Mineralogy and crystallography notes notes

1. 1
Olivine (Mg,Fe)2 [SiO4]
Orthorhombic symmetry
α 1.635-1.827
β 1.651-1.869
 1.670-1.879
 0.035-0.052
Optic axial angle (2V) 82o
- 134o
RI : =y, =z, =x (least, intermediate and greatest)
O.A.P (001)
Dispersion: red > violet, D: 3.22-4.39, H: 7-6.5
Cleavages: {010}, {100} imperfect in Mg-rich members, moderate in Fe-rich members
Twinning: {100}, {011}
Colour: Green, lemon-yellow, greenish yellow, yellow-amber;
Mg-rich members: colourless
Fe-rich members: pale yellow in thin section
Pleochroism: Fe-rich members- = pale yellow, β=orange yellow
SiO4 tetrahedra linked with divalent cation with octahedral co-ordination. Each cation has 6 nearest Oxygen neighbors. In
001 plane Oxygen are arranged in approx. hexagonal packing Tetrahedra point alternately either way in both x- and y-
directions
Mg2 SiO4 - Forsterite
Fe2 SiO4 - Fayalite
Mn2 SiO4 - Tephroite
CaMg SiO4 - Monticellite
CaMn SiO4 - Glauconite
CaFe SiO4 - Kirschsteinite
Diadochy between atomic pairs
Mg & Fe+2
; Fe+2
& Mn;
Replacement of Mg by Fe+2
is accompanied by linear increase in cell parameters.
Chemistry
Vary in composition from Mg2SiO4 (Forsterite) to Fe2SiO4 (Fayalite) Diadochy between Mg and Fe+2
Fo100-90 Forsterite Fo10-0 Fayalite
Based on Atomic % of Fe+2
(diadochy series)
0-19 Forsterite
20-39 Chrysolite
40-59 Hyalosiderite
60-79 Hortonolite
80-89 Ferro Hortonolite

2. 2
90-100 Fayalite
Ni is common in Mg olivine
Mn and Ca small amount in Fa and Fo
M.P of Mg2SiO4 – 1890o
C,
Fe2SiO4 – 1205o
C
Replacing smaller ion (Mg+2
– 0.66Å) with larger ion (Fe+2
– 0.74Å)
Mg rich olivine crystallize first, consequently Fe+2
ions are concentrated in residual liquids
Heat of solutions – a linear function of the molar composition, indicates perfect thermal equlibrium during Mg↔Fe+2
replacement
Phase diagram of MgO-SiO2 System
Olivine is very susceptible to hydrothermal alteration, low grade metamorphism and weathering
Common alteration products of olivine – serpentine, iddingsite, bowlingite, chlorite, amphibole, talc, carbonates and
iron oxides
Associated with chrysotile and antigorite
2Mg(OH)2+Mg6Si4O10(OH)8↔4Mg2SiO4 +6H2O
2 Brucite + Serpentine ↔ 4 Forsterite + 6 Water
3Mg2SiO4+4H2O+SiO2  2Mg3Si2O5(OH)4
3Olivine +4Water + Quartz  2Serpentine
2Mg3Si2O5(OH)4+3CO2 Mg3Si4O10(OH)2+3MgCO3+3H2O
2 serpentine talc magnesite
Optical properties
• Refractive indices vary linearly with composition
• α and  (least and greatest RI) increase by approx. 0.002 per unit mol % Fe2SiO4
• Optic axial angle (2V) increases from 82o
for Mg end member to 134o
for the Fe end member

3. 3
Distinguishing features
• Higher Birefringence
• Larger 2V
• Poor cleavage
• Common alteration
• Yellow green pleochroism in Fayalite
Paragenesis
• Olivine of Fo85 to Fo40 are very common in gabbro, dolerite and basalts
• Also in trachy-basalts, trachytes, theralites and teschenites
• Fe rich olivines are common in ferrogabbros and granophyre
• Fayalites are associated with quartz bearing syenites, hedenbergite and arfvedsonite
• Mg rich olivine occur in thermally metamorphosed impure limestone and dolomites
2CaMg(CO3)2+SiO2  Mg2SiO4 +2CaCO3+2CO2
• Fayalites also occurs in thermally metamorphosed sediments, more common in regionally metamorphosed Fe rich
sediments
• Olivines in metamorphosed basic rocks, surrounded by reaction rims – coronas, kelyphitic borders or corrosion
mantles
• Common zonal sequence is –
Olivine –opx –(amphibole+spinel) – plagioclase
Olivine – opx – garnet - plagioclase
GARNET
Isometric - Dodecahedron
Dispersion: Weak ; H:6-7.5
Cleavages: None ; Fractures: Subconchiodal
Twinning: Complex,
Zoning : visible in birefreingent varieties

4. 4
Colour: Red,brown, black, green, yellow or white
In thin section: colourless, pink, yellow or brown
• Characteristic minerals of Metamorphic rocks
• Also found in igneous rocks
• Detrital grains in sedimentary rocks
Garnets are Nesosilicates having the general formula X3Y2[SiO4]3 or X3Y2Si3O12
The X site is usually occupied by divalent cations (Ca2+
,Mg2+
, Fe2+
) and the Y site by trivalent cations (Al3+
, Fe3+
,
Cr3+
) in an octahedral/tetrahedralframework with [SiO4]4−
occupying the tetrahedra.
Garnets are most often found in the dodecahedral crystal habit.
Tetrahedra are linked to Octahedra by trivalent ions
Divalent metal ions are situated in the interstices within Si-Al network
Each divalent ion is surrounded by 8 oxygen’s Tetrahedra in the garnet unit cell. Tetrahedra are darker with increasing
distance below the plane of the diagram.
Divalent cations are in green, trivalent in blue. As we can see from the end-on rows, each row of tetrahedra lies along
a two-fold symmetry axis.
Divalent cations lie on the two-fold axes midway between tetrahedra.

5. 5
The trivalent cations are easiest to visualize because they are octahedrally coordinated with oxygen.
Garnets are divided into 2 series
1) PyrAlSpite – Pyrope, Almandine, Spessartine
1) UGrAndite – Uvarovite, Grossular, Andradite
Members ofGarnet Group
Pyrope : Mg3Al2 Si3O12
Almandine : Fe3
+2
Al2 Si3O12
Spessartine : Mn3Al2 Si3O12
Uvarovite : Ca3Cr2 Si3O12
Grossular : Ca3Al2 Si3O12
Andradite : Ca3(Fe+3
,Ti)2 Si3O12
Hydrogrossular : Ca3Al2Si2O8(SiO4)1-m(OH)4m
Chemistry – Pyrope Mg3Al2Si3O12
Pyrope – 75% pyrope molecules
High grade metamorphic rocks
Synthesized from kaolin, SiO2, MgO & MgCl2
30 kilobars pressure, 900o
C
Pyrope melts at 36 kb, 1775o
C
Retrograde – Hbl, plag & iron ore
Chemistry – Almandine Fe3
+2
Al2Si3O12
• Pyrope (Mg) & Spessartine (Mn) molecules
• Calcium & ferric ions are typically low
• 30 mol% ugrandite molecules from eclogites & glaucophane schists.
• Alteration products – Chlorite
• Synthesized from kaolin, Fe2O3,SiO2, FeCl2.4H2O
• 10 kilobars pressure, 900o
C
• Almandine melts at 36 kb, 1775o
C

6. 6
• Retrograde – Hbl, plag & iron ore
Chemistry – Spessartine Mn3Al2Si3O12
• 40 – 90% spessartine molecules
• Fairly readily synthesized from its component oxides – SiO2, Al(OH)3,Al(NO3)3.6H2O
• 10 kilobars pressure, 900o
C
• Also, from SiO2, Al2O3, MnCO3 at 410o
C, 200 – 1500 bars
• Surface alteration and oxidation to black manganese oxides and hydroxide
Chemistry – Grossular Ca3Al2Si3O12
• 96.8% grossular molecules
• Dominant substitutional molecules is andradite
• Typical of thermal metamorphism
• Synthesized from its glass - SiO2, CaO,CaCl2
• 20 kilobars pressure, 900o
C
• Decomposed to wollastonite, gehlenite, anorthite.
• Colour depends on iron and manganese content
Chemistry – Andradite Ca3(Fe+3
,Ti)2 Si3O12
• 94% grossular molecules
• Dark brown to black variety – Melanite 1 – 2% TiO2
• Schorlomite – 20% TiO2
• Kimzeyite – 29.9% ZrO2
• Goldmanite – 18.3% V2O3
• Synthesized from its glass – wollastonite, Fe2O3,FeCl3
• 20 kilobars, pressure,900o
C Associated with
Chemistry – Uvarovite Ca3Cr2Si3O12
• grossular and andradite
• Forms continuous series
• Synthesized from its powdered components
• 110 bars,525o
C
• Dark green to vivid emerald green
Chemistry – Hydrogrossular
• Hydrogarnets are all hydrogrossular
• Hydrothermal synthesis from its oxides
Distinguishing features
• High relief
• Isotropic
• Weakly birefringent
• Various species are determined by RI, D, Cell edges
• In-turn partial chemical data

7. 7
Paragenesis
• Pyrope – ultrabasic such as mica peridotites, kimberlites, assocaited serpentinites
In sands & gravels derived from above rocks
• Almandine – garnetiferous schists of regional metamorphism of argillaceous sediments
Breakdown of mica  garnet+ K feldspar
Staurolite+quartz  garnet+kyanite or sillimanite
Almandine occurs in aureoles of thermal or contact metamorphism
In plutonic rocks, contamination of granitic magma with argillaceous impurities
• Spessartine – common in granitic pegmatites, skarn deposits, associated with rhodonite, tephroite (Mn rich
assemblages) of metasomatic origin
• Grossular – found in thermally and regionally metamorphosed impure calcareous rocks. – marls, calcareous shale
and replacement of wollastonite
Found in zeolite bearing vesicles in metamorphosed basaltic lavas
Found with Dioside or scapolite resulting from Pneumatolysis with granite pegmatite (alteration of rock or
mineral crystallization effected by gaseous emanations from solidifying magma)
• Andradite – contact or thermally metamorphosed impure calcareous sediments
• 3CaCO3+Fe2O3+3SiO2 Ca3Fe2Si3O12 + 3CO2
• 4CaCO3+2Fe2O3+2FeO+5SiO2 Ca3Fe2Si3O12 + CaFeSi2O6+Fe2O3+4CO2
• Uvarovite – rare common anhydrous garnet species, associated with grossular, serpentinites and chromites
• Hydrogrossular – metamorphosed marls, altered gabbroic rocks
Al2SiO5 Group of Minerals
Sillimanite - Orthorhombic
Mullite - Orthorhombic
Andalusite - Orthorhombic
Kyanite - Triclinic
Sillimanite – Al2O3.SiO2
• α 1.654-1.661
• β 1.658-1.662
•  1.673-1.683
•  0.020-0.022
• Optic axial angle (2V) 21o
- 30o
• RI : =x, =y, =z (least, intermediate and greatest)
• O.A.P (010)
• Dispersion: red > violet, strong, D: 3.23-3.27, H: 7.5-6.5
• Cleavages: {010} good,
• Colour: Colourless to white, yellow, brown, greyish green
• Colourless in thin section

8. 8
• Pleochroism: = pale brown or pale yellow, =dark brown or blue, β=brown or greenish
Chemistry
• Al2O3. SiO2 = Al2SiO5
• Fe+3
is the common ion replacing Al+3
• Synthesized from its component oxides
• 300±50o
C, 8±0.5kb
• At 1545o
C, unstable, converted to mullite + liquid
• Alteration products-muscovite, sericite, pyrophyllite, kaolinite, montmorillonite
• Can be converted into Kyanite
Optical and Physical Properties
• Long prismatic crystals or as fibrous mat of fine crystalline material
• Relief is moderately high
• RI vary slightly, birefringence is comparatively strong
• Positive (length-slow) elongation
Paragenesis
• Higher grades of thermally metamorphosed argillaceous rocks
• Sillimanite-cordierite gneiss and biotite-sillimanite hornfels
• Regional metamorphism of pelitic rocks followed by thermal metamorphism
Mullite - 3Al2O3.2SiO2
• α 1.640-1.670
• β 1.642-1.675
•  1.651-1.690
•  0.012-0.028
• Optic axial angle (2V) 45o
- 61o
• RI : =x, =y, =z (least, intermediate and greatest)
• O.A.P (010)
• Dispersion: red > violet, strong, D: 3.15-3.26, H: 7-6
• Cleavages: {010} distinct,
• Colour: Colourless to white, yellow, pink or red, colourless or pinkish in thin section
• Pleochroism: = β colourless, =pinkish
Chemistry
• Deviation of composition with 60 mol% Al2O3
• Synthetic mullite 2Al2O3.SiO2 or 67 mol% Al2O3 has been produced
• Fe+3
and Ti may replace Al
Optical and physical properties
• RI can be raised by increasing Al2O3 and substitution of Fe+3
• Iron mullite with Fe2O : 35.93, TiO2 : 0.55  =1.690
Paragenesis

9. 9
• Pelitic xenoliths (buchites) in basic igneous rocks
• Iron mullite  thermally metamorphosed lateritic lithomarge
• Common refractory produce
Andalusite – Al2O3.SiO2
• α 1.629-1.649
• β 1.633-1.653
•  1.638-1.660
•  0.009-0.011
• Optic axial angle (2V) 73o
- 86o
• RI : =z, =y, =x (least, intermediate and greatest)
• O.A.P (010)
• Dispersion: red < violet, D: 3.13-3.16, H: 7.5-6.5
• Cleavages: {110} good,
• Colour: pink or red rose,grey violet yellow, green, or white,
• Colourless in thin section, but pink or green
• Pleochroism: in coloured varities weak, = rose pink, β & =greenish yellow
Chemistry
• Andalusite is relatively pure with Al2SiO5
• Ferric and manganese – replaceable ions
• Mn-rich variety – Manganandalusite (7% Mn2O3)
• Viridine – green variety of andalusite contain 9.6% Fe2O3,7.6% Mn
• Synthesized from kaolinite or from Al2O3+SiO2 at 450-650o
C, 5-8 kb
• Alter to sericite, sillimanite, kyanite
Optical and physical properties
• RI & Sp.gr. Increased by Ferric and Manganese ions
• Pleochroism related to Fe and Mn content
• Pink & red varieties contain Fe
• Green varities contain Mn
• 1450o
-1500o
C andalusite converted to mullite
• Used in refractories
Distinguishing feature
• Square cross section
• High relief
• Low birefringence
• Length fast
Paragenesis
• Typically in argillaceous rocks of contact aureoles around igneous intrusions
• Often associated with cordierite

10. 10
• Common detrital mineral in some sandstones
Gibbs' phase rule
Kyanite – Al2O3.SiO2
• α 1.712-1.718
• β 1.721-1.723
•  1.727-1.734
•  0.012-0.016
• Optic axial angle (2V) 82o
- 83o
• Dispersion: red > violet, weak,D: 3.53-3.65, H: 5.5-7
• Cleavages: {100} perfect,{010} good
• Colour: Blue to white, grey, green, yellow, pink or black
• Colourless to pale blue in thin section
• Pleochroism: weak, = colourless, =cobalt blue, β=violet blue
Chemistry
• Limited amount Fe+3
can enter structure
• Cr sometimes present in moderate amounts
• Synthesized at 900o
C, 20 kb
• Alteration products – pyrophyllite, muscovite, sericite
Optical and physical properties
• Relief is distinctly high
• Colourless in thin section
• Birefringence is moderate, higher orders of 1st
order colours
• Extinction angle 30o
to 0o
• OAP is perpendicular to {100}
Paragenesis
• Typical mineral of regional metamorphism of pelitic or rarely psammitic rocks
• Kyanite derived from pyrophyllite by dehydration of paragonite with addition of quartz

11. 11
• In thermal aureoles with staurolite
• Found in eclogites and amphibolites
• Also found in pegmatites by quartz-kyanite segregation veins
• Common detrital mineral in sedimentary rocks
Epidote group
• Crystallizes in Orthorhombic and Monoclinic
• X2Y3Z3(O,OH, F)13
X=Ca, Ce+3
,La+3
, Y+3
, Th, Fe+2
, Mn+3
,
Y=Al, Fe+3
, Mn+3
, Fe+2
, Mn+2
,
Z= Si
Epidote Group members
Orthorhombic
Zoisite: Ca2Al.Al2O.OH.Si2O7.SiO4
Monoclinic
Clinozoisite: Ca2Al.Al2O.OH.Si2O7.SiO4
Epidote: Ca2Fe+3
Al2O.OH.Si2O7.SiO4
Piemontite: Ca2(Mn+3
,Fe+3
,Al)3O.OH.Si2O7.SiO4
Allanite: (Ca,Mn+2
,Ce)2(Fe+2
,Fe+3
,Al)3O.OH.Si2O7.SiO4
Structure of Epidote
Chains of AlO6 and AlO4(OH)2 octahedra linked by independent SiO4 tetrahedra and Si2O7
Chemistry
• Replacement of Al by Fe+3
per unit cell
• Ca2Al3Si3O12(OH) to Ca2Fe+3
Al2Si3O12(OH)
• Ca2Fe+3
Al2Si3O12(OH) to Ca2Mn+3
Al2Si3O12(OH)
• Ca2Mn+3
Al2Si3O12(OH) to Ca2Mn2
+3
AlSi3O12(OH)
Paragenesis
• Regionally metamorphosed rocks
• Green schist to epidote-amphibolite facies
• Product of hydrothemal alteration – (saussuritization) of plagioclase feldspar, along joints and fissures, amygdales
and vugs
• 3 Chlorite + 10 Calcite + 21 Quartz ↔ 2 Clinozoisite + 3 Actinolite
+ 10 CO2 + 8 H2O
Pyroxene Group
General Formula: X1-pY1+p Z2 O6
X=Ca,Na,
Y=Mg, Fe+2
, Fe+3
,Mn, Li, Al, Cr, Ti

12. 12
Z= Si, Al
In Opx, p≈1
In Cpx, p varies from 0 to 1
Orthorhombic Minerals (Opx) compositional range: MgSiO3-FeSiO3
Monoclinic Minerals (Cpx) compositional range:
CaMgSi2O6-CaFeSi2O6-MgSiO3-FeSiO3
Orthorhombic Pyroxene
Enstatite – Orthoferrosilite (Mg, Fe+2
)2Si2O6
Monoclinic Pyroxene
Diopside-Hedenbergite-Johannsenite Ca(Mg, Fe+2
, Mn) Si2O6
Augite-Ferroaugite (Ca, Na,Mg, Fe+2
, Mn, Fe+3
, Al, Ti)2 (Si, Al)2O6
Pigeonite (Mg, Fe+2
, Ca)(Mg,Fe+2
) Si2O6
Aegirine-Aegirine augite Na Fe+3
Si2O6 – (Na,Ca)(Fe+3
Fe+2
, Mg) Si2O6
Spodumene LiAlSi2O6
Jadeite NaAlSi2O6
Mg SiO3 Enstatite (001)
Ca SiO3 Wollastonite (010)

13. 13
• SiO4 tetrahedra shares 2 of the 4 oxygens to form continuous chain
• Unit length of the chain is 5.3Å
• Laterallink is by cations (Ca,Fe, Mg, Na,Al)
Enstatite – Orthoferrosilite
• α 1.650-1.768
• β 1.653-1.770
•  1.658-1.788
•  0.007-0.020
• Optic axial angle (2V) 50o
- 125o
• RI : =y, O.A.P (100)
• Dispersion: red > violet, weak to strong, D: 3.21-3.96, H: 5-6
• Cleavages: {201} good ,
• Colour: Green, grey, yellow, brown, Mg-rich members: colourless
• Iron rich members – green or dark brown, reddish or greenish in thin section
• Pleochroism: Enstatite - none, other compositional members weak to strong, = pink, pale reddish brown,
purple-violet, smoky brown, β=yellow, pale greenish brown, pale reddish yellow, pale brown, =green, pale
green, smoky green
Chemistry
• Mg and Fe+2
are invariably present in the OPX
• Common ions – Ca, Mn, Ni, Fe+3
, Cr, Al, Ti (not exceeds 10 mol %)
• Enstatite –
• low temp. enstatite,
• high temp. protoenstatite,
• (orthorhombic) and clinoenstatite

14. 14
Paragenesis
• Mg OPXcommon in ultrabasic rocks – pyroxenites, hyrzburgites, Iherzolites, picrites
• Associated with Fo – olivine, diopsidic augite, Mg-spinel
• Opx – in layered complex,
• Ca(Mg,Fe)Si2O6+Al2SiO5 (Mg,Fe)SiO3+CaAl2Si2O8
Salite
• Chlorite+quartz Opx+cordierite+water
• Biotite+quartz Opx+orthoclase+water

15. 15
Amphibole Group
General Formula
W0-1X2-3 Y5Z8O22 (OH)2
Where W=Na, K
X= Ca,Na, K, Mn
Y= Mg, Fe+2
, Fe+3
, Al, Ti, Mn, Cr, Li, Zn
Z=Si, Al
Introduction
• The basic building block consists of a double chain (inosilicates) of silicon tetrahedra.
• In general cleavage in amphiboles is better than in the pyroxenes, which is about 56 ˚ and 124˚, rather than nearly
at right angles.
• Amphibole structure and classification are very similar to the pyroxenes.
• Si:O is 4:11.
Physical properties
Long prismatic, acicular, or fibrous crystal habit,
Mohs hardness between 5 and 6, and
Two directions of cleavage intersecting at approximately 56° and 124°.
The specific gravity values range from about 2.9 to 3.6.
Amphiboles yield water when heated in a closed tube and fuse with difficulty in a flame.
Colour ranges extensively from colourless to white, green, brown, black, blue, or lavender and is related to composition,
principally the iron content.
Mg-rich amphiboles such as anthophyllite, cummingtonite, and tremolite are colourless or light in colour.

16. 16
Many amphiboles occur in characteristic fibrous or acicular forms. Amphiboles are widespread in igneous and
metamorphic rocks, and certain types occur as detrital grains in sediments. Certain of the fibrous forms belong to the
group of minerals collectively known as asbestos
Classification
Structure

17. 17
(OH) groups in structure of amphibole decrease their thermal stability when compared with more refractive pyroxenes
thus amphiboles disintegrated to anhydrous minerals at elevated temperatures below the melting point. Amphibole
structure is consisted of double Si4O11 chain stack along c-axis, bonded by octahedral strips. A site has 10-, 12-
coordination with O and (OH) and host Na (K).
M4 site accommodate 6- , 8-coordination and host Xtype cations (Ca2+, Na+, Mn2+, Fe2+, Mg2+, Li+).
M1, M2, M3 octahedra host Y type cations (Mn2+
, Fe2+
, Mg2+
,Fe3+
,Al3+
, Ti4+
) and share edge to form octahedralband
parallel to c-axis.
Amphibole also form ‘t-o-t’ structure of which twice as wide as ‘t-o-t’ in pyroxene (along b-axis) because of doubled
chain width in amphibole. This wider geometry causes 56° and 124° cleavage.
Structure
• Double chain silicates with (Si,Al)O4 tetrahedra.

18. 18
• The OH group is an essential part of the structure.
• There are both orthorhombic and monoclinic amphiboles.
• The complex structure allows a large number of different ion substitutions, therefore, amphiboles occur in a very
diverse number of rock types.
• Eg hornblende, there is no simple composition and it occurs in many rock types, hence there are no simple charts
that can be developed to describe properties.
Chemistry
There are severalamphibole groups
• Anthophyllite-Cummingtonite (Ca+Na≅0)
• Calcium Amphiboles (Ca>Na)
• Alkali Amphiboles (Na>Ca)
Within these groups there are severalcontinuous series.
A typical series is:
• Tremolite → Actinolite → Ferro-Actinolite
Ca2Mg5Si8O22(OH)2 Ca2(Mg,Fe)5Si8O22(OH)2 Ca2Fe5Si8O22(OH)2
These are essentially metamorphic minerals and are found in all types of metamorphic regimes. They are usually found as
fibrous, radiating clusters.
• There are many such series for the amphiboles.
• There are also mixtures to Hornblende:
• (Ca,Na,K)2-3(Mg,Fe+2
,Fe+3
,Al)5[Si6(Si,Al)2O22](OH,F)2
• Higher temperatures allow Al to enter the amphibole structure and hornblende often occurs. However,depending
on Fe/Mg ration and pressure,you can get an miscibility gap between actinolite/tremolite and hornblende.
Amphibole Paragenesis
1) Thermal metamorphism of sediments (common reaction)
5CaMg(CO3)2+ 8SiO2+ H2O →Ca2Mg5Si8O22(OH)2+ 3CaCO3 + 7CO2
(dolomite) (silica) (tremolite) (calcite)
• If you increase the grade of metamorphism, tremolite breaks down to diopside and at very high temperatures
forsterite can form.
• All such reactions liberate CO2 and H2O and are therefore pressure dependent.

19. 19
2) Regional Metamorphism: Tremolite occurs where calcareous sediments dominate. Regional metamorphism of
Ultrabasic rocks
both tremolite and actinolite (common) + talk, carbonates and chlorite.
3) Hornblende-occurs in a wide variety of T-P conditions in both igneous and metamorphic rocks. Stable in most silicious
rocks and very common in intermediate plutonic rocks. Gabbros (Mg:Fe = 3:1), Syenites and granites, (Mg:Fe=5:95),
Diorites (Mg:Fe=1:15)
4) Basaltic Hornblende-only in igneous rocks-occur as phenocrysts in andesites
5) Kaersutite (Ti bearing amphibole)-occur as large phenocrysts (glomoporhyritic)in alkalic volcanic rocks such as
trachyte
6) Alkali Amphiboles -Ca (Na,K) Glaucophane -occur under high pressure, low temperature conditions such as
subduction zones (also large amounts of Na from seawater). Eg:Glaucophane schists. Riebeckite - occurs in alkali
igneous rocks (plutonic) such as quartz syenites, granites, trachyte and rhyolite.
Occurrence and Distinction ofthe Amphiboles
Tremolite - Occurs almost exclusively in low grade metamorphic rocks, particularly those with a high Ca concentration,
such as meta-dolomites, meta-ultrabasic rocks.
Tremolite in hand specimen is white in color and shows a fibrous habit and the
characteristic amphibole cleavage. In thin section it is distinguished from wollastonite and diopside by its amphibole
cleavage. In thin section it is clear with no pleochroism, which distinguishes it from other amphiboles. It shows high
relief, inclined extinction, and is optically negative with a 2V of about 85o
.
Actinolite - Also occurs almost exclusively in low grade metamorphic rocks, particularly in meta-basalts and
meta-gabbros where it is commonly associated with chlorite. It is green in hand specimen and shows the
characteristic amphibole cleavage, usually showing an elongated habit. In thin section it shows a characteristic pale
yellow to green pleochroism, has high relief, and is optically negative with a 2V of 60 to 85o
.
Hornblende - is a common mineral in both igneous and metamorphic rocks. In igneous rocks it is found in andesites,
dacites, and rhyolites, as well as in gabbros, diorites, and granites. In metamorphic rocks it is a common constituent of
meta-basalts that have been metamorphosed to intermediate grades of regional metamorphism (amphibolites). It is also
found in some ultrabasic rocks. In hand specimen it is dark brown to black in color and shows the characteristic
amphibole cleavage. In thin section, it shows high relief with a characteristic green - brown - yellow pleochroism. Optic
sign and 2V angle cover a wide range and not very useful in the distinction of hornblende.
Basaltic Hornblende (also called Oxy-hornblende)- is a dark brown to reddish brown variety of hornblende that results
from oxidation during crystallization of basalts, andesites, dacites, and rhyolites. It usually has a dark reaction rim that
consists of opaque oxide, and is characteristically pleochroic in yellow to brown to reddish brown
colors.
Anthophyllite - does not occur in igneous rocks, but is a constituent of metamorphic rocks. It is the only orthorhombic
amphibole so it is easily characterized by its parallel extinction relative to the {110} cleavage.
Cummingtonite - Grunerite - is more common in metamorphosed igneous rocks where members of the series occur with
hornblende. It has been found in siliceous volcanic rocks as well. Cummingtonite is optically positive, while grunerite is
optically negative. Members of this series can be distinguished from orthorhombic Anthophyllite by the inclined
extinction of the monoclinic Cummingtonite-Grunerite series, and can be distinguished from tremolite and actinolite by
the higher refractive indices and higher birefringence of the Cummingtonite Grunerite series.
Glaucophane - Riebeckite - Glaucophane is a common mineral in blueschist facies metamorphic rocks that result from
low temperature,high pressure metamorphism along ancient subduction zones. Riebeckite is found in alkali granites,
syenites, and peralkaline rhyolites. Glaucophane is easily distinguished from the other amphiboles by its characteristic
blue-lavender pleochroism. Glaucophane is length slow, whereas Riebeckite is length fast.

20. 20
Arfvedsonite - occurs most commonly in peralkaline volcanic rocks and alkaline plutonic igneous rocks, where it
typically occurs with the sodic pyroxene aegerine. Its blue green to yellow green pleochroism distinguish it from the other
amphiboles.
Cation distribution in Mg-end members
WX Y Z
1. Cummingtonite
(Anthophyllite) Mg2 Mg5Si8
2. Gedrite Mg2Mg3Al2 Si6Al2
1. Tremolite Ca2 Mg5Si8
2. Hornblende Ca2Mg4AlSi7Al
3. Tshermakite Ca2Mg3Al2Si6Al2
4. Edenite NaCa2Mg5Si7Al
5. Pargasite NaCa2Mg4AlSi6Al2
6. Richterite NaNaCaMg5Si8
7. Katophorite NaNaCaMg4Al
8. Mboziite NaNaCaMg3Al2Si6Al2
9. Glaucophane Na2Mg3Al2Si8
10. Eckermannite NaNa2Mg4AlSi8
Mica Group – Monoclinic
Sheet structures – Phyllosillicates
Platy morphology, perfect basal cleavage – layer atomic structures
Minerals in this group show considerable variation in chemical and physical properties.
Phlogopite, muscovite and lepidolite are of consideral economic importance
Phyllosilicates (sheet silicate or layered silicate)
• flaky,
• soft
• low density.
• Form at relatively low temperature
• Replace earlier minerals as a result of alteration
Mica Group
• Muscovite
• Paragonite
• Glauconite
• Lepidolite
• Phlogopite
• Biotite
• Zinnwaldite

21. 21
• Margarite
• Clinotonite
• Xanthophyllite
• Vermiculite
Most common micas- muscovite, paragonite, glauconite, lepidolite, biotite and zinnwaldite
General formula
X2Y4-6Z8O20(OH,F)4
X = K,Na,Ca, Ba, Rb, Cs
Y = Al, Mg, Fe, Mn, Cr, Ti, Li
Z = Si, Al
Structure
Layer of octahedrally co-ordinated cations is sandwiched between two identical layers of linked (Si,Al)O4 tetrahedra

22. 22
X Y Z
DI-OCTAHEDRAL MICAS
Common
Mica
Muscovite K2 Al4 Si6 Al2
Paragonite Na2 Al4 Si6 Al2
Glauconite (K, Na)1.2-2.0 (Fe, Mg, Al)4 Si7-7.6 Al1.0-0.4
Brittle Mica Margarite Ca2 Al4 Si4 Al4
TRI-OCTAHEDRAL MICAS
Common
Mica
Phlogopite K2 (Mg, Fe+2)6 Si6 Al2
Biotite K2 (Mg, Fe, Al)6 Si6-5 Al2-3
Zinnwaldite K2 (Fe, Li, Al)6 Si6-7 Al2-1
Lepidolite K2 (Li, Al)5-6 Si6-5 Al2-3
Brittle Mica Clintonite,
Xanthophyllite
Ca2 (Mg, Al)6 Si2.5 Al5.5
Paragenesis - Muscovite - Igneous rocks - Granites, Granitic Pegmatites and aplites.
Phlogopite, Peridotites Biotite - Gabbros, norites, diorites, granites, pegmatites
Lepidolite, Zinnwaldite - Pegmatites and high- temperature veins
Metamorphic Rocks - Muscovite, Paragonite and biotite: Phyllites, Schists and gneisses. Phlogopite: metamorphosed
limestones and dolomite Sedimentary Rocks Muscovite and Paragonite - Detrital and authigenic sediments. Glauconite :
greensands
Chemistry : MUSCOVITE (Monoclinic) One of the most common micas it can vary considerably in composition, as a
result of atomic substitutions. Na always present. Higher the replacement of K, higher temperature of formation. Na
content of muscovite may thus be a useful geological thermometers.
Al replaced by Mg and Fe. Less commonly by Cr (giving a bright green colour, var. Fuchsite or mariposite) Diagnositic
features:the perfect cleavage .
Sericite is a variety of muscovite which formed from the alteration of feldspar.
Uses of Muscovite
The combination of perfect cleavage,flexibility, elasticity, low thermal conductivity, and high dielectric strength makes
muscovite a unique mineral and one which is essential in industry. Used in electrical industry for condensers,as insulating
material between commutator segments and in heating elements.
Paragonite
• A mineral, related to muscovite.

23. 23
• Wide solvus separates muscovite from paragonite, such that there is little solid solution along the vector Na+
K+
and apparent micas of intermediate composition is most commonly a microscopic (or even sub-microscopic)
intergrowth of two distinct micas, one rich in K,and the other in Na.
• Paragonite is a common mineral in rocks metamorphosed under blueschist facies conditions along with other
sodic minerals such as albite, jadeite and glaucophane. During the transition from blueschist to greenschist facies,
paragonite and glaucophane are transformed into chlorite and albite.
Glauconite
• It is an iron-potassium phyllosilicate of characteristic green color with very low weathering resistance and very
friable.
• Its name is derived from the Greek glaucos (γλαυκος) meaning 'gleaming' or 'silvery', to describe the appearance
of the blue-green color, presumably relating to the sheen and blue-green color of the sea's surface. Its color ranges
from olive green, black green to bluish green.. It can be confused with chlorite (also of green color) or with some
other clay minerals.
• Glauconite is considered a diagnostic mineral indicative of continental shelf marine depositional environments
with slow rates of accumulation. For instance, it appears in Jurassic/lower Cretaceous deposits of greensand,so-
called after the coloration caused by glauconite.
• It develops as a consequence of diagenetic alteration of sedimentary deposits, bio-chemical reduction and
subsequent mineralogical changes affecting iron-bearing micas such as biotite,
• It is also influenced by the decaying process of organic matter degraded by bacteria in marine animal shells.
Glauconite forms under reducing conditions in sediments and such deposits are commonly found in near shore
sands,
• Glauconite has long been used in Europe as a pigmentation agent for artistic oil paint, especially in Russian
"icon paintings".
• It is also found as mineral pigment in wall paintings from the ancient Roman.
• Glauconite, a major component of greensand,is also a common source of potassium in plant fertilizers.
Margarite
• Calcium rich member of the mica group
• It forms white to pinkish or yellowish gray masses or thin laminae
• It occurs commonly as an alteration product of corundum, andalusite and other aluminous minerals.
• It has been reported as forming alteration pseudomorphs of chiastolite (variety of andalusite) along with
muscovite and paragonite.
• The margarite in this occurrence forms preferentially along the dark graphite rich inclusions with the chiastolite
crystals.
Phlogopite
• It is a yellow, greenish, or reddish-brown member.
• It is also known as magnesium mica.
• Phlogopite is the magnesium end member of the biotite solid solution series,
• Iron substitutes for magnesium in variable amounts leading to the more common biotite with higher iron content.
• Similar characteristic properties of biotite.
Paragenesis

24. 24
• Phlogopite is an important and relatively common end-member composition of biotite. Phlogopite micas are
found primarily in igneous rocks, although it is also common in contact metamorphic aureoles of intrusive
igneous rocks with Mg-rich country rocks.
• Basaltic association
• The basaltic occurrence of phlogopite is in association with picrite basalts and high-alumina basalts.
• Phlogopite is stable in basaltic compositions at high pressures and is often present as partially resorbed
phenocrysts or an accessory phase in basalts generated at depth.
• Ultrapotassic association
• Phlogopite mica is a commonly known phenocryst and groundmass phase within ultrapotassic igneous rocks such
as lamprophyre, kimberlite,lamproite,and other deeply sourced ultramafic or high-magnesian melts.
• Phlogopite in this association is a primary igneous mineral present because of the depth of melting and high vapor
pressures.
Biotite
• Common phyllosilicate mineral within the mica group,
More generally, it refers to the dark mica series,
Primarily a solid-solution series between the iron-end member annite and the magnesium-end member phlogopite
More aluminous end members include siderophyllite.
• It is sometimes called "iron mica" because it is more iron-rich than phlogopite. It is also sometimes called "black
mica”
• Biotite is used extensively to constrain ages of rocks, by either potassium-argon dating or argon-argon dating.
• Because argon escapes readily from the biotite crystal structure at high temperatures,
• These methods may provide only minimum ages for many rocks.
• Biotite is also useful in assessing temperature histories of metamorphic rocks, because the partitioning of iron and
magnesium between biotite and garnet is sensitive to temperature.
• Biotite occurs in greater variety of geological environments than any of the other micas. Formed under a wide
range of temperature and pressure conditions, and it occurs abundantly in many regionally and contact
metamorphosed sediments. Found in many igneous rocks but most commonly in the intermediate and acidic
families. Biotite of volcanic rocks are in generalpoorer in Fe + 2
and richer in Fe+3
and Ti than those in their
intrusive equivalents.
Lepidolite
- Commonly pale lilac colour
Can be distinguished from other micas by its lithium content (flame test) and by its lilac colour and common
occurrence in complex granite pegmatites.
Ore of lithium and also used as a raw material in glass and ceramics.
Zinnwaldite
• potassium lithium iron aluminium silicate hydroxide fluoride is a silicate mineral in the mica group.
• Occurrence
• It occurs in greisens, pegmatite, and quartz veins often associated with tin ore deposits.
• It is commonly associated with topaz, cassiterite, wolframite, lepidolite, spodumene, beryl, tourmaline, and
fluorite.
Clintonite

25. 25
• is a calcium magnesium aluminium phyllosilicate mineral.
• It is a member of the margarite group of micas, "brittle" micas.
• It occurs as variably colored, colorless, green, yellow, red, to reddish-brown masses and radial clusters.
• The brittle micas differ chemically from the micas in containing less silica and no alkalis,and from the chlorites
in containing much less water;
• In many respects,they are intermediate between the micas and chlorites.
• Clintonite and its iron-rich variety xanthophyllite are sometimes considered the calcium analogues of the
phlogopites.
• Typical formation environment is in serpentinized dolomitic limestones and contact metamorphosed skarns.
• It occurs with talc, spinel, grossular, vesuvianite, clinopyroxene, monticellite, phlogopite, chlorite, quartz, calcite
and dolomite.
Feldspar Group
Tectosilicate Minerals – Framework structure
Classified based on
NaAlSi3O8-KAlSi3O8-CaAlSi3O8
NaAlSi3O8-KAlSi3O8 Alkali feldspar
NaAlSi3O8-CaAlSi3O8 Plagioclase feldspar
• Pure Ca-end member – anorthite – Triclinic
• Sanidines – high temp. – Monoclinic
• Orthoclase – lower temp. – Monoclinic
• Microcline – lowest temp – Triclinic
Alkali feldspars classified based on optical properties
• High albite – High Sanidine
• High albite – Low Sanidine

26. 26
• Low albite – Orthoclase
• Low albite - Microcline
Plagioclase series
Albite (0 to 10) — NaAlSi3O8
oligoclase (10 to 30) — (Na,Ca)(Al,Si)AlSi2O8
andesine (30 to 50) — NaAlSi3O8 — CaAl2Si2O8
labradorite (50 to 70) — (Ca,Na)Al(Al,Si)Si2O8
bytownite (70 to 90) — (NaSi,CaAl)AlSi2O8
anorthite (90 to 100) — CaAl2Si2O8
Potash Feldspars
• Sanidine - Monoclinic
• Orthoclase - Monoclinic
• Microcline - Triclinic
• Perthites – Intergrowths of Na-rich feldspars in a K-rich feldspar
• Antiperthites - Intergrowths of K-rich feldspars in a Na-rich feldspar
• Potassium feldspar or "K-feldspar" varies in crystal structure depending on its crystallization temperature.
• Microcline is the stable form of potassium feldspar below about 400°C.
• Orthoclase and sanidine are stable above 500°C and 900°C, respectively
• Square cleavage is the mark of K-feldspar, along with a generally less translucent appearance and the
absence of striations along cleavage faces. It also commonly takes pinkish colors. Green feldspar is always K-
feldspar, a variety called amazonite.
Ca2Mg3Al4Si6O22(OH)2+SiO22CaAl2Si2O8+ 3MgSiO3+H2O
Amphibole anorthite OPX
NaCa2Mg3Fe+2Al3Si6O22(OH)2+4SiO2NaAlSi3O8+CaAl2Si2O8
Amphibole albite anorthite

27. 27
+ CaMgSi2O6+Mg2FeSi3O9+H2O
Diopside OPX
NaAlSi3O8+(Mg,Fe)2SiO4 NaAlSi2O6+2(Mg,Fe)SiO3
Albite Olivine Omphacite
CaAl2Si2O8+(Mg,Fe)2SiO4 Ca(Mg,Fe)2Al2Si3O12
Anorthite Olivine Garnet
silica mineral, any of the forms of silicon dioxide (SiO2), including quartz, tridymite, cristobalite, coesite,
stishovite, lechatelierite, and chalcedony. Various kinds of silica minerals have been produced synthetically;
one is keatite.
General considerations
Silica minerals make up approximately 12 percent of the Earth’s crust and are second only to the feldspars in
mineral abundance. Free silica occurs in many crystalline forms with a composition very close to that of silicon
dioxide, 46.75 percent by weight being silicon and 53.25 percent oxygen. Quartz is by far the most commonly
occurring form. Tridymite, cristobalite, and the hydrous silica mineral opal are uncommon, and vitreous
(glassy) silica, coesite, and stishovite have been reported from only a few localities. Several other forms have
been produced in the laboratory but have not been found in nature.
Physical and chemical properties
The crystallographic structures of the silica minerals, except stishovite, are three-dimensional arrays of linked
tetrahedrons, each consisting of a silicon atom coordinated by four oxygen atoms. The tetrahedrons are usually
quite regular, and the silicon-oxygen bond distances are 1.61 ± 0.02 Å. Principal differences are related to the
geometry of the tetrahedral linkages, which may cause small distortions within the silica tetrahedrons. High
pressure forces silicon atoms to coordinate with six oxygen atoms, producing nearly regular octahedrons in the
stishovite structure.
The silica minerals when pure are colourless and transparent and have a vitreous lustre. They are nonconductors
of electricity and are diamagnetic. All are hard and strong and fail by brittle fracture under an imposed stress.
Some important physical properties of the silica minerals are compared in the Table. All except low tridymite
and coesite (among the crystalline varieties) have relatively high symmetry. There is a linear relationship
between the specific gravity values listed in the Table and the arithmetic mean of the indices of refraction
(measures of the velocity of light that is transmitted in different crystallographic directions) for silica minerals
composed of linked tetrahedrons. This relationship does not extend to stishovite because it is not made up of
silica tetrahedrons. Melanophlogite is notable because it plots below vitreous silica on the graph. The specific
gravities of silica minerals are less than those of most of the dark-coloured silicate minerals associated with
them in nature; in general, the lighter-coloured rocks have lower specific gravity for this reason. Silica minerals
are insoluble to sparingly soluble in strong acids except hydrofluoric acid, in which there is a correlation
between specific gravity and solubility.
Quartz occurs in many varieties in almost all types of igneous, sedimentary, and metamorphic rocks. It has also
been found in meteorites and in some lunar rocks.
Quartz crystals lack a centre of symmetry or planes of symmetry and have one crystallographic axis (c)
perpendicular to three polar axes (a) that are 120° apart. One end of a polar axis is different from its other end;
when mechanical stress is applied on such an axis, opposite electrical charges develop on each end. This leads
to important applications in electronics as a frequency control and in pressure gauges and other devices. The

28. 28
lack of symmetry planes parallel to the vertical axis allows quartz crystals to occur as two types: left-handed or
right-handed (enantiomorphism). Left-handed quartz is less than 1 percent more abundant than right-handed
quartz. The structural tetrahedrons spiral upward through the crystal in the sense of the handedness parallel to
the c axis. Similarly, if polarized light is transmitted by a quartz crystal along the c-axis direction, the plane is
rotated in the direction of the handedness by tens of degrees per millimetre, the amount depending on the
wavelength of the light. This property is used in optical instruments such as monochromators.
The requirements for high-purity crystals for electronic frequency applications have led to the development of
industrial synthesis methods based on the differences in silica solubility with temperature. Nutrient crystals are
placed in the hot portion of a solution-filled autoclave, and an oriented seed crystal free from twinning is placed
in the cooler portion. Crystals weighing thousands of grams can be grown in a few weeks.
Quartz shows less range in chemical composition than do most other minerals, but it commonly contains tens to
hundreds of parts per million of aluminum atoms substituting for silicon atoms, with charge balance maintained
by the incorporation of small atoms, such as hydrogen, lithium, or sodium. Titanium, magnesium, or iron atoms
substituting for silicon atoms also have been reported, but anionic substitution (i.e., substitution for the negative
ion, oxygen) is limited because the linkage of the tetrahedrons is disrupted.
Coloured varieties of quartz are numerous and have many causes. Most colours result from mechanically
incorporated admixtures within fine-crystallized or granular quartz, but some coarse-crystallized varieties, such
as amethyst (violet), citrine (yellow), milky quartz, smoky quartz or morion (black), or rose quartz, may be
coloured by ions other than silicon and oxygen that occur within the crystal structure. Small fractions of 1
percent by weight of iron, aluminum, manganese, titanium, hydrogen, and small alkali atoms, such as lithium
and sodium, have been shown to be the cause of different colours. Heat treatment or various irradiation
treatments under oxidizing or reducing atmospheres are used to change one coloured variety to another. Citrine
is commonly produced by heat-treating amethyst at 250–400 °C (482–752 °F), for example.
Quartz may contain inclusions of other minerals, such as rutile (rutilated quartz), tourmaline, asbestiform
amphiboles, or platy minerals, such as mica, iron oxides, or chlorite (aventurine).
Chalcedony is a white, buff, or light tan, finely crystallized or fibrous quartz that forms rounded crusts,rinds, or stalactites
(mineral deposits suspended from the roofs of caverns) in volcanic and sedimentary rocks as a precipitate from moving
solutions. If chalcedony is conspicuously colour-banded, it may be called agate; onyx is agate with alternate bands of
white and black or dark brown. Some concentrically banded “eye” agate nodules contain cores of coarsely crystalline
quartz, and other agates are mottled or variegated in colour. Arborescent or dendritic (branching) dark-coloured patterns
set in a lighter field are called moss agate or Mocha stone. Translucent red chalcedony is called carnelian, and translucent
brown shades are referred to as sard; both are pigmented by admixed iron oxides.
Chalcedony
Chalcedony is a white, buff, or light tan, finely crystallized or fibrous quartz that forms rounded crusts, rinds, or
stalactites (mineral deposits suspended from the roofs of caverns) in volcanic and sedimentary rocks as a
precipitate from moving solutions. If chalcedony is conspicuously colour-banded, it may be called agate; onyx
is agate with alternate bands of white and black or dark brown. Some concentrically banded “eye” agate nodules
contain cores of coarsely crystalline quartz, and other agates are mottled or variegated in colour. Arborescent or
dendritic (branching) dark-coloured patterns set in a lighter field are called moss agate or Mocha stone.
Translucent red chalcedony is called carnelian, and translucent brown shades are referred to as sard; both are
pigmented by admixed iron oxides.
Chrysoprase, plasma, and prase are names for green varieties of chalcedony coloured by admixed green
minerals, such as chlorite, fibrous amphiboles, or hydrous nickel silicates. Bloodstone and heliotrope are green
chalcedony with red spots.
Jasper, chert, and flint

29. 29
Jasper is opaque red, brown, or yellow quartz that is pigmented by admixed iron oxides. Chert and flint are
finely crystallized varieties of gray to black quartz that occur as nodules or bands in sedimentary rocks.
High quartz (β-quartz)
High quartz, or β-quartz, is the more symmetrical form quartz takes at sufficiently high temperatures (about 573
°C at one atmosphere of pressure), but the relationship is pressure-sensitive. High quartz may be either left- or
right-handed, and its c axis is one of sixfold symmetry rather than threefold; thus, many twin laws of ordinary
quartz cannot occur. High quartz twins typically involve inclined sets of axes. High quartz can form directly
from silicate magma or from high-temperature gases or solutions. It invariably undergoes the transition to
ordinary quartz (low quartz) on cooling, and all ordinary quartz, when heated above the transition temperature,
is transformed into high quartz. The transformation involves displacement of the linkage between the
tetrahedrons; no bonds are broken.
Tridymite
Tridymite may occur as a primary magmatic phase (i.e., as a direct result of crystallization from a silicate melt)
in siliceous rocks but is most abundant in voids in volcanic rocks where it probably was deposited metastably
from hydrous gases. Tridymite also forms in contact-metamorphosed rocks. It has been found in meteorites and
is common in lunar basalts. It occurs in quantity in firebricks and other siliceous refractories. Natural tridymite
has no specific commercial use.
Cristobalite
Cristobalite is probably more abundant in nature than tridymite, although it seldom forms as distinctive crystals.
The devitrification (transformation from the glassy to the crystalline state) of siliceous volcanic glasses yields
abundant tiny crystallites of cristobalite, and the mineral is also deposited metastably from hot hydrous gases in
cavities and cracks of many volcanic rocks. It has been found in lunar basalts and in meteorites and is common
in silica refractories exposed to very high temperatures.
Opal
Opal is poorly crystalline or amorphous hydrous silica that is compact and vitreous and most commonly
translucent white to colourless. Precious opal reflects light with a play of brilliant colours across the visible
spectrum, red being the most valued. Opal forms by precipitation from silica-bearing solutions near the Earth’s
surface. Electron microscopy has shown that many opals are composed of spheres of tens to a few thousand
angstroms in size that are arranged in either hexagonal or cubic close packing. The spheres are composed of
hydrous silica that may be either almost cristobalite-like, tridymite-like, mixtures of both, or random and
nondiffracting. The specific gravity and refractive index are lower than those of pure silica minerals. The play
of colours in precious opal arises from the diffraction of light from submicroscopic layers of regularly oriented
silica spheres. When heated, opal may lose as much as 20 percent of its weight of water, fracture, and then
crystallize to one of the silica minerals described above.
Opal usually contains 4 to 9 percent water, but lower and much higher values have been observed. The contents
of alumina, ferric oxide, and alkalis are variable but may amount to several percent in light-coloured opals and
more if pigmenting minerals are also present. Precious opal has been synthesized. Opaline silica is a friable
hydrous silica found near hot springs and geysers.
Vitreous silica
Vitreous silica, lechatelierite, is supercooled liquid silica. It has been observed in nature as the result of fusion
of quartz by lightning strikes (fulgurites) or by shock associated with large meteorite impacts and may approach
artificial, very pure silica glass in composition and physical properties.
Melanophlogite

30. 30
Melanophlogite is a tetragonal or cubic silica mineral with a gas-hydrate structure containing many large voids.
In nature these are filled with 6 to 12 percent by weight of compounds of hydrogen, carbon, and sulfur, which
may be necessary for mineral growth. If these compounds are destroyed by heating, they do not cause the
crystal to collapse, but the free carbon formed does darken it. Melanophlogite occurs with bitumen and forms at
temperatures below 112 °C. It has been found on native sulfur crystals in Sicily and Santa Clara county, Calif.
Keatite
Keatite is a tetragonal form of silica known only from the laboratory, where it can be synthesized metastably in
the presence of steam over a temperature range of 300 to 600 °C and a pressure range of 400 to 4,000 bars
(standard atmospheric pressure at sea level is 1,013.3 millibars, or slightly more than 1 bar, which equals 760
millimetres of mercury). It has negative thermal expansion along the a axis and positive thermal expansion
along the c axis, so that the overall expansion is very low or negative.
Coesite and stishovite are rare dense forms of silica. They are observed in nature only where quartz-bearing
rocks have been severely shocked by a large meteorite impact, such as Meteor Crater in Arizona, U.S. Coesite is
found in ultrahigh-pressure metamorphic rocks such as in Dora Maira, Italy, and the Dabie Mountains, China.
Coesite is made up of tetrahedrons arranged like those in feldspars. Stishovite is the densest form of silica and
consists of silicon that is octahedrally coordinated with oxygen. Both coesite and stishovite have been
synthesized and found to be stable only at high pressures.
Originand occurrence
Silicon and oxygen are the two most abundant elements in the Earth’s crust, in which they largely occur in
combination with other elements as silicate minerals. Free silica (SiO2) appears as a mineral in crystallizing
magma only when the relative abundance of SiO2 exceeds that of all other cations available to form silicates.
Silica minerals thus occur only in magmas containing more than about 47 percent by weight of SiO2 and are
incompatible with minerals with low cation:silica ratios—such as olivine, nepheline, or leucite. Basaltic and
alkalic igneous magmas therefore can crystallize only minor amounts of silica minerals, and sometimes none
are produced. The gas released from such rocks can dissolve the silica components, however, and later
precipitate silica minerals upon cooling. The amount of silica minerals crystallized from magma increases with
increasing silica content of magma, reaching 40 percent in some granites and rhyolites.
Solubility of silica minerals
The solubility of silica minerals in natural solutions and gases is of great importance. The solubility of all silica
minerals increases regularly with increasing temperature and pressure except in the region of 340–550 °C and
0–600 bars, where retrograde solubility occurs because of changes in the physical state of water. The solubility
of silica increases in the presence of anions such as OH- and CO2-/3, which form chemical complexes with it.
Quartz is the least soluble of the forms of silica at room temperature. In pure water its solubility at 25 °C is
about 6 parts per million, that of vitreous silica being at least 10 times greater. Typical temperate-climate river
water contains 14 parts per million of silica, and enormous tonnages of silica are carried away in solution
annually from weathering rocks and soils. The amount so removed may be equivalent to that transported
mechanically in many climates. Silica dissolved in moving groundwater may partially fill hollow spheroids and
precipitate crystals to form geodes, or it may cement loose sand grains together to form concretions and nodules
or even entire sedimentary beds into sandstone, which, when all pore space is eliminated by selective solution
and nearby deposition during metamorphism, form tough, pore-free quartzite.
Gases or solutions escaping from cooling igneous rocks or deep fractures commonly are saturated with silica
and other compounds that, as they cool, precipitate quartz along their channelways to form veins. It may be
fine-grained (as chalcedony), massive granular, or in coarse crystals as large as tens of tons. Most natural
colourless quartz crystals, “rock crystal,” were formed in this way.

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The emergence of heated silica-bearing solutions onto the surface results in rapid cooling and the loss of
complexing anions. Rapid precipitation of fine-grained silica results in formation of siliceous sinter or geyserite,
as at Mammoth Hot Springs in Yellowstone National Park in the western United States.
Quartz is mechanically resistant and relatively inert chemically during rock weathering in temperate and cold
climates. Thus, it becomes enriched in river, lak e, and beach sediments, which commonly contain more than
one-half quartz by weight. Some strata consist almost entirely of quartz over large lateral distances and tens or
hundreds of metres in thickness. Known as glass sands, these strata are important economic sources of silica for
glass and chemical industries. Quartz-bearing strata are abundant in metamorphic terrains. The reincorporation
of free silica into complex silicates and the solution and redeposition of silica into veins is characteristic of such
terrains.
The silica phase diagram
In diagrams of pressure-temperature fields of stability of silica minerals, stability fields are not shown for
keatite, melanophlogite, opal, or the low forms of tridymite and cristobalite because they have not been
demonstrated. Quartz is the stable phase of silica under the physical conditions that prevail over most of the
Earth’s crust. Coesite occurs at depths of about 100 kilometres (60 miles) in the Earth’s mantle. Stishovite
would require even greater depths of burial, and no rocks that occur on the terrestrial surface have been buried
so deeply. Stishovite is reported only in a few localities that were subjected to very high pressures from
meteorite impact events.
Uses
Quartz is the only natural silica mineral used in significant quantities; millions of tons are consumed annually
by many industries. The sand that is an essential ingredient of concrete and mortar is largely quartz, as are the
sandstone and quartzite used as building stones. Crushed sandstone and quartzite are used for road and railway
construction, roofing granules, and riprap—erosion-control linings of river channels. Quartz is hard (7 on the
Mohs scale) and resists fracture because it lacks easy cleavage. These properties, combined with its ready
availability, lead to its use as a sandpaper abrasive and in sandblasting; for polishing and cutting glass, stone,
and metal; and for providing traction on stairs, streets, and rails. Large amounts of relatively pure quartz are
used in refractory products, such as insulation and firebricks, foundry molds, and electrical insulators, because
of the combination of its high melting temperatures, low coefficients of expansion, inertness of the high-
temperature forms of silica, and low costs.
Relatively pure quartz is required in large tonnages as an ingredient for glass and porcelain manufacture. High
purity quartz is fused to make premium grades of chemical and optical glass for which one or more of its
desirable properties of low thermal expansion, high-shape stability, elasticity, low solubility, and transparency
to various kinds of light can justify the greatly increased costs involved. Fibres of vitreous silica are essential
for precision instruments, such as balances, galvanometers, and gravimeters. Tons of quartz of various qualities
are used as raw materials for processes in which silica is not the final product. These include the production of
water glass, or sodium silicate, various sols—very fine dispersions of solids in liquids—that are used as
hydrophobic (water-repelling) coatings, organic silicates and silicones, silicon carbide, silicon metal, smelting
flux, and alloying in metallurgy.
Quartz and its varieties have been used since antiquity as semiprecious gems, ornamental stones, and collector’s
items. Precious opal, a hydrous form of silica, has been a gemstone since Roman times.