Download Inosilicates (Chain silicates)

Inosilicates (Chain silicates)
 Common Fe/Mg-bearing silicates
 two common groups:
- pyroxenes: single chain
- amphiboles: double chain
 pyroxenes more common because of MORB
 amphiboles more common on continents because of weathering
Pyroxene Group – XYZ2O6
 Single chains extend along c axis
 Z/O ratio = 1/3
 Z cations usually Si, occasionally Al
 Chains are stacked along a axis, alternating:
- base faces base
- apex faces apex
 X cations in M2 sites
- between bases of tetrahedrons
- distorted 6- and 8- fold coordination
- depends stacking and the size of the cations
 Y cations in M1 sites
- 6-fold coordination between apical oxygen
 “I-beams”
- consist of two chains connected by Y cations
- closeness of apical oxygen and 6-fold coordination make bonds strong
 I-beams held together by X cations in M2 site
- coordination depends on how chains line up
- 6-fold coordination gives orthorhombic symmetry
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- 8-fold coordination gives monoclinic symmetry
 Crystal shapes:
- blocky prisms, nearly square
- elongate along c axis
 cleavage controlled by I-beams:
- cleavage typically between 87º and 93º
- viewed down the c axis
Classification of pyroxenes
 Based on two linked things:
- cations in M2 site
- symmetry
 Most plot on ternary diagram, apices:
- Wollastonite, Wo
- Enstatite, En
- Ferrosilite, Fe
 Three major groups
- orthopyroxenes (opx) – orthorhombic
- low-Ca orthopyroxenes (opx) – orthorhombic
- Ca-rich clinopyroxenes (cpx) – monoclinic
 Orthopyroxenes: Fe and Mg, little Ca
- both M1 and M2 are octahedral (small)
- larger Fe ion fits mostly in M2 site (larger)
 Low-Ca orthopyroxene: more Ca, but no solid solution with clinopyroxene
- mineral species is Pigeonite
- Ca restricted to M2 sites, these still mostly Fe and Mg
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- M1 sites all Mg and Fe
 Ca-clinopyroxene
- diopside to hedenbergite
- M2 site contains mostly Ca
- M1 site contains mostly Fe and Mg
 Most common specie is augite:
- Al substitutes in M1 sites, for Si in tetrahedral site
- Na, Fe or Mg substitutes for Ca in M2 site
 Identification in hand-sample difficult
- mostly based on occurrence
- also color can be indicative
- optical properties distinguish clino- from ortho-pyroxenes
- if composition is important, need chemical analysis
Geology of Pyroxenes
 Igneous
- common igneous pyroxenes: augite, pigeonite, and opx
- augite most common
- usually in mafic and intermediate volcanics
- both intrusive and extrusive
- zoning common: magma becomes enriched in Fe because of partition of Mg into crystals
• Exsolution important
• Mechanisms
• Augite original crystallization:
- Ca substitution in M2 sites restricted
- as cool, Ca reorganizes
- generally find exsolution lamellae of opx within host augite – parallel to (100)
- also pigeonite parallel to (001)
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- two lamellae
• Opx crystallize at high T with excess Ca
- slow cooling allows Ca expelled to form exsolution of augite
- single lamellae of augite parallel to (100)
- “Bushveld” variety – S. Africa type location
• Pigeonite grow in mafic magma
- up to 10% Ca in M2 site
- cooling causes Ca to expel and form augite
- single lamellae parallel to (001)
• if slow enough pigeonite converts to opx
- Pigeonite only preserved where cooling fast (volcanic)
- generates second set of lamellae
- second set augite parallel to (100)
- “Stillwater type”
• Metamorphic
• carbonate rocks, typically diopside because of Ca and Mg from calcite and dolomite
- amphibolite common association (water)
• Na and Ca clinopyroxenes
- typically restricted to high T and low P conditions
- found at subduction zones (blue schist facies)
• Opx also in granulite facies rocks
- hot enough to remove water.
- derived from amphiboles
• Sedimentary
- not stable (anhydrous)
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- convert to clay minerals
Pyroxenoid Group
• Similar to Pyroxenes
- single chains
- Si/O ratio 1/3
• Differ in repeat distance along c axis
- Pyroxenes – 2 tetrahedron repeat (5.2Å)
- Pyroxenoid – 3 or more repeat (more than 7.3Å)
- difference is the pyroxenes are straight, pyroxenoids are “kinked”
• Caused by larger linking cations
• Only few minerals
- most common Wollastonite - Ca
- others are Rhodonite – Mn,
- Pectolite – Ca and Na
• Wollastonite
- Composition: Ca with some Mn and Fe substitution
- common in altered carbonate rocks, particularly with reaction with qtz
- useful industrial mineral, replacing asbestos, also used in paints and plastics
Amphibole Group
• Structure, composition, and classification similar to pyroxenes
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• Primary difference is they are double chains
• Z/O ratio is 4/11
• Structure
- Chains extend parallel to c axis
- Stacked in alternating fashion like pyroxenes
- points face points and bases face bases
• Geometry produces five different structural sites
- M1, M2, and M3 between points of chains
- M4 and A sites between bases of chains
• TOT layers
- two T layers (tetrahedral layers with Z ions)
- intervening O layer (octahedron) with M1, M2, and M3 sites
- Form “I-beams” similar to pyroxenes
• TOT layers
- bonded by the cations in the M4 and A sites
- these bonds weaker than bonds within “I-beams”
- cleavage forms along the weak bonds
- “I-beams” wider than pyroxenes
- cleavage angles around 56º and 124º
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• Composition – W0-1X2Y5Z8O22(OH)2
• each cation fits a particular site
• W cation:
- occurs in A site
- has ~10 fold coordination
- generally large, usually Na+
• X cations:
- located in M4 sites
- analogous to M2 sites in pyroxenes
- have 6 or 8 fold coordination depending on arrangement of chains
- if 8-fold, X usually Ca
- if 6 fold, X usually Fe or Mg
• Y cations:
- Located in M1, M2, and M3 sites; O cations in TOT strips
- usually Mg, Fe2+, Fe3+, Al
• Z cations
- usually Si and Al
• Composition
- most common amphiboles shown on ternary diagram
- wide variety of substitution, simple and coupled
- divided into ortho and clino amphiboles
- depends on cations in X site (largely Ca), distorts structure
- reduces symmetry from orthorhombic to monoclinic
• Identification
- hand sample and thin section difficult
- best method is association
- Ca and Na amphiboles commonly dark green to black, pleochroic: usually Hornblende
- White or pale green amphiboles usually called tremolite
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Geology of Amphiboles
• Several important aspects:
- hydrous – as part of their structure
- not stable in anhydrous environments
- dehydrate at high temperature
- High Si/O ratio (4/11) mean they should occur in Si-rich rocks
• Generalizations:
(1) Not common in mafic and ultramafic rocks
- crystallize late in magmatic history; melt rich in Si and H2O
- overgrowths of amphibole on pyroxenes common
(2) Common in felsic to intermediate rocks
- Fe and Mg minerals eitheramphibole or biotite
- depends on abundance of K (biotite) and Ca/Na (amphibole)
- generally amphibole tends toward intermediate rocks; biotite toward felsic
(3) Amphiboles common in regional metamorphism of intermediate to mafic rocks
- usually water rich from breakdown of clay and micas
- metamorphic rocks with abundant amphiboles called amphibolite facies
- at high T, amphiboles break down to pyroxenes
Note: these are generalities are likely to be wrong
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