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Feldspar Group

Feldspar Group. Most abundant mineral in the crust  6 of 7 most common elements Defined through 3 end-members  Albite (Na), Anorthite (Ca), Orthoclase (K) Comprised of 2 series: Albite-anorthite (Na-Ca) Albite-orthoclase (Na-K). Tectosilicates.

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Feldspar Group

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  1. Feldspar Group • Most abundant mineral in the crust  6 of 7 most common elements • Defined through 3 end-members  • Albite (Na), Anorthite (Ca), Orthoclase (K) • Comprised of 2 series: • Albite-anorthite (Na-Ca) • Albite-orthoclase (Na-K)

  2. Tectosilicates Substitute Al3+ for Si4+ allows Na+ or K+ to be added Albite-Orthoclase Feldspars Substitute two Al3+ for Si4+ allows Ca2+ to be added Albite-Anorthite Albite: NaAlSi3O8

  3. 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

  4. 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 Michel-Levy technique for both sides of a twin form

  5. Staining technique • Stains that attach to K really well (Like Co(NO3)2 ) will higlight the K-feldspars quickly and easily in hand specimen or thin section

  6. monalbite anorthoclase 1100 high albite sanidine 900 intermediate albite Temperature (ºC) 700 orthoclase low albite microcline 500 Miscibility Gap 300 10 70 30 50 90 Orthoclase KAlSi3O8 Albite NaAlSi3O8 % NaAlSi3O8 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

  7. Liquid 1100 monalbite anorthoclase 900 high albite sanidine intermediate albite Temperature (ºC) 700 orthoclase low albite microcline 500 Miscibility Gap 300 10 70 30 50 90 Orthoclase KAlSi3O8 Albite NaAlSi3O8 % 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

  8. Alkali Feldspar lamellae

  9. 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, Si-poor) because they react with SiO2 to form feldspars

  10. Feldspathoids, Cont. • Nepheline • Important feldspathoid mineral • Indicates undersaturated magma

  11. 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

  12. 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

  13. 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

  14. 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)

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

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

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

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

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

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

  21. 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)

  22. M1 octahedron Inosilicates: single chains- pyroxenes

  23. M1 octahedron Inosilicates: single chains- pyroxenes

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

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

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

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

  28. (+) (+) (+) (+) (+) 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

  29. (+) (+) (+) (+) (+) 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 Orthopyroxenes have alternating (+) and (-) orientations

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

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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

  37. 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

  38. b Tremolite: Ca2Mg5 [Si8O22] (OH)2 a sin Inosilicates: double chains- amphiboles Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg) yellow = M4 (Ca)

  39. b Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 a sin Inosilicates: double chains- amphiboles Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na) little turquoise ball = H

  40. Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 Same I-beam architecture, but the I-beams are fatter (double chains) Inosilicates: double chains- amphiboles Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe)

  41. (+) (+) (+) (+) (+) b Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 Same I-beam architecture, but the I-beams are fatter (double chains) a sin Inosilicates: double chains- amphiboles All are (+) on clinoamphiboles and alternate in orthoamphiboles Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na) little turquoise ball = H

  42. Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 M1-M3 are small sites M4 is larger (Ca) A-site is really big Variety of sites  great chemical range Inosilicates: double chains- amphiboles Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na) little turquoise ball = H

  43. Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 (OH) is in center of tetrahedral ring where O is a part of M1 and M3 octahedra Inosilicates: double chains- amphiboles (OH) Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na) little turquoise ball = H

  44. Amphibole Chemistry • See handout for more information • General formula: • W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2 • W = Na K • X = Ca Na Mg Fe2+ (Mn Li) • Y = Mg Fe2+ Mn Al Fe3+ Ti • Z = Si Al • Again, the great variety of sites and sizes  a great chemical range, and hence a broad stability range • The hydrous nature implies an upper temperature stability limit

  45. Amphibole Chemistry Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes) Tremolite Ferroactinolite Actinolite Ca2Mg5Si8O22(OH)2 Ca2Fe5Si8O22(OH)2 Clinoamphiboles Cummingtonite-grunerite Anthophyllite Fe7Si8O22(OH)2 Mg7Si8O22(OH)2 Orthoamphiboles Al and Na tend to stabilize the orthorhombic form in low-Ca amphiboles, so anthophyllite  gedrite orthorhombic series extends to Fe-rich gedrite in more Na-Al-rich compositions

  46. Amphibole Chemistry • Hornblende has Al in the tetrahedral site • Geologists traditionally use the term “hornblende” as a catch-all term for practically any dark amphibole. Now the common use of the microprobe has petrologists casting “hornblende” into end-member compositions and naming amphiboles after a well-represented end-member. • Sodic amphiboles • Glaucophane: Na2 Mg3 Al2 [Si8O22] (OH)2 • Riebeckite: Na2 Fe2+3 Fe3+2 [Si8O22] (OH)2 • Sodic amphiboles are commonly blue, and often called “blue amphiboles”

  47. Inosilicates + + + + + + + a + + + + + + + + + + + - - - - - - Clinopyroxene Clinoamphibole + + + + + a + - - - - - - Orthopyroxene Orthoamphibole • Pyroxenes and amphiboles are very similar: • Both have chains of SiO4 tetrahedra • The chains are connected into stylized I-beams by M octahedra • High-Ca monoclinic forms have all the T-O-T offsets in the same direction • Low-Ca orthorhombic forms have alternating (+) and (-) offsets

  48. pyroxene amphibole Inosilicates b a Cleavage angles can be interpreted in terms of weak bonds in M2 sites (around I-beams instead of through them) Narrow single-chain I-beams  90o cleavages in pyroxenes while wider double-chain I-beams  60-120o cleavages in amphiboles

  49. Tectosilicates After Swamy and Saxena (1994)J. Geophys. Res., 99, 11,787-11,794.

  50. Tectosilicates Low Quartz 001 Projection Crystal Class 32

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