• two thirds of crust is framework silicates
• Quartz and feldspars are most common
• All will similar structures
- TO4 tetrahedral framework
- T = Si or Al
- each oxygen is shared with another tetrahedral
- sharing of highly charged cations make structures very open

 
• Consequences of open framework
- compositional: accommodate large cations, Ca, Na, and K
* charge balance maintained generally by exchange Si for Al
- physical properties: specific gravity lower than most other minerals
* e.g. quartz = 2.65, forsterite = 3.27
* not stable at high P
* restricted to crust
 
• Four major groups:
- Silica group (SiO₂)
- Feldspars
- Feldspathoids
- Zeolites

 
Silica Group
• eight natural polymorphs, and other synthetic varieties
• Rare forms – High P varieties:
- stishovite
- coesite
• Most common:
- quartz
- tridymite
- cristobalite
 
• Structures
- quartz, tridymite, and cristobalite are all distinct
- reconstructive polymorphs
- each has two varieties a (low) and b (high)
- two varieties are displacive polymorphs
- b polymorph stable at higher T and has higher symmetry than a

• a-b transitions are not quenchable
- b variety never found at room temperature
- a variety are never stable at any P and T conditions
- they represent original crystallization of b form and conversion during cooling
 
Quartz

• structure: interlocking spirals of tetrahedron
- spirals may be right or left handed, entantiomorphic pair
- b-quartz is hexagonal
- cooling kinks a-quartz

• composition: relatively pure SiO₂
- some subsitution of Fe³⁺ and Al³⁺
- charge balance with monovalent cations
- creates numerous varieties

 
• Varieties:

(1) Microcrystalline varieties
- Chert –occurs as nodules or beds in limestone, black called flint, red called jasper
- Chalcedony – firbrous microcrystalline quartz, color bands or irregular color called agate

 
(2) Coarsely crystalline varieties
- amethyst – Violet or purple from trace amounts of Fe
- Rose quartz – pink colored, may be caused by mineral inclusion
- citrine – yellow quartz from Fe, radiation or combination
- Smoky quartz – irradiation with minutes about of Al
- Milky quartz – from minute fluid inclusions

 
• Occurrence - common
- igneous rocks: felsic to intermediate volcanics and intrusives, e.g. granitic pegmatites, granite, granodiorite, quartz diorite, rhyolite, rhyodacite, dacite, and small amounts in diorite, gabbro, syenite, and their volcanic equivalents
- metamorphic rocks: slate phyllite, schist, gneiss, and quartzite
- sedimentary rocks: most clastic sedimentary rocks, both as detrital grains and cements
- Hydrothermal veins and replacement deposits
 
Feldspar Group

• Three principal compositional end members
- K-feldspar (KAlSi₃O₈) – Ks (or Or)
- albite (NaAlSi₃O₈) – Ab
- anorthite (CaAl₂Si₂O₈) – An

• Plagioclase feldspars are An and Ab

• Alkali feldspars are Ab and Ks (Or)

 
• Plagioclase feldspars
- at high T, continuous solid solution
- CaAl substitutes for NaSi
- Generally described as fraction of An, assuming An + Ab = 100%
- Composition divided into ranges and given names:

Albite:   An₀ – An₁₀
Oligoclase:  An₁₀ – An₃₀
Andesine:   An₃₀ – An₅₀
Labradorite:  An₅₀ – An₇₀
Bytownite:   An₇₀ – An₉₀
Anorthite:   An₉₀ – An₁₀₀

 
• Alkali feldspars
- also continuous solid solution
- K and Na same charge, which compensates for difference in size
- K and Ca don’t have solid solution because different size and charge
 
• Structure
- 4 member rings
- linked to each other to form chains by sharing oxygen
- chains are linked to each other by sharing oxygens
- remaining oxygen have 9-fold coordination with large cations

 
• Al/Si order-disorder polymorphism
- controlled by distribution of Al and Si in the 4-member rings
- 4 different modes of ordering, depending on Al/Si ratio:
(1) Al/Si = 1/3
* Al and Si randomly distributed
* Al preferentially located in half of tetrahedral sites
* Al restricted to one tetrahedral site
(2) Al/Si – 2/2
* Al and Si occupy alternating tetrahedral sites
 
• K-spars
- three polymorphs: sanidine, orthoclase, and microcline
- sanidine: complete disorder, 25% chance of Al in any Si site, plane of symmetry present, monoclinic
- microcline: complete order, only one of the T1 site has Al; reduces symmetry to triclinic; creates twins
- Orthoclase: intermediate; created as Al diffuses out of T2 sites into T1 sites; preferentially enters to the T1o site

 
• Significance – record of thermal history of rocks
- Sanidine-bearing rocks were quenched rapidly
- microcline with substantial twinning must have cooled rapidly
- easily to determine degree of ordering using x-ray diffraction

 
• Plagioclase order/disorder phenomenon
- depends on composition

• Na plagioclase similar to K-spars
- conversions simpler than with K-spars, so typically disorder structure found only in volcanic rocks
- most commonly well ordered, triclinic albite in plutonic and metamorphic rocks (slow cooling).

 
• Ca-plagioclase
- ordering so that Al equally distributed in tetrahedral sites
- Distribution of alternating Al in difference sites

 
• Exsolution – K spars

- Na (1.32 Å) and K (1.65 Å) different ionic radii
- complete solid solution at high T, immiscible at low T
- perthite: albite lamellae in K-spar
- antiperthite: K-spar lamellae in albite
- extent and size of lamellae controlled by cooling
- sanidine and orthoclase may have perthitic texture
- microcline cools slowly, may have perthitic texture observable in hand sample
 
• Exsolution - Plagioclase
- controlled by order-disorder rather than size
- Al – Si ordering controls exsolution
- generally too small to be seen in hand sample or optically
- generally several hundred nanometer, about wavelength of light
- creates iridescent, called labradorescence
 
• Other common intergrowths
- Myrmekite – wormlike quartz in Na-plagioclase
- Granophyric – irregular quartz intergrowths in K-spar or Na-plag
- Graphic – large crystals of perthitic K-spar enclose smaller volumes of quartz

• Twinning
- about 20 different twin laws
- albite and pericline twin laws very common in triclinic feldspars, polysynthetic twins
- Carlsbad, Baveno, and Manebach common single twins

 
Feldspathoids
• similar to feldspars
• common minerals
- nepheline Na₃K(Al₄Si₄O₁₆)
- leucite KAlSi₂O₆
- sodalite group Na₈Al₆Si₆O₂₄Cl₂
- nepheline is the most common

 
• chemically distinct from feldspars by having less Si relative to Na and K
- rarely found in association with quartz
- typically occur in alkali-rich, silica-poor igneous rocks

• Structurally similar to feldspars
- 4 and 6 member rings
- linked to form framework
- more open than feldspars, lower specific gravity than feldspars
 
Zeolites

• Very common group of minerals
• Over 40 naturally occurring varieties, over 600 synthetic ones
• largest single group of silicate minerals
• Most commonly seen in vesicles of basaltic and andesitic volcanic rocks
• commonly too fine grained for identification on basis of physical properties
- requires x-ray diffraction
- often considered “clay minerals”, not really

 
• Composition
- hydrated framework silicates
- general formula:
M_xD_y(Al_x+2ySi_n-x-2yO_2n)•mH₂O
- Si/Al ratio varies from 1 to 6
- M usually monovalent Na or K
- D usually divalent Ca, Mg or others
 
• Structure
- open framework of Al/Si tetrahedral
- link to form open channels and voids
- geometry varies from one to the other
- water and cations often in voids and weakly bonded
- create important properties of minerals

 
• Occurrence
- all but analcime are secondary
- analcime may be a primary igneous mineral, late crystallization in basalts
- Environments of formation (increasing depth of burial):
* Weathering with high pH
* diagenesis of ash, lakes and marine
* alteration from ground water
* Hydrothermal alteration
* contact metamorphism
* burial and low grade regional metamorphism
- low-T geothermometers
 
• Uses – very important group (thus synthetic ones)
- Desiccants: hydrated, but water easily exchanges so can dessicate gasses such as CO₂, freon, and organic chemicals
- molecular sieves: if dehydrated, other molecules fill voids, eg. Separate N from O, purify O
- 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 NH4 from wastewater and cat litter
- 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
- Petroleum refining, cleaning spills, filters in paper processing