Framework SilicatesFramework Silicates 2/3 of crust is framework silicates2/3 of crust is framework silicates Quartz and feldspars are most commonQuartz and feldspars are most common All with similar structuresAll with similar structures

TO4 tetrahedral frameworkTO4 tetrahedral framework T = Si or AlT = Si or Al Each oxygen is shared with another Each oxygen is shared with another

tetrahedrontetrahedron Sharing of highly charged cation (SiSharing of highly charged cation (Si4+4+) make ) make

open structuresopen structures

Consequences of open Consequences of open frameworkframework

CompostionalCompostional Accommodate large cations – Ca, Na, Accommodate large cations – Ca, Na,

and Kand K Charge balance maintained by Charge balance maintained by

exchange of Al and Siexchange of Al and Si

Physical propertiesPhysical properties Specific gravity lower than most other Specific gravity lower than most other

mineralsminerals E.g. quartz = 2.65, olivine = 3.27E.g. quartz = 2.65, olivine = 3.27 Not stable at high PNot stable at high P Restricted to crustRestricted to crust

Four major groups:Four major groups: Silica group (SiO2)Silica group (SiO2) FeldsparsFeldspars FeldspathoidsFeldspathoids ZeolitesZeolites

Silica groupSilica group Eight possible polymorphsEight possible polymorphs one other synthetic varietyone other synthetic variety

Rare forms – High PRare forms – High P Stishovite – Si in octahedral coordinationStishovite – Si in octahedral coordination CoesiteCoesite

Common formsCommon forms Quartz, Quartz, Tridymite, Tridymite, Cristobalite, Cristobalite,

Fig. 12-1Fig. 12-1

StructuresStructures

Reconstructive polymorphism Reconstructive polymorphism between qtz, tridymite and between qtz, tridymite and cristobalitecristobalite

Displacive polymorphism between Displacive polymorphism between and and varieties varieties

Tridymite and Cristobalite not stable Tridymite and Cristobalite not stable at surface P and Tat surface P and T Will convert to quatzWill convert to quatz

Varieties of quartzVarieties of quartz

Microcrystalline varietiesMicrocrystalline varieties Chert – occurs as nodules or beds in Chert – occurs as nodules or beds in

limestonelimestone Black version – flintBlack version – flint Red version – jasperRed version – jasper

Chalcedony – fibrous microcrystalline Chalcedony – fibrous microcrystalline quartzquartz

Color bands or irregular color called agateColor bands or irregular color called agate

Coarsely crystalline Coarsely crystalline varietiesvarieties

Amethyst – biolet or purple from trace Amethyst – biolet or purple from trace amounts of Feamounts of Fe

Rose quartz – pink colored, may be caused Rose quartz – pink colored, may be caused by mineral inclusionsby mineral inclusions

Citrine – yellow quartz from Fe, radiation or Citrine – yellow quartz from Fe, radiation or combinationcombination

Smoky quartz – irradiation and small Smoky quartz – irradiation and small amounts of Alamounts of Al

Milky quartz – minute fluid inclusionsMilky quartz – minute fluid inclusions

Feldspar GroupFeldspar Group

Three compositional end membersThree compositional end members K-feldspar (KAlSiK-feldspar (KAlSi33OO88) – Ks or Or) – Ks or Or

SanidineSanidine OrthoclaseOrthoclase MicroclineMicrocline

Albite (NaAlSiAlbite (NaAlSi33OO88) – Ab) – Ab Anorthite (CaAlAnorthite (CaAl22SiSi22OO88) – An) – An

Plagioclase – An and AbPlagioclase – An and Ab Alkali – Ab and Ks (Or)Alkali – Ab and Ks (Or)

Plagioclase feldsparsPlagioclase feldspars

At high T, continuous solid solutionAt high T, continuous solid solution CaAl substitutes for NaSiCaAl substitutes for NaSi Generally described as fraction of An, Generally described as fraction of An,

assuming An + Ab = 100%assuming An + Ab = 100%

Compositions divided into ranges and Compositions divided into ranges and given namesgiven names Albite: An0 – An10Albite: An0 – An10 Oligoclase: An10 – An30Oligoclase: An10 – An30 Andesine: An30 – An50Andesine: An30 – An50 Labradorite: An50 – An70Labradorite: An50 – An70 Bytownite: An70 – An90Bytownite: An70 – An90 Anorthite: An90 – An100Anorthite: An90 – An100

Alkali feldsparsAlkali feldspars

Continuous solid solution at high TContinuous solid solution at high T K and Na same charge – K and Na same charge –

compensates for difference in sizecompensates for difference in size K and Ca don’t have solid solution K and Ca don’t have solid solution

because different size and chargebecause different size and charge

Fig. 12-5Fig. 12-5

Names of compositional variations caused by solid solution in feldspar minerals

Influence of temperature on the allowed solid solution of feldspar minerals

Fig. 5-23Fig. 5-23

Origin of miscibility gap – variations in solid solutions

FeldspathoidsFeldspathoids

Similar to feldsparsSimilar to feldspars Common mineralsCommon minerals

Nepheline NaNepheline Na33K(AlK(Al44SiSi44OO1616)) Leucite KAlSiLeucite KAlSi22OO66

Sodalite group NaSodalite group Na88AlAl66SiSi66OO2424ClCl22 Nepheline is the most commonNepheline is the most common

Chemically different from feldsparsChemically different from feldspars Less Si relative to Na and KLess Si relative to Na and K Rarely found with quartz (too much Si)Rarely found with quartz (too much Si) Found in alkali-rich, silica poor igneous Found in alkali-rich, silica poor igneous

rocksrocks

Structurally similar to feldsparsStructurally similar to feldspars 4 and 6 member rings4 and 6 member rings Linked to form frameworkLinked to form framework More open than feldsparsMore open than feldspars Lower specific gravity than feldsparsLower specific gravity than feldspars

Feldspathoid minerals compared to feldspars minerals Shaded regions are allowed solid solution

ZeolitesZeolites

Very common group of mineralsVery common group of minerals Over 40 naturally occurring varietiesOver 40 naturally occurring varieties Over 600 synthetic onesOver 600 synthetic ones Largest single group of silicate Largest single group of silicate

mineralsminerals Most commonly alteration products of Most commonly alteration products of

basaltic and andasitic volcanic rocksbasaltic and andasitic volcanic rocks

Commonly too fine grained for Commonly too fine grained for identification from physical identification from physical propertiesproperties Requires x-ray diffractionRequires x-ray diffraction Often considered “clay minerals’Often considered “clay minerals’

Tectosilicates, not phyllosilicatesTectosilicates, not phyllosilicates

CompositionComposition

Hydrated framework silicatesHydrated framework silicates General formulaGeneral formula

MMxxDDyy(Al(Alxx+2ySi+2ySin-x-2yn-x-2yOO2n2n) MH) MH2200 Si/Al ratio varies from 1 to 6Si/Al ratio varies from 1 to 6 M usually monovalent Na or KM usually monovalent Na or K D usually divalent Ca, Mg or othersD usually divalent Ca, Mg or others

StructureStructure Open framework of Al/Si tetrahedralOpen framework of Al/Si tetrahedral Link to form open channels and voidsLink to form open channels and voids Geometry varies from one to the otherGeometry varies from one to the other Water and cations often in voids and Water and cations often in voids and

weakly bondedweakly bonded Create important properties of mineralsCreate important properties of minerals

Fig. 12-22Fig. 12-22

Heulandite

OccurrenceOccurrence

All but analcime are secondaryAll but analcime are secondary Analcime may be a primary igneous Analcime may be a primary igneous

mineral, late crystallization in basaltsmineral, late crystallization in basalts Can be used as low-T Can be used as low-T

geothermometersgeothermometers

Variety of UsesVariety of Uses

Desiccants: hydrated, but water easily Desiccants: hydrated, but water easily exchanges so can dessicate gasses such exchanges so can dessicate gasses such as COas CO22, freon, and organic chemicals, freon, and organic chemicals

Molecular sieves: if dehydrated, other Molecular sieves: if dehydrated, other molecules fill voids, e.g. separate N from molecules fill voids, e.g. separate N from O, purify OO, purify O

Water softener: Na-rich zeolites will Water softener: Na-rich zeolites will remove Ca from water and replace with remove Ca from water and replace with NaNa

Water purification: heavy metals in Water purification: heavy metals in acid mine drainage, isotopes from acid mine drainage, isotopes from radioactive waste, contaminated radioactive waste, contaminated soils, remove NHsoils, remove NH44 from wastewater from wastewater and cat litterand cat litter

Soil conditioner: agriculture for water Soil conditioner: agriculture for water and cations, slow release of N, carrier and cations, slow release of N, carrier of pesticidesof pesticides

Feed: pigs, cattle, chicken, turkeys, Feed: pigs, cattle, chicken, turkeys, improve feed efficiency, reduce improve feed efficiency, reduce waste smell, increase N retentionwaste smell, increase N retention

Petroleum refining, cleaning spills, Petroleum refining, cleaning spills, filters in paper processingfilters in paper processing

Environments of formation (increasing Environments of formation (increasing depth of burial)depth of burial) Weathering with high pHWeathering with high pH Diagenesis of ash, lakes and marineDiagenesis of ash, lakes and marine Alteration from ground waterAlteration from ground water Hydrothermal alterationHydrothermal alteration Contact metamorphismContact metamorphism Burial and low grade regional Burial and low grade regional

metamorphismmetamorphism