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Back to silicate structures: nesosilicates inosilicates tectosilicates phyllosilicates cyclosilictaes sorosilicates.

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Presentation on theme: "Back to silicate structures: nesosilicates inosilicates tectosilicates phyllosilicates cyclosilictaes sorosilicates."— Presentation transcript:

1 Back to silicate structures: nesosilicates inosilicates tectosilicates phyllosilicates cyclosilictaes sorosilicates

2 Nesosilicates: independent SiO 4 tetrahedra Olivine (100) view blue = M1 yellow = M2 b c projection

3 Inosilicates: single chains- pyroxenes TM1T Creates an “I-beam” like unit in the structure (+)

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

5 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 (+) (+)

6 The pyroxene structure is then composed of alternation I-beams Orthoopyroxenes have I-beams oriented in alternate direction in different layers (-) (+) (+) Inosilicates: single chains- pyroxenes (-) (-)

7 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 Inosilicates: single chains- pyroxenes c a (+) M1 (+) M2

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

9 Pyroxene Chemistry The general pyroxene formula: W 1-P (X,Y) 1+P Z 2 O 6 Where F W = Ca Na F X = Mg Fe 2+ Mn Ni Li F Y = Al Fe 3+ Cr Ti F Z = Si Al Anhydrous so high-temperature or dry conditions favor pyroxenes over amphiboles

10 Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Diopside Diopside CaMgSi 2 O 6 Hedenbergite CaFeSi 2 O 6 CaFeSi 2 O 6 Wollastonite Ca 2 Si 2 O 6 Enstatite Mg 2 Si 2 O 6 Ferrosilite Fe 2 Si 2 O 6 orthopyroxenes clinopyroxenes pigeonite Orthopyroxenes – solid soln between Enstatite-FerrosiliteOrthopyroxenes – solid soln between Enstatite-Ferrosilite – solid soln between Diopside-HedenbergiteClinopyroxenes – solid soln between Diopside-Hedenbergite Joins – lines between end members – limited mixing away from join

11 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) Diopside Diopside CaMgSi 2 O 6 Hedenbergite CaFeSi 2 O 6 CaFeSi 2 O 6 Wollastonite Ca 2 Si 2 O 6 Enstatite Mg 2 Si 2 O 6 Ferrosilite Fe 2 Si 2 O 6 orthopyroxenes clinopyroxenes pigeonite (Mg,Fe) 2 Si 2 O 6 Ca(Mg,Fe)Si 2 O 6 pigeonite clinopyroxenes orthopyroxenes Solvus 1200 o C 1000 o C 800 o C OPX CPX CPX OPX

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

13 Pyroxene Chemistry “Non-quad” pyroxenes Jadeite NaAlSi 2 O 6 Ca(Mg,Fe)Si 2 O 6 Aegirine NaFe 3+ Si 2 O 6 Diopside-Hedenbergite Ca-Tschermack’s molecule CaAl2SiO 6 Ca / (Ca + Na) Omphacite aegirine- augite Augite Spodumene: LiAlSi 2 O 6

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

15 Back to silicate structures: nesosilicates inosilicates tectosilicates phyllosilicates cyclosilictaes sorosilicates

16 Inosilicates: double chains- amphiboles Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg) yellow = M4 (Ca) Tremolite: Ca 2 Mg 5 [Si 8 O 22 ] (OH) 2 b a sin 

17 Inosilicates: double chains- amphiboles Hornblende: (Ca, Na) 2-3 (Mg, Fe, Al) 5 [(Si,Al) 8 O 22 ] (OH) 2 b a sin  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

18 Inosilicates: double chains- amphiboles Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) Hornblende: (Ca, Na) 2-3 (Mg, Fe, Al) 5 [(Si,Al) 8 O 22 ] (OH) 2 Same I-beam architecture, but the I-beams are fatter (double chains)

19 Inosilicates: double chains- amphiboles b a sin  (+) (+) (+) (+) (+) Same I-beam architecture, but the I-beams are fatter (double chains) 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 Hornblende: (Ca, Na) 2-3 (Mg, Fe, Al) 5 [(Si,Al) 8 O 22 ] (OH) 2

20 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 Hornblende: (Ca, Na) 2-3 (Mg, Fe, Al) 5 [(Si,Al) 8 O 22 ] (OH) 2 M1-M3 are small sites M4 is larger (Ca) A-site is really big Variety of sites  great chemical range

21 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 Hornblende: (Ca, Na) 2-3 (Mg, Fe, Al) 5 [(Si,Al) 8 O 22 ] (OH) 2 (OH) is in center of tetrahedral ring where O is a part of M1 and M3 octahedra (OH)

22 See handout for more information General formula: W 0-1 X 2 Y 5 [Z 8 O 22 ] (OH, F, Cl) 2 W = Na K X = Ca Na Mg Fe 2+ (Mn Li) Y = Mg Fe 2+ Mn Al Fe 3+ 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 Amphibole Chemistry

23 Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes) Amphibole Chemistry 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 Tremolite Ca 2 Mg 5 Si 8 O 22 (OH) 2 Ferroactinolite Ca 2 Fe 5 Si 8 O 22 (OH) 2 Anthophyllite Mg 7 Si 8 O 22 (OH) 2 Fe 7 Si 8 O 22 (OH) 2 Actinolite Cummingtonite-grunerite Orthoamphiboles Clinoamphiboles

24 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: Na 2 Mg 3 Al 2 [Si 8 O 22 ] (OH) 2 Riebeckite: Na 2 Fe 2+ 3 Fe 3+ 2 [Si 8 O 22 ] (OH) 2 Sodic amphiboles are commonly blue, and often called “blue amphiboles” Amphibole Chemistry

25 Tremolite (Ca-Mg) occurs in meta-carbonates Actinolite occurs in low-grade metamorphosed basic igneous rocks Orthoamphiboles and cummingtonite-grunerite (all Ca-free, Mg-Fe-rich amphiboles) are metamorphic and occur in meta-ultrabasic rocks and some meta-sediments. The Fe-rich grunerite occurs in meta-ironstones The complex solid solution called hornblende occurs in a broad variety of both igneous and metamorphic rocks Sodic amphiboles are predominantly metamorphic where they are characteristic of high P/T subduction-zone metamorphism (commonly called “blueschist” in reference to the predominant blue sodic amphiboles Riebeckite occurs commonly in sodic granitoid rocks Amphibole Occurrences

26 Inosilicates Pyroxenes and amphiboles are very similar: F Both have chains of SiO 4 tetrahedra F The chains are connected into stylized I-beams by M octahedra F High-Ca monoclinic forms have all the T-O-T offsets in the same direction F Low-Ca orthorhombic forms have alternating (+) and (-) offsets a a Clinopyroxene Orthopyroxene Orthoamphibole Clinoamphibole

27 Inosilicates 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  90 o cleavages in pyroxenes while wider double- chain I-beams  o cleavages in amphiboles pyroxeneamphibole a b

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

29 Tectosilicates Low Quartz 001 Projection Crystal Class 32

30 Tectosilicates High Quartz at 581 o C 001 Projection Crystal Class 622

31 Tectosilicates Cristobalite 001 Projection Cubic Structure

32 Tectosilicates Stishovite High pressure  Si VI

33 Tectosilicates Low Quartz Stishovite Si IV Si VI

34 Igneous Minerals l Quartz, Feldspars (plagioclase and alkaline), Olivines, Pyroxenes, Amphiboles l Accessory Minerals – mostly in small quantities or in ‘special’ rocks F Magnetite (Fe 3 O 4 ) F Ilmenite (FeTiO 3 ) F Apatite (Ca 5 (PO 4 ) 3 (OH,F,Cl) F Zircon (ZrSiO 4 ) F Titanite (CaTiSiO 5 ) F Pyrite (FeS 2 ) F Fluorite (CaF 2 )


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