ordered
structure produced on the surface during growth is not
always the same as the three-dimensionally ordered
structure which take place by a phase transition or after
growth process. At the surface of a growing crystal, the
equivalent
sites
become
geometrically
and
energetically non-equivalent. This gives rise to an
ordered distribution of the atoms involved in the solid
solution. At low temperature and pressure, the
aluminium silicate minerals grow by adding atomic
clusters to the growing surface. If the environment of
the growth process changes, it can lead to various
sectors and lamellar textures.
On the other hand, Antao (2013 a, b) proposed a
solution to the birefringence problem in the silicate
garnets by establishing a relation between
birefringence and multi-cubic phases as heteroepitaxial or epitaxial intergrowth. A general solution to
the birefringence is mismatch of cubic unit-cell
parameters in a multi-phase intergrowth that gives rise
to strain-induced birefringence. If the intergrowth
occurs on a fine scale, the separate phases are not
detected by EPMA but are easily observed by
HRPXRD (high resolution powder x-ray diffraction).
The structural and chemical differences between
different phases produce strain and cause birefringence
in garnet. The Jeffrey mine grossular of this study
displayed complicated pattern on the bottom area in
Figure 1b, that might be attributed to strain in the
crystal, developed during cooling. Therefore, it is
possible that the birefringence in this Jeffrey mine
grossular resulted from strain during growth or after
growth as another cause.
This study of a birefringent garnet of near-endmember grossular in composition from the Jeffrey
mine, Asbestos, Quebec, Canada, represents the
investigation about its birefringence and lamellar
texture. This research serves to broaden our knowledge
of the texture, crystal chemistry and true symmetry of
the birefringent natural calcic garnets. For better
insight into the optical properties of real crystals,
methods of crystallographic analysis, scanning probe
microscopy and synchrotron high resolution powder xray diffraction are proposed.
Acknowledgement
This research work was supported under the cultural
scholarship scheme of Ministry of Education of
Pakistan and MONBUSHO (Ministry of Education) of
Japan.
In this Jeffrey mine grossular sample, out of the
reasons mentioned in the introduction, Points 2 and 6
are ruled out in near-end-member grossular and
andradite as the X site is mostly filled with Ca and a
little substituted by other atoms. Points 3 and 4 are not
favored as such features are common in many minerals
but no display of birefringence is observed. This
grossular garnet crystallized at about 300ºC and 1- 1.5
G Pa (Evans et al., 1976). This condition cannot allow
cation diffusion. It is well known that high pressure
favors an increase in the miscibility between pyropealmandine-spessartine and grossular-andradite garnets
(Nemec, 1967). This suggests that specific pressure
and temperature conditions are important in
determining whether a garnet of a particular chemical
composition capable of ordering, will order in fact.
The sample under this study occurs as euhedral
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Optical anomaly in near-end-member grossular garnet from the

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