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b c Nesosilicates: independent SiO4 tetrahedra projection Olivine (100) view blue = M1 yellow = M2

b M1 in rows and share edges M2 form layers in a-c that share corners Some M2 and M1 share edges Nesosilicates: independent SiO4 tetrahedra a Olivine (001) view blue = M1 yellow = M2

b Nesosilicates: independent SiO4 tetrahedra c M1 and M2 as polyhedra Olivine (100) view blue = M1 yellow = M2

Olivine – complete solid solution Forsterite-Fayalite  FoxFay Fayalite – Fe end-member Forsterite – Mg end-member Olivine Occurrences: Principally in mafic and ultramafic igneous and meta-igneous rocks Fayalite in meta-ironstones and in some alkalic granitoids Forsterite in some siliceous dolomitic marbles Monticellite CaMgSiO4 Ca  M2 (larger ion, larger site) High grade metamorphic siliceous carbonates

Olivine minerals • Solid solution forsterite-fayalite, tephroite-glaucochroite, monticellite-kirschsteinite • Not in between  no forsterite-tephroite series Larnite – Ca2SiO4

Distinguishing Forsterite-Fayalite • Petrographic Microscope • Index of refraction  careful of zoning!! • 2V different in different composition ranges • Pleochroism/ color slightly different • Spectroscopic techniques – many ways to determine Fe vs. Mg • Same space group (Pbnm), Orthorhombic, slight differences in unit cell dimensions only

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b Diopside: CaMg [Si2O6] a sin Where are the Si-O-Si-O chains?? Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

b a sin Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

Perspective view Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

SiO4 as polygons (and larger area) IV slab VI slab IV slab a sin VI slab Inosilicates: single chains- pyroxenes IV slab VI slab IV slab b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

M1 octahedron Inosilicates: single chains- pyroxenes

(+) M1 octahedron Inosilicates: single chains- pyroxenes (+) type by convention

M1 octahedron This is a (-) type (-) Inosilicates: single chains- pyroxenes

T M1 T Creates an “I-beam” like unit in the structure. Inosilicates: single chains- pyroxenes

(+) T M1 T Creates an “I-beam” like unit in the structure Inosilicates: single chains- pyroxenes

(+) (+) (+) (+) (+) Inosilicates: single chains- pyroxenes The pyroxene structure is then composed of alternating I-beams Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation Note that M1 sites are smaller than M2 sites, since they are at the apices of the tetrahedral chains

(+) (+) (+) (+) (+) Inosilicates: single chains- pyroxenes The pyroxene structure is then composed of alternation I-beams Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation

Inosilicates: single chains- pyroxenes Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms

Inosilicates: single chains- pyroxenes The tetrahedral chain above the M1s is thus offset from that below The M2 slabs have a similar effect The result is a monoclinic unit cell, hence clinopyroxenes (+) M2 c a (+) M1 (+) M2

Inosilicates: single chains- pyroxenes Orthopyroxenes have alternating (+) and (-) I-beams the offsets thus compensate and result in an orthorhombic unit cell c (-) M1 (+) M2 a (+) M1 (-) M2

Pyroxene Chemistry The general pyroxene formula: W1-P (X,Y)1+P Z2O6 Where • W = Ca Na • X = Mg Fe2+ Mn Ni Li • Y = Al Fe3+ Cr Ti • Z = Si Al Anhydrous so high-temperature or dry conditions favor pyroxenes over amphiboles

Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Wollastonite Ca2Si2O6 • Orthopyroxenes – solid soln between Enstatite-Ferrosilite • Clinopyroxenes – solid soln between Diopside-Hedenbergite Hedenbergite CaFeSi2O6 Diopside CaMgSi2O6 clinopyroxenes Joins – lines between end members – limited mixing away from join pigeonite orthopyroxenes Ferrosilite Fe2Si2O6 Enstatite Mg2Si2O6

Wollastonite Ca2Si2O6 Hedenbergite CaFeSi2O6 Diopside CaMgSi2O6 clinopyroxenes pigeonite orthopyroxenes Ferrosilite Fe2Si2O6 Enstatite Mg2Si2O6 Orthopyroxene - Clinopyroxene OPX and CPX have different crystal structures – results in a complex solvus between them Coexisting opx + cpx in many rocks (pigeonite only in volcanics) pigeonite 1200oC orthopyroxenes clinopyroxenes 1000oC CPX Solvus 800oC (Mg,Fe)2Si2O6 Ca(Mg,Fe)Si2O6 OPX OPX CPX

Orthopyroxene – Clinopyroxenesolvus T dependence • Complex solvus – the ‘stability’ of a particular mineral changes with T. A different mineral’s ‘stability’ may change with T differently… • OPX-CPX exsolution lamellae  Geothermometer… CPX CPX Hd Di Di Hd augite augite Subcalcic augite Miscibility Gap Miscibility Gap pigeonite pigeonite orthopyroxene orthopyroxene Fs En Fs En OPX OPX 800ºC 1200ºC Pigeonite + orthopyroxene

Pyroxene Chemistry Jadeite Aegirine NaAlSi2O6 “Non-quad” pyroxenes NaFe3+Si2O6 0.8 Omphacite aegirine- augite Spodumene: LiAlSi2O6 Ca / (Ca + Na) Ca-Tschermack’s molecule 0.2 CaAl2SiO6 Augite Diopside-Hedenbergite Ca(Mg,Fe)Si2O6

17.4 A 12.5 A 7.1 A 5.2 A Pyroxenoids “Ideal” pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sites Pyroxene 2-tet repeat Wollastonite (Ca  M1)  3-tet repeat Rhodonite MnSiO3  5-tet repeat Pyroxmangite (Mn, Fe)SiO3  7-tet repeat

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