Framework Silicates 2/3 of crust is framework silicates 2/3 of crust is framework silicates Quartz and feldspars are most common Quartz and feldspars are most common All with similar structures All with similar structures TO4 tetrahedral framework TO4 tetrahedral framework T = Si or Al T = Si or Al Each oxygen is shared with another tetrahedron Each oxygen is shared with another tetrahedron Sharing of highly charged cation (Si 4+ ) make open structures Sharing of highly charged cation (Si 4+ ) make open structures

Consequences of open framework Compostional Compostional Accommodate large cations – Ca, Na, and K Accommodate large cations – Ca, Na, and K Charge balance maintained by exchange of Al and Si Charge balance maintained by exchange of Al and Si

Physical properties Physical properties Specific gravity lower than most other minerals Specific gravity lower than most other minerals E.g. quartz = 2.65, olivine = 3.27 E.g. quartz = 2.65, olivine = 3.27 Not stable at high P Not stable at high P Restricted to crust Restricted to crust

Four major groups: Four major groups: Silica group (SiO2) Silica group (SiO2) Feldspars Feldspars Feldspathoids Feldspathoids Zeolites Zeolites

Silica group Silica group Eight possible polymorphs Eight possible polymorphs one other synthetic variety one other synthetic variety Rare forms – High P Rare forms – High P Stishovite – Si in octahedral coordination Stishovite – Si in octahedral coordination Coesite Coesite Common forms Common forms Quartz,  Quartz,  Tridymite,  Tridymite,  Cristobalite,  Cristobalite, 

Fig. 12-1

Structures Reconstructive polymorphism between qtz, tridymite and cristobalite Reconstructive polymorphism between qtz, tridymite and cristobalite Displacive polymorphism between  and  varieties Displacive polymorphism between  and  varieties Tridymite and Cristobalite not stable at surface P and T Tridymite and Cristobalite not stable at surface P and T Will convert to quatz Will convert to quatz

Varieties of quartz Microcrystalline varieties Microcrystalline varieties Chert – occurs as nodules or beds in limestone Chert – occurs as nodules or beds in limestone Black version – flint Black version – flint Red version – jasper Red version – jasper Chalcedony – fibrous microcrystalline quartz Chalcedony – fibrous microcrystalline quartz Color bands or irregular color called agate Color bands or irregular color called agate

Coarsely crystalline varieties Amethyst – biolet or purple from trace amounts of Fe Amethyst – biolet or purple from trace amounts of Fe Rose quartz – pink colored, may be caused by mineral inclusions Rose quartz – pink colored, may be caused by mineral inclusions Citrine – yellow quartz from Fe, radiation or combination Citrine – yellow quartz from Fe, radiation or combination Smoky quartz – irradiation and small amounts of Al Smoky quartz – irradiation and small amounts of Al Milky quartz – minute fluid inclusions Milky quartz – minute fluid inclusions

Feldspar Group Three compositional end members Three compositional end members K-feldspar (KAlSi 3 O 8 ) – Ks or Or K-feldspar (KAlSi 3 O 8 ) – Ks or Or Sanidine Sanidine Orthoclase Orthoclase Microcline Microcline Albite (NaAlSi 3 O 8 ) – Ab Albite (NaAlSi 3 O 8 ) – Ab Anorthite (CaAl 2 Si 2 O 8 ) – An Anorthite (CaAl 2 Si 2 O 8 ) – An Plagioclase – An and Ab Plagioclase – An and Ab Alkali – Ab and Ks (Or) Alkali – Ab and Ks (Or)

Plagioclase feldspars At high T, continuous solid solution At high T, continuous solid solution CaAl substitutes for NaSi CaAl substitutes for NaSi Generally described as fraction of An, assuming An + Ab = 100% Generally described as fraction of An, assuming An + Ab = 100%

Compositions divided into ranges and given names Compositions divided into ranges and given names Albite: An0 – An10 Albite: An0 – An10 Oligoclase: An10 – An30 Oligoclase: An10 – An30 Andesine: An30 – An50 Andesine: An30 – An50 Labradorite: An50 – An70 Labradorite: An50 – An70 Bytownite: An70 – An90 Bytownite: An70 – An90 Anorthite: An90 – An100 Anorthite: An90 – An100

Alkali feldspars Continuous solid solution at high T Continuous solid solution at high T K and Na same charge – compensates for difference in size K and Na same charge – compensates for difference in size K and Ca don’t have solid solution because different size and charge K and Ca don’t have solid solution because different size and charge

Fig Names of compositional variations caused by solid solution in feldspar minerals Influence of temperature on the allowed solid solution of feldspar minerals

Fig Origin of miscibility gap – variations in solid solutions

Feldspathoids Similar to feldspars Similar to feldspars Common minerals Common minerals Nepheline Na 3 K(Al 4 Si 4 O 16 ) Nepheline Na 3 K(Al 4 Si 4 O 16 ) Leucite KAlSi 2 O 6 Leucite KAlSi 2 O 6 Sodalite group Na 8 Al 6 Si 6 O 24 Cl 2 Sodalite group Na 8 Al 6 Si 6 O 24 Cl 2 Nepheline is the most common Nepheline is the most common

Chemically different from feldspars Chemically different from feldspars Less Si relative to Na and K Less 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 rocks Found in alkali-rich, silica poor igneous rocks

Structurally similar to feldspars Structurally similar to feldspars 4 and 6 member rings 4 and 6 member rings Linked to form framework Linked to form framework More open than feldspars More open than feldspars Lower specific gravity than feldspars Lower specific gravity than feldspars

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

Zeolites Very common group of minerals Very common group of minerals Over 40 naturally occurring varieties Over 40 naturally occurring varieties Over 600 synthetic ones Over 600 synthetic ones Largest single group of silicate minerals Largest single group of silicate minerals Most commonly alteration products of basaltic and andasitic volcanic rocks Most commonly alteration products of basaltic and andasitic volcanic rocks

Commonly too fine grained for identification from physical properties Commonly too fine grained for identification from physical properties Requires x-ray diffraction Requires x-ray diffraction Often considered “clay minerals’ Often considered “clay minerals’ Tectosilicates, not phyllosilicates Tectosilicates, not phyllosilicates

Composition Hydrated framework silicates Hydrated framework silicates General formula General formula M x D y (Al x +2ySi n-x-2y O 2n ) MH 2 0 M x D y (Al x +2ySi n-x-2y O 2n ) MH 2 0 Si/Al ratio varies from 1 to 6 Si/Al ratio varies from 1 to 6 M usually monovalent Na or K M usually monovalent Na or K D usually divalent Ca, Mg or others D usually divalent Ca, Mg or others

Structure Structure Open framework of Al/Si tetrahedral Open framework of Al/Si tetrahedral Link to form open channels and voids Link to form open channels and voids Geometry varies from one to the other Geometry varies from one to the other Water and cations often in voids and weakly bonded Water and cations often in voids and weakly bonded Create important properties of minerals Create important properties of minerals

Fig Heulandite

Occurrence All but analcime are secondary All but analcime are secondary Analcime may be a primary igneous mineral, late crystallization in basalts Analcime may be a primary igneous mineral, late crystallization in basalts Can be used as low-T geothermometers Can be used as low-T geothermometers

Variety of Uses Desiccants: hydrated, but water easily exchanges so can dessicate gasses such as CO 2, freon, and organic chemicals Desiccants: hydrated, but water easily exchanges so can dessicate gasses such as CO 2, freon, and organic chemicals Molecular sieves: if dehydrated, other molecules fill voids, e.g. separate N from O, purify O Molecular sieves: if dehydrated, other molecules fill voids, e.g. separate N from O, purify O Water softener: Na-rich zeolites will remove Ca from water and replace with Na Water softener: Na-rich zeolites will remove Ca from water and replace with Na

Water purification: heavy metals in acid mine drainage, isotopes from radioactive waste, contaminated soils, remove NH 4 from wastewater and cat litter Water purification: heavy metals in acid mine drainage, isotopes from radioactive waste, contaminated soils, remove NH 4 from wastewater and cat litter Soil conditioner: agriculture for water and cations, slow release of N, carrier of pesticides Soil conditioner: agriculture for water and cations, slow release of N, carrier of pesticides

Feed: pigs, cattle, chicken, turkeys, improve feed efficiency, reduce waste smell, increase N retention Feed: pigs, cattle, chicken, turkeys, improve feed efficiency, reduce waste smell, increase N retention Petroleum refining, cleaning spills, filters in paper processing Petroleum refining, cleaning spills, filters in paper processing

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