TECTOSILICATE/ FRAMEWORK SILICATE

• More than 2/3 of the earth’s crust is composed of framework silicate minerals, Qtz, Plag, K-fdspar
• Each oxygen atom is shared with other another tetrahedra. Tetrahedra is occupied by Si+4 or Al+3
• Every oxygen is shared with an tetrahedra causes mutual repulsion of the high charged cations making the structure of the framework minerals are fairly open. This results in two imp. Properties-�

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• 1) Compositional property: Open structure can easily house large cations such as Ca2+, Na+, K+. These ions do not readily fit in minerals having maximum octahedral site. These elements appear due to substitution of Si+4 by Al+3 in tetrahedral site.
• 2) Physical property: Specific gravity for most framework silicate is significantly lower than the minerals whose anion arrangements can be approximated by close packing

Qtz sp. gravity 2.65, Fo sp. gravity 3.27

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Quartz

SiO2 Group

• Between different polymorphs of SiO2 the common one in Qtz, Tridymite and Cristabolite.
• Stishovite & Coesite are quite rare
• Stishovite stable at extremely high P (rock who suffered meteoric impact), resembles structure of Rutile (TiO2) where Si is octahedrally coordinated (6-fold) in contrast to 4fold at low P polymorph

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Qtz-Trigonal

Tridymite-Hexagonal

Cristobalite-Tetragonal

Coesite-Monoclinic

Stishovite-Tetragonal

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• Qtz, tridymite and cristabolite have their own distinct structures and conversion from one to other requires the chemical bonds to be broken. They are reconstructive polymrphs.
• Their structure is represeted by ∞ (low) & ß (high) varieties. ∞& ß varieties are displacive polymorphs (reversible because they are related to each other by a distortion in the crystal lattice). The ß polymorph is always stable aat higher T compared to the ∞

Alpha Qtz

Beta Qtz

Quartz varieties

• Amethyst-violet or purple
• Rose Qtz- Pink or rose red
• Itrine- Yellow
• Milky Qtz-Milky white
• Smoky Qtz-Smoky brown to black

Microcrystalline variety & Qtz-

• Cherts-Microcrystalline aggregate of quartz
• Jasper-Red or brown
• Flint-Black or dark gray
• Agate-usually cherts

Amorphous SiO2-

• Quartz

Rock types

Occurence

• Next to feldspar Qtz is ost abundant mineral in the earth’s crust.
• Igneous-Acidic rocks (Granite, Tonalite etc.), less common in intermediate rocks compared to acidic, <5% in basic rocks. Not found in SiO2 undersaturated rocks with feldspathoid.
• Sedimentary-Chemically and physically resistant so abundant mineral in sedimentary rocks. In oolite can form as nucleus. Also forms as cementing material in sedimentary rocks (in sst, lst).
• Metamorpic rocks-Metamorphic equivalent of qtz bearing sedimentary or igneous rock (metaacidic rocks). Sometimes formed as a product of reaction in metamorphic rocks. Cpx+Pl=Grt+Qtz

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Feldspar Group

• Feldspars are the most abundant minerals in the earth’s crust. make up about 50% of the Earth’s crust.
• Found in nearly all igneous rocks, most metamorphic rocks, imp. Constituent of sedimentary rocks.

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• K+, Na+ has same charge but different size, so continuous solid solution is limited to high T only.

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• K-feldspar and An lack solid solution because of substantially different size and charge.

Couple substitution in Plagiclase

• When a substitution takes place, in a mineral structure, electrical neutrality must be maintained.
• In feldspars, this is illustrated by members of the plagioclase series:
• from Na Al Si₃O₈ to CaAl₂Si₂O₈
• Na ⁺ is replaced by Ca ²⁺
• Si ⁴⁺ is replaced by Al ³⁺
• When three or more ions are involved in the substitution to preserve electrical neutrality, we call it a coupled substitution.
• In Alkali feldspar series-the substitution is between Na+ and K+

Bowen’s reaction series

This type of diagram illustrates the chemical changes that take place during the crystallization of liquids of intermediate composition between Ab and An.

This type of diagram illustrates the chemical changes that take place during the crystallization of liquids of intermediate composition between albite and anorthite.

Pure albite

crystallizes

at this temperature

Pure anorthite

crystallizes

at this temperature

Liquid of

intermediate

composition A

starts to crystallize at T1, to give a bit of plagioclase, composition B.

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T1

As liquid cools, the composition of the plagioclase changes. At T2, the plagioclase of composition D. The liquid has lost more Ca than Na, and is now of composition C.

T2

The last drop of liquid, composition E crystallizes at T3. If cooling was slow, all the plagioclase is now at composition F. It has the same composition as the original liquid (A).

T3

The core of the crystal is more calcium-rich than the edges.

The slow cooling as described ends up with a plagioclase similar to the original liquid because ions are exchanged back and forth between the growing crystals and the molten liquid.

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If cooling is fast, ions cannot leave or enter the crystal in time to keep adjusting the plagioclase composition.

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The result is concentric zoning.

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(This also happens in rapidly cooled olivines and pyroxenes, the Mg-rich variety crystallizes at a higher temperature than the Fe-rich variety because Mg-O bond is shorter, stronger).

Structure

• Al/Si order disorder
• Depending on distribution of Si and Al in T1 and T2 sites lead to the distinction among different order-disorder polymorphs.

Figure 12.6 Idealized feldspar structure: T₁ and T₂ refer to different tetrahedral sites.

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 12.7 Ordering of Al and Si in tetrahedral sites in the feldspars.

Al/Si Order-Disorder

Al: Si ratio = 1:3 – present in Alkali feldspars

• Mode I: Al and Si randomly distributed in the tetrahedral sites. 25% of sites occupied by Al.
• Mode II ( Orthoclase): Al are only in T1 sites occupying ½ of each T1 site, none in T2 sites.
• Mode III (Low Microcline) : All Al are only in one of the T1 sites: highest degree of ordering.

Al:Si ratio 2:2 (Anorthite):

• Mode IV: Al and Si occupy alternate sites.

Thermal implication of 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. Must have quenched rapidly.
• Mode II ( Orthoclase): Al are only in T1 sites occupying ½ of each T1 site, none in T2 sites. Real Orthoclase is a mixture of Mode II and Mode III.
• Mode III (Microcline) : All Al are only in one of the T1 sites: highest degree of ordering. Seen in slowly cooled igneous or metamorphic rocks.

Thermal implication of order-disorder

Al: Si ratio = 1:3 – in albitic plagioclase

• Mode I (Monalbite) : Al and Si randomly distributed in the tetrahedral sites. 25% of sites occupied by Al. Forms at high temp. like sanidine. Must have quenched rapidly.

Monoclinic

Triclinic

Analbite (triclinic) shows lower symmetry than Monalbite.

Reason for change in structure- 1) Al-Si ordering 2) Smaller Na can prop the site (which can house K) at high T, but structure collapses with ordering

• Mode II-III (tricilinic-high albite/low albite) : Seen in slowly cooled igneous or metamorphic rocks. High albite is less ordered compare to low albite

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• Al:Si ratio 2:2 (Anorthite):
• Mode IV: Al and Si occupy alternate sites
• Al avoidance rule-mode IV ordering Al and Si tend to avoid having Al in adjacent tetrahedra
• Al avoidance rule become a problem for intermediate composition

Ab (mode III)

An (mode IV)

• So forms submicroscopic domains Na rich and Ca rich Plag- ‘e’ Plag

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EXSOLUTION

Exsolution is a chemical unmixing of the solid crystal during cooling. It occurs by diffusion of ions within the solid (but still hot) crystal, because the mineral structure readjusts and contracts during cooling. It becomes less tolerant of impurities that have the wrong size.

Exsolution in alkali feldspar

• Na+ and K+ has significantly different ionic radii (1.32 vs.1.62 A), on cooling with ordering acceptance of Na permissibility in K-feldspar and K permissibility in Na-feldspar decreases. EXSOLUTION results-Perthite, Antiperthite
• Exsolution development depens on rate of cooling
• Slower cooling larger lamellae
• Microcline will develop visible exsolution lamellae than sanidine/orthoclase- True or False?

Moonstone-Pale blue iridescence because of interaction of light with submicroscopic exsolution lamellae

Exsolution in Plag

• Why not common?
• Doesn’t depend on ion size but order disorder
• Peristerite-composition An2 to An17, one lamellae will be Ab (mode III) another will be ‘e’ Pl (An20-An25)
• Boggild intergrowth-composition An46 to An60, one lamellae will be ‘e’ Pl
• Huttenlocher intergrowth-composition An65 to An85, one lamellae will be will be ‘e’ Pl (An20-An25) another An (mode IV)

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Other Intergrowth

• Myrmekite: worm-like quartz in sodic plagioclase. Forms at the contact of K-Feldspar and Plagioclase in granitic rocks

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Graphic intergrowth

• Graphic : cuneiform quartz in perthitic K-feldspar. Forms when the two crystallize simultaneously.

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• Rapakivi: sodic plagioclase mantles K- Feldspar. Antirapakivi – the opposite.

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Alkali Feldspar

Rock types

Occurence of Alkali feldspar

• Igneous-Acidic igneous rocks, Pegmatite,

Less common in basic rocks can occur in ultrapotassic basic rocks

Volcanic-Sanidine, Plutonic-Microcline

• Sedimentary-Found as detrital grain, authigneic mineral, commonly convert to clay mineral by hydration
• Metamorphic-Metaacid igneous rocks

Found in metapelite at very high grade as Mus and Bt breaks down

Plagioclase

Rock types

Occurence of Plagioclase

• Igneous-basic and intermediate rocks (Anorthosite), Na fldspr dominates in acidic rock also.
• Metamorphic-metabasic rocks (Na rich in lower grade, Ca rich in higher grade)

Not stable in eclogite (metabasalt in composition)

metaacidic to intermediate rocks (e.g. Charnockite series) contain Na rich Plag

Metamorphosed impure calcsilicate

sodic plag sometime found in metapelite

• Sedimentary-common authigenic mineral

Hydrous Tectosilicates

• Analcime (Zeolite Gp) NaAlSi₂O₆·H₂O

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• Natrolite (Zeolite Gp) Na₂Al₂Si₃O₁₀·2H₂O

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• Heulandite (Zeolite Gp) CaAl₂Si₇O₁₈·6H₂O

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• Stilbite (Zeolite Gp) NaCa₂Al₅Si₁₃O₃₆·14H₂O

Feldspathoids (Si-poor feldspars)

Common in alkaline (Si-undersaturated) igneous rocks

Leucite – KAlSiO₄

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Nepheline – (Na,K)AlSiO₄

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Sodalite – Na₈(AlSiO₄)₆Cl₂

Feldspathoid

• Orthoclase- https://www.youtube.com/watch?v=FLII1L68gWI&list=PLB8NRY_JSTcLsqUyZSnm-CFPbjjKMzL6j

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• Plagioclase

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https://www.youtube.com/watch?v=1OdYmVq9r2g&index=2&list=PLB8NRY_JSTcLsqUyZSnm-CFPbjjKMzL6j

• Microcline

https://www.youtube.com/watch?v=qp6cgtWoFTg&list=PLB8NRY_JSTcLsqUyZSnm-CFPbjjKMzL6j&index=10

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• Quartz

https://www.youtube.com/watch?v=r0RPz0i7Rww&list=PLB8NRY_JSTcLsqUyZSnm-CFPbjjKMzL6j&index=7