NESOSILICATE MINERALS.docx

ASSAM UNIVERSITY, SILCHAR
DEPARTMENT OF EARTH SCIENCE
HOME ASSIGNMENT
ESCCC-103
MINERALOGY
DESCRIPTIVE MINERALOGY OF
NESOSILICATE MINERALS
SUBMITTED TO
Dr. NAGENDRA PANDEY
(Professor)
DEPARTMENT OF EARTH SCIENCE
ASSAM UNIVERSITY, SILCHAR
SUBMITTED BY:
KUKI MONJORI BORUAH
1ST
SEMESTER
ROLL NO-202112

CONTENT
SL.NO. TOPIC PAGE NO.
1. INTRODUCTION 1
2 NESOSILICATE MINERALS 2
3 OLIVINE GROUP 3-7
4 GARNET GROUP 8-12
5 ALUMINOSILICATE GROUP 13-17
6 CONCLUSION 18
7 REFERANCE 19

INTRODUCTION:-
Silicate minerals are rock-forming minerals made up of silicate groups. They are the
largest and most important class of minerals and make up approximately 90% of earth’s crust.
All silicate minerals have [SiO4] tetrahedra as building blocks in their structure. Each silicon
atom is the center of an ideal silicon-oxygen tetrahedron. Two adjacent tetrahedra may share a
vertex, meaning that the oxygen atom is a bridge connecting the two silicon atoms. An
unpaired vertex represents an ionized oxygen atom, covalently bound to a single silicon atom,
that contributes one unit of negative charge to the anion.
In mineralogy silicate minerals are classified into seven major groups according to
the structure of their silicate anion.
MAJOR GROUP STRUCTURE CHEMICAL FORMULA SI:O RATIO EXAMPLE
Nesosilicate Isolated Silicon
Tetrahedra
[SiO4]4-
1:4 Olivine Group,
Garnet Group,
Aluminosilicate
Group
Sorosilicates Double Tetrahedra [Si2O7]6-
2:7 Epidote
Cyclosilicate Rings [SinO3n]
2n-
1:3 Beryl
Inosilicate Single Chain [SinO3n]2n-
1:3 Pyroxene Group
Inosiliate Double Chain [Si4nO11n]6n-
4:11 Amphibole Group
Phyllosilicate Sheets [Si2nO5n]2n- 2:5 Micas And Clays
Tectosilicates 3D Framework [AlxSiyO(2x+2y)]x-
1:2 Quartz, Feldspars,
Zeolites

NESOSILICATE MINERALS
Nesosilicates or orthosilicates, have the orthosilicate ion, which constitute isolated
(insular) [SiO4]4-
tetrahedra that are connected only by interstitial cations. The atomic packing
of the nesosilicate structure is generally dense which causes the mineral of this group to have
relatively high specific gravity and hardness. The crystal habit of thee mineral is generally
equidimensional and they have poor cleavage. The simplest structure in nesosilicates have
mineral forsterite Mg2[SiO4]. These properties also that nesosilicates incorporate a
considerable number of gemstones. The most important minerals from the nesosilicates are
shown in the table below.
OLIVINE GROUP GARNET GROUP ALUMINOSILICATE GROUP
Forsterite(Mg2SiO4)
Fayalite(Fe2SiO4)
Pyrope[Mg3Al2(SiO4)3
Almandine[Fe3Al2(SiO4)3
Spessartine[Mn3Al2(SiO4)3
Grossular[Ca3Al2(SiO4)3
Andradite[Ca3Fe2(SiO4)3
Uvarovite[Ca3Cr2(SiO4)3
Andalusite(Al2SiO4)
Kyanite(Al2SiO5)
Sillimanite(Al2SiO5)

OLIVINE GROUP:-
GENERAL FORMULA:
The olivine group includes a number of minerals with its general formula
X2SiO4 where X = Divalent Cations (Mg2+
, Fe2+
, Mn2+
, Ca2+
or mixture of such cations).
The name “olivine” commonly refers to Mg-Fe members of the family although there are
other less common end members in the group.
STRUCTURE:
Minerals of Olivine Group all possess Orthorhombic symmetry. , the structures
of all the minerals of the group consist of independent SiO4 tetrahedra linked by divalent
atoms in six-fold coordination. As is common in other nesosilicate minerals, Si is not
replaced by Al and the octahedral positions in the structures are occupied almost exclusively
by divalent ions. Trivalent ions Al and Fe3+
are either absent, or present in very small
amounts. Mg-Fe olivine may contain minor amounts of Ni, Co, Ca and Mn. The olivine
structure is based on isolated SiO4 tetrahedra which link chains of (Fe, Mg)O6 octahedra.

There are two octahedral cation sites: M1 and M2. Both sites accommodate Fe2+
and Mg2+
cations and normally there is complete disorder of Fe and Mg over the M1 and M2 sites.
Olivine is orthorhombic and therefore will show parallel or symmetric extinction under
crossed polarized light.
COMPOSITIONAL CHARACTERISTICS:
There is complete solid solution between forsterite and fayalite (Fig. 2) and natural
mineral data attests to that. In many natural occurrences, and particularly in the more iron-
rich members of the series there is a little replacement of (Mg,Fe)2 by Mn and Ca. At the
magnesium-rich end of the series Cr and Ni, generally in small amounts, are usually
present. Fo olivine is considered to be the major mineral in the mantle.
There is an inverse relationship between Mn content Fo in natural olivines- Mn is
higher in fayalitic olivines. Ni content is higher in forsterite-rich olivines from ultrabasic
rocks. Cr content in olivine is the highest in komatiites and in olivine occurring as inclusion
in diamond. These olivines are very rich in forsterite. Ca content in olivine is slightly higher
in fayalite-rich varieties. In some ultrabasic rocks, platinum group elements are present in
FIG1: STRUCTURE OF OLIVINE

Mg-rich olivines. It may be noted that concentrations of the above elements in natural
olivines do not exceed 1 wt.% and often found in trace contents.
HIGH-PRESSURE TRANSFORMATIONS OF OLIVINE:
α- olivine converts to two high-pressure polymorphs, known as wadsleyite (βolivine)
and ringwoodite (γ- olivine) at high pressures. Such transformations are significant in
the context of mineralogy of the mantle below the depth of approximately 400 km.
Wadsleyite is orthorhombic and with a formula of (Mg,Fe2+)
2(SiO4), its cell
parameters are as follows: a = 5.7Å, b = 11.7Å and = 8.24Å, space group Pmmb.
Wadsleyite has both a single (SiO4) and coupled tetrahedral (Si2O7). Because of
oxygens not bound to silicon in the Si2O7 groups of wadsleyite, it leaves oxygens
unoccupied, and as a result, these oxygens are hydrated easily. As a result, there are
high concentrations of hydrogen atoms in the mineral. Hydrous wadsleyite is a
considered a potential site for water storage in the Earth’s mantle due to its low
electrostatic potential. With increasing pressure, wadsleyite transforms to a cubic
FIG.2: Phase diagram in the Fo-Fa series of olivines showing complete solid solution.

phase called ringwoodite, also known as γ-olivine or silicate spinel. Ringwoodite has
the crystallographic parameter of a=8.113Ao
with space group Fd3m. Structure of
ringwoodite is shown in Fig. 3. Here, Si atoms occupy the tetrahedral sites and Fe and
Mg atoms occupy the edge-sharing octahedral sites.
Ringwoodite is also described from meteorites and also from a rare ultra-deep diamond
from Brazil, where it occurs as inclusion. Ringwoodite is also capable of hosting (OH)
and therefore can contribute towards water content in the mantle. High pressure
experiments in the system Mg2SiO4-Fe2SiO4 show that the β-olivine is restricted to Mg-
rich compositions, while the γ-olivine is stable in Fe-rich compositions as well. Seismic
discontinuities in the Earth’s mantle are believed to be caused by different types of phase
transitions in the olivine structure. α-olivine is stable up to 410 Km depth, where it is
converted to β-olivine, which is subsequently converted to γolivine at ~ 525 Km depth. γ-
olivine subsequently breaks down to silicate perovskite and periclase at 660 km seismic
discontinuity .
FIG.3: STRUCTURE OF WADSLEYITE AND RINGWOODITE

OCCURRENCE:
Olivine is a major constituent of common ultrabasic rocks, such as dunite and peridotite,
where the composition is typically >Fo90. It is abundant in ultrabasic nodules occurring
in alkali basalts and kimberlites. Olivine is dominant in dunite and peridotite occurring in
ophiolite complexes. It is often a phenocrysts phase in olivine basalts, and present in
stony meteorites and lunar and Martian basalts. Fayalite is present in ironstones and
rhyolites. The most magnesian olivine (almost pure forsterite) occurs in metamorphosed
calc-magnesian rocks, particularly in contact aureoles. Olivine alters easily to serpentine
and brucite. The gem variety of olivine is known as peridot.
PHYSICAL PROPERTIES:
1.Crystal System: Orthorhombic
2.Common Form: Granular masses, Scattered grains, Rarely as prismatic crystals
3.Color: Shades of green, pale green, greyish green, brownish, olive green
4.Streak: Colorless (no streak)
5.Hardness: 6 – 7
6.Luster: Vitreous
7.Fracture: Conchoidal
8.Cleavage: Imperfect in Forsterite; Moderate in Fayalite
9.Sp. Gr.: Forsterite 3.2; Fayalite 4.3

GARNET GROUP:
Garnet is a group of six minerals which are isomorphous in nature and never occur pure as
represented by their chemical composition. It is a hard silicate mineral which occurs in
many rocks but it is especially common in some metamorphic rock like schist. The garnets
vary only slightly in physical properties and some of the minerals may be so similar that
they are indistinguishable from one another without a X-ray analysis.
GENERAL FORMULA:
General formula of garnet is X3Y2(SiO4)3. Where X = Mg+2
, Fe+2
, Mn+2, Ca+2
and Y =
Al+3
, Fe+3
, Cr+3
, Ti+4
.
CLASSIFICATION:
Garnet Minerals Chemical
Composition
Cell Dimensions (Å)
Pyrope Mg3
Al2
(SiO4
)3
11.46
Almandine Fe3
Al2
(SiO4
)3
11.53
Spessartine Mn3
Al2
(SiO4
)3
11.62
Grossularite Ca3
Al2
(SiO4
)3
11.85
Andradite Ca3
Fe
3+
2
(SiO4
)3
12.05
Uvarovite Ca3
Cr2
(SiO4
)3
11.97 / 12.00
STRUCTURE:
Garnet consist of groups of independent distorted SiO4 tetrahedrons, each of which is
linked by sharing corners to distorted AO6 (e.g., aluminium and/or iron centered)
octahedorons, thus forming a three dimensional framework. Within these there are cavities
containing triangular dodecahedra that contain the B cations. Each oxygen is coordinated
by one Si, one A and two B cations. Two edges of each tetrahedron and six of each
octahedron are shared with dodecahedra and four dodecahedral edges are shared with
other dodecahedra. Garnets commonly occurs as well developed crystal. The typical form
of the crystals have 12 or 24 sides and are called dodecahedron and trapezohedron
respectively. Crystal habits can be co-related with chemical composition i.e.

dodecahedron are most likely to be grossular rich, trapezohedron tend to be pyrope,
almandine or spessartine and combination of dodecahedron and trapezohedron are
generally andradite rich.
CHEMISTRY:
Although the six species of anhydrous garnet (pyrope, almandine, spessartine,
grossular, andradite and uvarovite) are the most abundant in natural silicate garnets,
another molecule knorringite (Mg3Cr2Si3O12) has been reported in certain kimberlitic
garnets. Ti may be present in some andradites, which is referred to as melanite. Rarer
elements in garnet include vanadium and zirconium. Presence of structural H2O in
rare garnet occurrences has been recognized and the corresponding species is
hydrogrossular.
PYROPE:
The composition of pure pyrope is Mg3Al2(SiO4)3, although typically other elements
are present in at least minor proportions—these other elements include Ca, Cr, Fe and
Mn. Pyrope forms a solid solution series with almandine and spessartine, which are
collectively known as the pyralspite garnets (pyrope, almandine, spessartine). Iron and
FIG.4: STRUCTURE OF GARNET

manganese substitute for the magnesium in the pyrope structure. The resultant, mixed
composition garnets are defined according to their pyrope-almandine ratio. The semi-
precious stone rhodolite is a garnet of ~70% pyrope composition. The origin of most
pyrope is in ultramafic rocks, typically peridotite from the Earth's mantle: these
mantle-derived peridotites can be attributed both to igneous and metamorphic
processes. Pyrope also occurs in ultrahigh-pressure (UHP) metamorphic rocks.
Retrograde changes cause its breakdown to a mixture of Hbl + Plag + Iron ore, often
in the form of a light green Kelyphitic intergrowth or to a fibrous amphibole and green
biotite.
ALMANDINE:
Almandine is an iron alumina garnet, of deep red color, inclining to purple. The
almandine crystal formula is: Fe3Al2(SiO4)3. Magnesium substitutes for the iron with
increasingly pyrope-rich composition. Almandine, Fe2
+3
Al2Si3O12, is the ferrous iron
end member of the class of garnet minerals representing an important group of rock-
forming silicates, which are the main constituents of the Earth's crust, upper mantle
and transition zone. Almandine crystallizes in the cubic space group Ia3d, with unit-
cell parameter a ≈ 11.512 Å at 100 K.
SPESSARTINE:
Spessartine is manganese aluminium garnet species, Mn2+
3Al2(SiO4)3. Ideally forms at
10,000 atm. P / 900°C T, but can also be produced at PT conditions as low as 410°C /
200-1500 bars using MnCO3, Al2O3 and SiO2. Spessartine forms a solid solution series
with the garnet species almandine. Spessartine, always occurs as a blend with other
species. Gems with high spessartine content tend toward a light orange hue, while
almandine prevalence induces red or brownish hues.
GROSSULARITE:
Grossular has the chemical formula of Ca3Al2(SiO4)3 but the calcium may, in part, be
replaced by ferrous iron and the aluminium by ferric iron. Typical of thermal
metamorphism. Gem varieties include Cinnamon stone and Hessonite.

ANDRADITE:
Andradite is a mineral species of the garnet group with formula Ca3Fe2Si3O12.
Andradite includes three varieties:
 Melanite: Black in color, referred to as "titanian andradite".
 Demantoid: Vivid green in color, one of the most valuable and rare stones in
the gemological world.
 Topazolite: Yellow-green in color and sometimes of high enough quality to
be cut into a faceted gemstone, it is rarer than demantoid.
It occurs in skarns developed in contact metamorphosed impure limestones or
calcic igneous rocks; in chlorite schists and serpentinites and in alkalic igneous rocks
(typically titaniferous). Associated minerals include vesuvianite, chlorite, epidote,
spinel, calcite, dolomite and magnetite.
UVAROVITE:
Uvarovite is a chromium-bearing garnet group species with the formula:
Ca3Cr2(SiO4)3. Uvarovite is the rarest of the common members of the garnet
group,[6] and is the only consistently green garnet species, with an emerald-green
color. It occurs as well-formed fine-sized crystals. Uvarovite most commonly occurs
in solid solution with grossular or andradite, and is generally found associated with
serpentinite, chromite, metamorphic limestones, and skarn ore-bodies.
Pyrope Almandine Spessartine

 Color : Virtually all colors but no bluish shades
 Crystal habit: Dodecahedra & Trapezohedral crystals
 Crystal system: Cubic
 Cleavage: None, may exhibit parting
 Fracture: Conchoidal to uneven
 Tenacity: Brittle
 Hardness: 6.5 – 8.0
 Luster: Vitreous, adamantine & resinous
 Streak: Undeterminable
 Specific gravity: 3.5 - 4.3
OPTICAL PROPERTIES:
 Single refractive, often anomalous double refractive (Violation of Cubic
Symmertry in O-H stretching region, Strain induced by Comp. Zoning).
 Refractive index: 1.72 - 1.94
 Pleochroism: None
 Birefringence: None
OCCURRENCE:
Garnet group of minerals are very common in metamorphic rocks of practically all
bulk compositions (pelitic, basic, quartzofeldspathic, calcareous) and are present in a
variety of igneous rocks. Garnet occurs quite commonly as detrital grains in clastic
sediments. In natural occurrences garnet is mostly solid solutions of different end
ite
Grossularite Andradite Uvarovite

members, one notable exception being nearly pure pyrope in ultrahigh pressure
metamorphosed pelitic rocks. Almandine-rich garnets (with variable solid solutions
of pyrope, spessartine and grossular contents) occur in regionally metamorphosed
pelitic rocks (defining garnet isograds in Barrovian zonal sequence) as index
mineral. Garnet becomes progressively Mg-rich with increasing grades of
metamorphism and occurs abundantly in the granulite and eclogite facies rocks.
Almandine-rich garnets also occur in some basic plutonic rocks and as rare
phenocrysts in acidic volcanic rocks. Spessartine-rich garnets are widespread in
metamorphosed Mn-rich sediments, such as in gondite, and occur rarely in granitic
pegmatites. Mn-rich bulk compositions in manganese silicate rocks allow
spessartine-rich garnets to form in greenschist facies also. Pyrope-rich garnets (with
variable grossular and almandine contents) are common in garnet peridotite
xenoliths in kimberlites, in some ultrabasic rocks and in ultrahigh pressure
metamorphosed pelitic and basic rocks. Grossular-rich garnets are common in both
regionally and thermally metamorphosed calcareous rocks, and, depending on the
ambient oxygen fugacity, may contain variable amount of andradite. Andradite-rich
garnets are common in thermally metamorphosed calcareous rocks and in skarns
associated with such metamorphism. Ti-rich varieties are found in alkaline igneous
rocks. Uvarovite-rich garnets (with grossular) commonly occur in serpentinites and
in association with chromite deposits in basic-ultrabasic rock series. Hydrossular is
reported from altered gabbroic rocks and in some skarn deposits.

ALUMINOSILICATE GROUP:-
Aluminosilicate minerals are a group of minerals that contain the
compounds alumina (Al2O3) and silica (SiO2), hence the name aluminosilicate. The
Al2SiO5 group of Nesosilicates are widespread minerals in aluminous rocks of the
Earth’s crust. The Al2SiO5 group of minerals can be found in medium- to high-grade
metamorphic rocks rich in aluminum. The group consists of three polymorphs:
Kyanite, Sillimanite, and Andalusite. The three Polymorphs serve as a classical
“Thermobarometry” tool in regards to the Metamorphic rocks.
GENERAL FORMULA:
The aluminosilicate group is composed of minerals of the general formula
Al2SiO5. When the formula is written as AlAlO(SiO4) it is clear that the minerals
belong to the Nesosilicates and have two different Al structural sites as well as
isolated [SiO4] tetrahedra.
CRYSTAL STRUCTURE:
The structures of the three polymorphs share a number of common features. In all
three minerals straight chains of edge-sharing AlO6 - Octahedra extend along the C-
axis. These octahedra contain half of the Al in the structural formula. The remaining
Al atoms are in coordination which is different in each mineral: 6-fold sites in
Kyanite, 5-fold sites (Trigonal bipyramidal / Square pyramidal) in Andalusite and 4-
fold sites in Sillimanite. These Al-polyhedra alternate with [SiO4] tetrahedra, also
along the C-axis, linking together the AlO6 chains. Sillimanite and Andalusite are both
Orthorhombic, whereas Kyanite is Triclinic.
FIG.6: STRUCTURE OF ANDALUSITE
FIG.5:STRUCTURE OF KYANITE

CHEMISTRY:
SILLIMANITE:
Sillimanite is orthorhombic with a good {010} cleavage. It generally occurs in
long fibrous crystals that are length slow, with extinction parallel to the {010}
cleavage. In sections lying on {001}that show well-developed {110} forms, the
cleavage is usually seen to cut across the crystal as shown here. Maximum
birefringence is generally seen to be between 2o
yellow to 2o
red. Sillimanite is biaxial
positive with a 2V of 21 – 310
.
ANDELUSITE:
Andalusite is also orthorhombic , but shows a length fast character. It generally tends
to occur as euhedral blocky crystals with a maximum birefringence in thin section
between 1o
yellow and 1o
red. It sometimes shows weak pleochroism with α = rose-
pink, β = γ = greenish yellow. Some varieties show a cross, termed the chiastolite
cross, which is made up of tiny carbonaceous inclusions oriented along
crystallographic directions. Andalusite generally occurs as euhedral crystals with an
almost square prism. It is biaxial negative with 2V = 73 - 86o
.
FIG.7: STRUCTURE OF SILLIMANITE

KYANITE:
Kyanite is triclinic and thus shows inclined extinction relative to its good {100}and
{010}cleavages and {001} parting. In hand specimen kyanite is commonly pale blue in
color, but is clear to pale blue in thin section. Because of its good cleavages and
parting, two cleavages or partings are seen in any orientation of the crystal in thin
section. These cleavages intersect at angles other than 90o and thus look like
parallelograms in two dimensions. Because Kyanite has high relief relative to other
minerals with which it commonly occurs, it stands out in thin section and sometimes
appears to have a brownish color. This color is more due to its high relief and
numerous cleavages rather than due to selective absorption. Kyanite is biaxial negative
with 2V = 78 -83o
.
PROPERTIES SILLIMANITE ANDALUSITE KYANITE
Color Colorless,
White, Yellow,
Brown, Blue,
Green
Reddish brown,
Olive green, white
to gray
Blue, white, gray,
green, colorless
Streak Indeterminable White White, colorless
Luster Vitreous Vitreous Vitreous, pearly
Diaphaneity Transparent
to Translucent
Transparent to
nearly opaque
Transparent
to translucent
Cleavage Perfect Good Perfect in
two directions,
faces sometimes
striated
Mohs Hardness 6.5 to 7.5 6.5 to 7.5 Long
bladed crystals
having a
hardness of
4.5 along the
length of
the crystals and
6.5 to 7 across
the width of
the crystal.
Specific Gravity 3.2 to 3.3 3.17 3.5 to 3.7

Diagnostic
Properties
Slender
crystals,
fibrous habit
Crystal form,
Associated minerals,
Strongly pleochoric
Symmetrical
inclusions
Color, cleavage,
bladed crystals
Chemical
Composition
Al2SiO5 Al2SiO5 Al2SiO5
Crystal System Orthorhombic Orthorhombic Triclinic
FIG: SILIMANITE
FIG: KYANITE

CONCLUSION:
 In the nesosilicates structure SiO4-tetrahedra are not directly connected with
mutual oxygen ion only by interstitial cations.
 The simplest structure in nesosilicates has the mineral forsterite [Mg2(SiO4)].
 The most important minerals from the nesosilicates are olivine group, garnet
group, aluminosilicate group.
 The olivines consist of a complete solid solution between Mg2SiO4 (forsterite,
Fo) and Fe2SiO4 (fayalite, Fa).
 Minerals of Olivine Group all possess Orthorhombic symmetry and a structure
consisting of independent [SiO4]4-
tetrahedra linked by divalent atoms in
octahedral co-ordination.
 Garnets are nesosilicates, and therefore based on the SiO4 structural unit. The
general formula for garnets is A3B2(Si3O12).
 The unit cell of Garnet contains eight formula units. In it the silicon – oxygen
tetrahedra exit as independent groups linked to octahedra of the trivalent ions,
while the divalent metal ions are situated in the interstices within the Si – Al
network, each divalent ion being surrounded by eight oxygens.
 The Al2SiO5 group of Nesosilicates are widespread minerals in aluminous rocks
of the Earth’s crust.
 The aluminosilicate group is composed of minerals of the general formula
Al2SiO5.

REFERANCES:
 https://www2.tulane.edu/~sanelson/eens211/Silicate_Structures.pdf
 http://epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/S000448GO/P
000595/M022502/ET/1505371199EText-Nesosilicates.pdf
 https://www.researchgate.net/publication/321706338_Silicate_Minerals_
An_Overview

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