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CHAPTER 12
Framework Silicates
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2. Predominance of Silicate Minerals in the Earth’s Crust
27% of all known minerals are silicates
40% of common minerals are silicates
>90% minerals in the earth’s crust are silicates

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Framework silicates
More than 2/3rd of earth’s crust is made up of framework silicates
Quartz, Feldspars (alkali and plagioclase), Felspathoids (Nepheline, Sodalite and Leucite), Zeolites, Scapolites
TO4 structure; T = Si4+ or Al 3+
Every Oxygen shared between two strongly charged cations – the mutual repulsion between the cations ensures open structure
Physical consequence: Lower density e.g., Quartz (SiO2)= 2.65, Olivine (Mg2SiO4) = 3.27
Compositional consequence: open framewok silicates can accommodate large ions like K+, Na+, Ca2+ octahedral sites between the tetrahedra
Charge balance is maintained by replacing Si4+ with Al3+ in tetrahedral sites
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Silica Group: SiO2: Many polymorphs: Quartz, Tridymite and Cristobalite are common, Coesite and Stishovite are high pressure polymorphs – found in mantle rocks or rocks shocked by meteorite or nuclear bomb impacts.
Quartz, Tridymite and Cristobalite are reconstructive polymorphs. And each has α (low) and β (high) varieties – and these are displacive polymorphs. β (high) varieties have higher symmetry and stable at higher temperature
Quartz can grow either has α or β variety
Tridymite grows only as β variety but converts to lower energy α variety by displacive polymorphism which survives because transformation to lower energy quartz requires reconstructive polymorphism which is slow.
β quartz, typical of felsic volcanics, has a stubby prismatic face topped by pyramidal phase – this shape is retained even when it converts to α variety.
Figure 12.1 Stability fields of the silica minerals. Adapted from Griffen (1992).
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Figure 12.2 Quartz structure viewed down the c axis.
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Microcrystalline varieties: Chert, Chalcedony (chert + moganite) Moganite
Clear
Rock Crystal
Violet
Amethyst
Pink
Rose Quartz
Yellow
Citrine
Milky
Milky Quartz
Smoky brown or black
Smoky Quartz
Red, microcrystalline
Jasper
Black microcrystalline
Flint
Agate
mineraloid
Opal
Figure 12.3
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Figure 12.4 Approximate variation of nω of chalcedony with density, which is a function of pore space between the fibers. After Frondel (1982).
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Feldspars are the most abundant mineral in the Earth’s crust
Two solid solution series between three end members
K-feldspar( KAlSi3O8) – Albite (NaAlSi3O8): Alkali Feldspar series
K+ and Na+ has same charge but size differs. Complete solid solution at high temperatures but the phases exsolve at lower temperature forming perthite (or anti-perthite)
Albite (NaAlSi3O8)– CaAl2Si2O8( Anorthite): Plagioclase series
Ca2+ and Na+ has same size but charges differ -- charge balance is maintained by replacing Si4+ with Al3+
Since Ca2+ and K+ has very dissimilar size, hence no composition intermediate between Alkali feldspar and Anorthite
Figure 12.5 Composition range of common feldspars.
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Structure:
4 member rings – 2 pointing up and two pointing down. One of each is T1 and the other is T2 site
The rings are joined with other rings by sharing oxygen forming chains
Chains joined with each other laterally by sharing other oxygens
Large space between chains that can accommodate large ions like Ca2+, Na+, K+ which are coordinated with 9 oxygen ions
Figure 12.6 Idealized feldspar structure: T1 and T2 refer to different tetrahedral sites.
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10. Al: Si ratio = 1:3 – present in Alkali feldspars
Al/Si Order-Disorder
Al: Si ratio = 1:3 – present in Alkali feldspars
Mode I (high Sanidine) : Al and Si randomly distributed in the tetrahedral sites. 25% of sites occupied by Al. Forms only above 1000 C. Plane of symmetry || (010). Must have quenched rapidly.
Mode II ( Orthoclase): All Al are only in T1 sites, none in T2 sites. Real Orthoclase is a mixture of Mode II and Mode III. 2V indicates degree of ordering.
Mode III (Low Microcline) : All Al are only in one of the T1 sites: highest degree of ordering. Seen in slowly cooled igneous or metamorphic rocks. Mirror symmetry is detroyed – so, Triclinic system. Microcline adapts to Monoclinic to triclinic transformation at about 450ºC by developing twins in both with {010} twin plane – Albite twins and by rotation along b axis (pericline twins) – this developing polysynthetic twins
Al:Si ration 2:2 (Anorthite):
Mode IV: Al and Si occupy alternate sites.
Figure 12.7 Ordering of Al and Si in tetrahedral sites in the feldspars.

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Microcline – “Cross hatched” or grid twinning. Twin lamellae are spindle shaped
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13. Plagioclase:
Na rich Plags show the same order/disorder of Modes I, II and III as K-feldspars
On colling below 980 C, high temp form Monalbite (monoclinic) coverts polymorphically to triclinic structure because of a. potential ordering of Al and b. collapse of oxygen ions around smaller Na.
Triclinic Na-Plag can be high albite or low albite (highly ordered)– conversion occurs naturally and easily. High Albite found only in volcanic rocks.
With Mode IV ordering in Anorthite, the collapse of Oxygen ions around Ca2+ causes triclinic symmtery.
Aluminum Avoidance Principle: Al does not occupy two adjacent tetrahedra.
Figure 12.8 Arrangement of oxygen anions about K+ in sanidine (left) and the distorted or collapsed arrangement around Na+ in albite (right).
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14. Figure 12.9 Perthitic exsolution.
In Alkali Feldspar:
Ionic radius of Na+ = 1.32 A, of K+ = 1.65 A. Na rich and K rich domains exsolve on cooling; Albite exsolved in K feldspar host is perthite. K feldspar in Albite host is antiperthite.
Slow cooling helps exsolution hence Microcline more likely to develop perthite compared to orthoclase or sanidine.
In Plagioclase: less common (why?)
Intergrowth:
Myrmekite: worm-like quartz in sodic plagioclase. Forms at the contact of K-Feldspar and Plagioclase in granitic rocks
Graphic : cuneiform quartz in perthitic K-feldspar. Forms when the two crystallize simultaneously.
Rapakivi: sodic plagioclase mantles K- Feldspar. Antirapakivi – the opposite.
Figure 12.9 Perthitic exsolution.
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Figure Quartz–feldspar intergrowths.
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Polysynthetic twins (only in Triclinic Feldspars)
Albite twins: formed by reflection on {010} and composition plane is parallel to {010}
Pericline twin is produced by 2 fold rotation on b axis. Composition plane is close to (001)
Both form by growth, deformation, and order/disorder conversion from monoclinic to triclinic
Single twins
Carsbad, Baveno and manebach are single twins:
Carsbad twin is most common and forms by 2 fold rotation on {001}
Figure Common twins in the feldspars.
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Figure Plagioclase in thin section.
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Figure Optical properties of low (plutonic) and high (volcanic) plagioclase. Adapted from Smith (1958) and Burri and others (1967).
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Figure Index of refraction of the fast ray (nα’) for plagioclase fragments lying on cleavages. The diagram is constructed for fragments on (001) cleavage surfaces but also may be used for fragments on (010) cleavage surfaces. After Morse (1968).
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Figure Michel-Lévy method.
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Figure Diagram for use with the Michel-Lévy method.
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Figure Carlsbad–albite method.
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Figure Diagrams for use with the Carlsbad–albite method.
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Figure Variation of 2Vx and indices of refraction for common K-feldspar as a function of the degree of Si–Al order.
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Figure K-feldspar.
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27. Feldspathoids (Si-poor feldspars)
Common in alkaline (Si-undersaturated) igneous rocks
Leucite – KAlSiO4
Nepheline – (Na,K)AlSiO4
Sodalite – Na8(AlSiO4)6Cl2

28. Hydrous Tectosilicates
Analcime (Scapolite Gp) NaAlSi2O6·H2O
Natrolite (Zeolite Gp) Na2Al2Si3O10·2H2O
Heulandite (Zeolite Gp) CaAl2Si7O18·6H2O
Stilbite (Zeolite Gp) NaCa2Al5Si13O36·14H2O

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Figure Photomicrograph of nepheline (N) in thin section with plagioclase (P) and microcline (M) in nepheline syenite. Crossed polarizers.
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Figure Typical zeolite structure. Heulandite viewed down the c axis. The open channels provide space for water molecules and mono- and divalent cations. Structural data from Gunter and others (1994).
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Figure Optical properties and specific gravity of scapolite. After Shaw (1960), Ulbricht (1973), and Graziani and Lucchesi (1982).
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