TECTOSILICATES - 3D Framework

TECTOSILICATES - 3D Framework

64% of the crust is made up of Tectosilicates (p. 457)

SiO2 GROUP (p. 475)

STABILITY RELATIONS OF SiO2 POLYMORPHS P. 478

QUARTZ - hexagonal 32

·  Occurs in all geologic environments (except marbles and basalts with olivine)

·  Microcrystalline varieties

Chalcedony fibrous, from aqueous fluids (carnelian, chrysoprase, agate=banded, onyx)

·  Flint and chert – granular (jasper)

Use: gems (amethyst, agate, citrine, tigers eye = pseudomorph after crocidolite, flux, abrasive, sand, optics, radios)

Tridymite - orthorhomic to hexagonal

·  Occurs in high-Temperature volcanic rocks: rhyolite, obsidian, andesite with sanidine and cristobalite

·  In stony meteorites and lunar basalts

Cristobalite - tetragonal to isometric (high temperature)

·  Occurs in siliceous volcanic rocks

Coesite / stishovite

·  In impact craters

Opal - SiO2.nH2O

·  Amorphous

·  Deposited by hot springs, meteoric waters or low temperature hypogene solutions

·  Gems

FELDSPAR GROUP P. 478

KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8

Orthoclase albite anorthite

Microcline

Sanidine

BaAl2Si2O8

(celsian / hyalophane)

Alkali feldspar structure

·  structure depends on temperature of crystallization and cooling rate p 481

Sanidine - high T structure; monoclinic 2/m

·  T1 and T2 sites are completely disordered

·  K+ is 10 co-ordinated

·  Complete solid solution with high albite (high temperature)

(K,Na)AlSi3O8

·  Occurs as phenocrysts in extrusive igneous rocks (rhyolites)

_

Microcline - KAlSi3O8 triclinic 1

·  In granites, syenites, pegmatites, gneisses, arkose

·  Distinguished by “tartan” twinning in thin section

·  Porcelain, gems

Orthoclase - KAlSi3O8 monoclinic 2/m

·  In granites, granodiorites, syenites

Adularia - KAlSi3O8 monoclinic-triclinic

·  Hydrothermal mineral *datable

·  K-feldspar are frequently twinned

PLAGIOCLASE FELDSPARS - essentially complete Solid Solution series

Triclinic T

·  Albite - NaAlSi3O8 with alkali feldspar in granites, rhyolites, syenites, pegmatites (variety cleavelandite)

·  Oligoclase - granodiorites, monzonites, quartz monzonites

·  Andesine - diorites, andesites

·  Labradorite - gabbros, basalts, anorthosites

·  Bytownite - rarer, in gabbros , in lunar rocks

·  Anorthite - CaAl2Si2O8 in basalts, lunar rocks, metamorphosed limestones

There is very limited S.S between CaAl2Si2O8 and KalSi3O8; at high temperatures albite – K-spar Solid solution is incomplete

Feldspathoids - anhydrous framework silicates chemically similar to feldspars, but with only 2/3 the amount of silica (great structure, less filling?)

Form from igneous melts rich in alkalis (Na+K) and poor in silica

Fig. 11.57

Nepheline (Na,K)AlSiO4 hexagonal 6

·  Structure is similar to tridymite, but every other ring is squashed

·  T1 + T4 – Al T2 + T3 - Si

·  K is 9 co-ord; Na is 7 or 8 co-ord

·  Syenites

·  Good source rock for bauxites (Al!) because of low Si content

·  Cancrinite Na6Ca(CO3)-(AlSiO4)6.2H2O is a rare mineral with similar occurrence and associations

_

Sodalite Na8(AlSiO4)6Cl2 isometric 4 3m

·  Large cavities in structure can accommodate Na+ or Cl-

·  Other unusual feldspathoids contain:

CO3 cancrinite Na6Ca(CO3)(AlSiO4)6.2H2O

Cl, S, SO4 lazurite (Na,Ca)8(AlSiO4)(SO4,S,Cl)2

Cl, S, SO4 noselite Na8(AlSiO4)6SO4

Leucite KAlSi2O6 tetragonal 4/m

·  K+ is 12 co-ord

·  Occurs as phenocrysts in Si-undersaturated lavas

Petalite Li(AlSi4O10) monoclinic 2/m

·  In pegmatites with spodumene, tourmaline, lepidolite

·  Ore of Li

Scapolites Tetragonal 4/m

Marialite Meionite

3NaAlSi3O8.NaCl « 3CaAl2Si2O8.CaSO4 (or CaCO3)

framework of SiO4 and AlO4 tetrahedra with large cavities containing the (Ca,Na) ions and (CO3,Cl2,SO4) anionic groups

·  Occurrence usually derived by alteration of plagioclase feldspar in schists, gniesses, amphibolites and granulites

·  Also in metamorphosed limestone with diopside, garnet, tremolite metamorphosed evaporates!

· 

Analcime NaAlSi2O6.H2O isometric 4/m 3 2/m

·  usually trapezohedrons

·  hydrothermal alteration of basaltic cavities

Zeolite Group

Hydrated aluminosilicates with principal cations (K, Na, Ca) in large voids of the framework

~46 different minerals with different densities of structures

most common

Natrolite Na2Al2Si3O10.2H2O fibrous

Chabazite Ca2Al2Si4O12.6H2O platy equant

Heulandite CaAl2Si7O18.6H2O platy

Stilbite CaAl2Si7O18.7H2O fibrous

Occurrences in cavities in basalts with calcite

Low T metamorphism of mafic rocks (zeolite facies)

Use: molecular sieves

Cation exchange properties (water softeners)

Ca2+ (water) « Na (zeolite)