Feldspar Group Most abundant mineral in the crust 6 of 7 most common elements Defined through 3 endmembers Albite Na Anorthite Ca Orthoclase K Comprised of 2 series Albite anorthite Na Ca Albite orthoclase Na K Tectosilicates Feldspars Substitute Al3 for Si4 allows Na or K to be added Albite Orthoclase Substitute two Al3 for Si4 allows Ca2 to be added Albite Anorthite Albite NaAlSi3O8 Feldspar Group Albite Anorthite series Complete solid solution Plagioclase Feldspars 6 minerals Albite Na Oligoclase Andesine Labradorite Bytownite Anorthite Ca Albite Anorthite double duty End members Pure Na or Ca Minerals 90 99 99 Na or Ca Notation AnxAby An20Ab80 Oligoclase Feldspar Group Albite Anorthite series Optical techniques to distinguish between plagioclase feldspars Michel Levy Method uses extinction angles of twinned forms to determine An Ab content Combined Carlsbad Albite Method uses MichelLevy technique for both sides of a twin form Staining technique Stains that attach to K really well Like Co NO3 2 will higlight the Kfeldspars quickly and easily in hand specimen or thin section Feldspar Group Albite Orthoclase series Several minerals Alkali Feldspars High T minerals Sanidine Anorthoclase Monalbite High Albite Low Temperature exsolution at solvus Chicken soup separation Forms 2 minerals in igneous rocks these are typically intergrowths or exsolution lamellae perthitic texture monalbite anorthoclase 1100 Temperature C high albite 900 700 500 sanidine intermediate albite orthoclase low albite microcline Miscibility Gap 300 10 Orthoclase KAlSi3O8 30 50 NaAlSi3O8 70 90 Albite NaAlSi3O8 Alkali Feldspar Exsolution Melt cools past solvus line defining miscibility gap Anorthoclase that had formed through liquidus solidus separates if cooling is slow enough to form orthoclase and low albite In hand sample schiller effect play of colors caused by lamellae Liquid 1100 monalbite anorthoclase Temperature C 900 high albite sanidine intermediate albite 700 500 orthoclase low albite microcline Miscibility Gap 300 10 Orthoclase KAlSi3O8 30 50 NaAlSi3O8 70 90 Albite NaAlSi3O8 Alkali Feldspar lamellae Feldspathoid Group Very similar to feldspars and zeolites Include Nepheline Analcime and Leucite Also framework silicates but with another Al substitution for Si Only occur in undersaturated rocks no free Quartz Sipoor because they react with SiO2 to form feldspars Feldspathoids Cont Nepheline Important feldspathoid mineral Indicates undersaturated magma Nesosilicates independent SiO4 tetrahedra b M1 in rows and share edges a M2 form layers in a c that share corners Some M2 and M1 share edges Olivine 001 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 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 Inosilicates single chains pyroxenes b a sin Diopside CaMg Si2O6 Where are the Si O Si O chains Diopside 001 view blue Si purple M1 Mg yellow M2 Ca Inosilicates single chains pyroxenes a sin b Diopside 001 view blue Si purple M1 Mg yellow M2 Ca Inosilicates single chains pyroxenes a sin b Diopside 001 view blue Si purple M1 Mg yellow M2 Ca Inosilicates single chains pyroxenes a sin b Diopside 001 view blue Si purple M1 Mg yellow M2 Ca Inosilicates single chains pyroxenes a sin b Diopside 001 view blue Si purple M1 Mg yellow M2 Ca Inosilicates single chains pyroxenes a sin b Diopside 001 view blue Si purple M1 Mg yellow M2 Ca Inosilicates single chains pyroxenes Perspective view Diopside 001 view blue Si purple M1 Mg yellow M2 Ca Inosilicates single chains pyroxenes IV slab SiO4 as polygons and larger area VI slab a sin IV slab VI slab IV slab VI slab IV slab b Diopside 001 view blue Si purple M1 Mg yellow M2 Ca Inosilicates single chains pyroxenes M1 octahedron Inosilicates single chains pyroxenes M1 octahedron Inosilicates single chains pyroxenes M1 octahedron type by convention Inosilicates single chains pyroxenes 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 Clinopyroxenes have all I beams oriented the same all are in this orientation The pyroxene structure is then composed of alternating I beams Note that M1 sites are smaller than M2 sites since they are at the apices of the tetrahedral chains Inosilicates single chains pyroxenes Clinopyroxenes have all I beams oriented the same all are in this orientation The pyroxene structure is then composed of alternation I beams Orthopyroxenes have alternating and orientations Inosilicates single chains pyroxenes Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms Inosilicates single chains pyroxenes M2 The tetrahedral chain above the M1s is thus offset from that below c a M1 M2 The M2 slabs have a similar effect The result is a monoclinic unit cell hence clinopyroxenes Inosilicates single chains pyroxenes Orthopyroxenes have alternating and I beams c M1 M2 a M1 M2 the offsets thus compensate and result in an orthorhombic unit cell 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 Diopside CaMgSi2O6 clinopyroxenes Orthopyroxenes solid soln between Enstatite Ferrosilite Clinopyroxenes solid soln between Diopside Hedenbergite Hedenbergite CaFeSi2O6 Joins lines between end members limited mixing away from join pigeonite orthopyroxenes Enstatite Mg2Si2O6 Ferrosilite Fe2Si2O6 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