Systematic Mineralogy
Silicates
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Relative abundance of different minerals in the earth’s crust (Fig. 1)
Crystal chemistry of different silicates
Coordination numbers of the different cations (Table 1)
Silicate Groups
I- Tectosilicates
Reading: pages 475 - 490
1) SiO2 minerals
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There are 9 different polymorphs of SiO2 (Table 2; Fig. 2).
Transitions from one polymorph to another: reconstructive vs. displacive
Tridymite: Orthorhombic, +, 2V = 35°, n<b. Occurs as a secondary mineral in
volcanic rocks.
Cristobalite: Pseudoisometric. n < b, moderate relief. Occurs as a secondary?
mineral in volcanic rocks (particularly in spherulites).
Coesite: Monoclinic, +, 2V = 54°, characteristic of ultrahigh pressure terranes.
Stishovite: Tertragonal, forms in impact craters.
Varieties of Quartz:
 Chalcedony: microcrystalline and fibrous. Secondary mineral in igneous rocks;
also in sedimentary rocks. Principal constituent of chert.
 Jasper: microcrystalline quartz with interstitial hematite. Red in color.
Sedimentary environments.
 Agate: Banded chalcedony
2) Feldspars
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Compositions and the ternary system: Ab – Or – An (Fig. 3)
Plagioclase feldspar series:
 albite (0 – 10% An), oligoclase (10 – 30% An), andesine (30 – 50% An),
labradorite (50 – 70% An), bytownite (70 – 90% An), anorthite (90 – 100% An).
 Common properties: Colorless in thin section, triclinic, weak birefringence, low
relief, polysynthetic twinning, 2 directions of cleavage at 83 - 90°, variable
extinction angles, and 2V.
 Optical orientations (Fig. 4).
 Zoning
 Estimating the An content of the Plagioclase:
a) Michél Lévy method (Fig. 5):
Precautions: (i) maximum extinction angles used; (ii) differences between the
2 angles < 4°; (iii) all lamellae should show the same interference color; (iv) if
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b)
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extinction angle measured is < 18°, use optic sign, 2V, or refractive indeces to
tell the An%; (v) need to carry out on at least 6 grains
Schuster method (Fig. 6): extinction angles measured against cleavage.
Using the Carlsbad + Polysynthetic twins (Fig. 7). Will work with one
measurement only; 2 extinction angles should not differ by more than 15°;
may need to be used in conjunction with the Michél Lévy method.
Relief and refractive indeces (Fig. 8)
2V and optic sign (Fig. 9).
Alkali Feldspars
 Ab – Or solid solution: limited (Figs. 10 & 11).
 Perthite and antiperthite
 Thermal stability of the different species (Fig. 12).
 Common properties:
Monoclinic or triclinic, colorless, low relief, n < b, 2 cleavages (83 – 90°); weak
birefringence; extinction angles are generally small; twinning is very common; 2V
variable, useful for telling the different species apart; optic sign: Albite is positive, all
others are negative!
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Optic orientations (Fig. 13).
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Twinning: (Fig. 14).
Orthoclase: KalSi3O8
Most common variety in igneous plutonic and metamorphic rocks. Simple twinning is
common. Extinction angle: 5 – 12°, 2V = 69 – 72°.
Microcline: KalSi3O8
Common in plutonic rocks that cooled slowly. Cross-hatched twinning (combination of
albite & pericline). 2V is large (84°). Extinction angle: 5° with cleavage.
Anorthoclase: (K,Na)AlSi3O8
Rare, seen in Na – rich igneous rocks (volcanics as well as plutonics), extinction angle: 1
– 10°, 2V = 43 – 54°.
Sanidine: (K,Na)AlSi3O8
Common in volcanic rocks (rhyolites). Small extinction angle (5 – 15°), very small 2V.
Unlike orthoclase, crystals tend to be clear!
Adularia:
A variety of orthoclase that occurs in veins!
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3) Feldspathoids
Common properties:
Isotropic to weak birefringence, low relief, usually colorless in thin section, but a few
species of the sodalite group are either blue or brown. They all occur in silica deficient
rocks, and therefore are never in equilibrium with quartz.
Leucite: KalSi2O6
Low to moderate relief, n < b, pseudoisometric and therefore pseudoisotropic,
multidirectional polysynthetic twinning is very common for coarse crystals, present in
SiO2 deficient volcanic rocks only.
Nepheline: NaAlSiO4
Hexagonal, uniaxial negative, low relief, n < b, lengthfast.
Sodalite: 3NaAlSiO4. NaCl
Isotropic, low to moderate relief, colorless.
Hauyne: 3NaAlSiO4. CaSO4
Blue in thin section, isotropic, low to moderate relief.
Nosean: 6NaAlSiO4. Na2SO4
Brown in thin section, isotropic, low to moderate relief.
4) Scapolite Group minerals
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Two end-members: Marialite (3(NaAlSi3O8).NaCl) - Meionite
3(CaAl2Si2O8).CaCO3
Tetragonal, Uniaxial negative, moderate to high relief, n > b, colorless in thin
section, yellowish, grayish, or bluish in hand specimen, strong birefringence, 2
directions of cleavage (90°). Indeces of refraction and birefringence increase with
the increase in Meionite end-member.
Common mineral in contact metamorphic aureoles, skarns, and other
metamorphosed calcareous and psammitic sediments. Associated with diopside,
actinolite, garnet, and calcite.
5) Analcime and the Zeolite group minerals
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Zeolites are hydrated aluminum silicates of alkali and alkaline earth elements.
They are characterized by open structures with large tubular channels where the
H2O (bonded primarily through H-bonds) and some of the cations reside (Fig. 15).
Different species have different sized channels.
H2O is easily driven off by heating (mostly at T < 350°C). Dehydrated samples
can be easily rehydrated when in contact with water.
Characterized by a large exchange capacity.
Shapes and habits: either fibrous, platy, or equidimensional.
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General optical properties (Table 3): low indeces of refraction (n < b), very
weak birefringence (or isotropic).
Composition of different species and Physical properties: Table 4.
Paragenesis: Low temperature environments in (a) basic volcanic rocks filling
amygdules; (b) deep sea sediments, (c) products of diagenesis to burial
metamorphism, (d) hydrothermal alteration of basic to intermediate tuffs (e)
analcime has a higher T stability limit than zeolites, and can also occur as a
primary igneous phase in subsilicic plutonic rocks (e.g. alkali syenites).
Uses: (a) molecular sieves, (b) exchange columns, (c) desiccants, (d) water
softeners.