Inosilicates chain n n Common FeMg bearing silicates
Inosilicates (chain) n n Common Fe/Mg – bearing silicates Two common groups Pyroxenes: single chains n Amphiboles: double chains n n Pyroxenes are common in MORB Amphiboles more common on continents because of weathering A third group is call pyroxenoids
Pyroxene group n n n General formula: XYZ 2 O 6 Z/O ratio = 1/3 Z cations usually Si, occasionally Al Single chain extend along c axis Chains are stacked along a axis, alternating: Base faces base n Apex faces apex n
View down a axis View down c axis Two distinct sites, depending on location relative to chains M 1 and M 2 Base facing base Apex facing Apex Fig. 14 -1
n X cations in M 2 sites Between bases of tetrahedrons n Distorted 6 - and 8 - fold coordination n Depends on stacking and the size of the cations n n Y cations in M 1 sites n 6 -fold coordination between apical oxygen
n “I-beams” n Consist of two chains connected by Y cations n n Located in M 1 sites Closeness of apical oxygen and 6 -fold coordination make bonds strong Apex pointed at apex I-beam
n I-beams held together by X cations in M 2 site Coordination number depends on how chains line up n 6 -fold coordination gives orthorhombic symmetry - OPX n 8 -fold coordination gives monoclinic symmetry - CPX n
Crystallographic and optical axes align C crystallographic axis at 32 to 42º angle to the Z optical axis OPX – Orthorhombic Parallel extinction Pigeonite – CPX – Monoclinic Inclined extinction
n Crystal shapes Blocky prisms, nearly square in the (001) face n Elongate along c axis, e. g. , [001] n n Cleavage controlled by I-beams Cleavage typically between 87º and 93º n Only when viewed down the c axis n Mineral grain must be cut parallel to (001) n
I beams – tightly bonded Weak zones between faces of I beams Cleavage angles are 87º and 93º Weak planes between “I beams” = cleavage Fig. 14 -1
Classification n Based on two linked things Which cations occurs in M 2 sites (facing bases of tetrahedron) n Cation determines symmetry n n Most plot on ternary diagram with apices: Wollastonite, Wo (a pyroxenoid) n Enstatite, En n Ferrosilite, Fe n
n Three major groups Orthopyroxenes (opx) – orthorhombic n Low-Ca clinopyroxenes (cpx) – monoclinic n Ca-rich clinopyroxenes (cpx) – monoclinic n n The amount of Ca in the mineral controls the extinction angle
n Orthopyroxenes: Fe and Mg, but little Ca Both M 1 and M 2 are octahedral n Larger Fe ion more concentrated in the larger M 2 site n
n Low-Ca clinopyroxene: more Ca, but no solid solution with Hi-Ca clinopyroxene Mineral species is Pigeonite n Ca restricted to M 2 sites, these still mostly Fe and Mg n M 1 sites all Mg and Fe n
n Ca- clinopyroxene Diopside Mg(+Ca) to Hedenbergite Fe (+Ca) n M 2 site contains mostly Ca n M 1 site contains mostly Fe and Mg n n Most common specie is augite Al substitutes in M 1 site, and for Si in tetrahedral site n Na, Fe or Mg substitutes for Ca in M 2 site n
n Other common pyroxenes Jadeite Na. Al. Si 2 O 6 n Spodumene Li. Al. Si 2 O 6 n
Possible ranges of solid solutions “Augite” Clinopyroxene Orthopyroxenes Fig. 14 -2
Additional Pyroxenes n Contain Na
Amphibole Group n n n Structure, composition, and classification similar to pyroxenes Primary difference is they are double chains Z/O ratio is 4/11
Structure n n n Chains extend parallel to c axis Stacked in alternating fashion like pyroxenes Points face points and bases face bases
O shared with Si n n n Chains are linked by sheets of octahedral sites Three unique sites: M 1, M 2, and M 3 O not n Depend on location shared relative to Si with Si tetrahedron Also contain hydroxyl ion – hydrous mineral OH Fig. 14 -12
n TOT layers Two T layers (tetrahedral layers with Z ions) n Intervening O layer (octahedron) with M 1, M 2, and M 3 sites n Form “I-beams” similar to pyroxenes n
n Geometry produces five different structure sites M 1, M 2, and M 3 between points of chains n M 4 and A sites between bases of chains n
Bonds at M 4 and A sites weaker than bonds within “I-beams” n Cleavage forms along the weak bonds n “I-beams” wider than pyroxenes n Cleavage angles around 56º and 124º n Weak planes between “I beams” = cleavage
Composition W 0 -1 X 2 Y 5 Z 8 O 22(OH)2 n n Each cation fits a particular site W cation Occurs in A site n Has ~10 fold coordination n Generally large, usually Na+ n
W 0 -1 X 2 Y 5 Z 8 O 22(OH)2 n X cations Located in M 4 sites n Analogous to M 2 sites in pyroxenes n Have 6 or 8 fold coordination depending on arrangement of chains n If 8 -fold, X usually Ca n If 6 -fold, X usually Fe or Mg n
W 0 -1 X 2 Y 5 Z 8 O 22(OH)2 n Y cations Located in M 1, M 2, and M 3 sites; Octahedral cations in TOT strips n Usually Mg, Fe 2+, Fe 3+, Al n n Z cations n n Usually Si and Al Note: Z/O ratio is 4/11, contains water
n Composition Most common amphiboles shown on ternary diagram n Wide variety of substitution, simple and coupled n Divided into ortho and clino amphiboles n Depends on X cations in M 4 site (largely amount of Ca), distorts structure n Reduces symmetry from orthorhombic to monoclinic n
W 0 -1 X 2 Y 5 Z 8 O 22(OH)2 Tremolite Anthophylite Orthorhombic Ferroactinolite ~30% Ca exactly 2/7 of sites available for Ca Grunerite Monoclinic Fig. 14 -13
Pyroxenes and Amphiboles
Pyroxenoid Group n Similar to pyroxenes Single chains n Z/O ratio 1/3 n n Differ in repeat distance along c axis Pyroxene – 2 tetrahedron repeat (5. 2 Å) n Pyroxenoid – 3 or more repeat (more than 7. 3 Å) n Difference: n pyroxenes are straight chains n pyroxenoids are kinked chains n n Caused by larger linking cations
Pyroxenes Rhodenite - Mn Wollastonite - Ca
n Only a few minerals Most common Wollastonite – Ca n Others are Rhodonite – Mn n Pectolite – Ca and Na n
n Wollastonite Composition: Ca with some Mn and Fe substitution n Common in altered carbonate rocks, particularly with reaction with qtz n Useful industrial mineral, replacing asbestos, also used in paints and plastics n
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