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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 1 - 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 2 - 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) 3 - 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) 4 - 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 5 • 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 6 • 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 7 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 8