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AMPHIBOLES (double chains)

AMPHIBOLES (double chains). General formula:  X 2 Y 5 Z 8 O 22 (OH) 2 the box is a site that may be vacant and takes Na and K X = Ca, Mg, Fe, Na; Y = Mg, Fe, Al; Z = Si, Al Taking the first possibility for each site gives Ca 2 Mg 5 Si 8 O 22 (OH) 2 tremolite

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AMPHIBOLES (double chains)

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  1. AMPHIBOLES (double chains) • General formula:  X2Y5Z8O22(OH) 2 the box is a site that may be vacant and takes Na and K • X= Ca, Mg, Fe, Na; Y= Mg, Fe, Al; Z= Si, Al • Taking the first possibility for each site gives • Ca2Mg5Si8O22(OH)2 tremolite • Ca2(Mg,Fe)5Si8O22(OH)2 actinolite • (Mg,Fe)7Si8O22(OH)2 cummingtonite • Na2(Mg,Fe)3Al2Si8O22(OH)2 glaucophane • Na2Fe2+3Fe3+2Si8O22(OH) 2 riebeckite

  2. Amphibole double chains • Tremolite Ca2Mg5Si8O22(OH)2 • Chains of Si4O11(each SiO4 tetrahedron shares two oxygens to form the chain)

  3. OPTICS • Most amphiboles are monoclinic and most sections cut parallel to C have inclined extinction • Coloured amphiboles are trichroic (glaucophane & riebeckite strongly so) • Well formed crystals common in igneous rocks • In low grade metamorphic rocks amphiboles tend to be fibrous but are more polygonal at high grades

  4. Igneous hornblende • Hornblende with plagioclase inclusions in gabbro from Ben Bullen • Similar hornblende with plagioclase inclusions in mafic igneous rock

  5. Hornblende • Brown hornblende from Zone 3 Broken Hill • Green hornblende partially replacing pyroxene in meta-andesite

  6. Where do they occur? • Tremolite (colourless and fibrous) in very Mg-rich metamorphic rocks (meta-dolomites, meta-peridotites) • Actinolite (green & fibrous) in low grade meta-basalts • Glaucophane (lavender) in high pressure meta-basalts • Riebeckite (pleochroic dark blue – yellow - green) in trachytes, quartz syenites and alkali granites and in meta-BIFs in part as the fibrous variety crocidolite • Cummingtonite (c’less or pale green, multiple twins) occurs in some igneous rocks (rhyolites) and in some amphibolites

  7. ACTINOLITE/HORNBLENDE • Hornblende is like actinolite with added Al, Na & K • Ca2(Mg,Fe)5Si8O22(OH)2 actinolite • Hornblende has Na+K in the vacant site, balanced by Al replacing Si and Al+Al substituting for Mg+Si. Some Ti and Fe3+ also in the Y site • NaCa2(Mg,Fe)4Al Si6Al2O22(OH)2hornblende • Most diorites,some gabbros and many I-type granodiorites have hornblende. It also occurs as a phenocryst in some andesites and dacites. Some pyroxene is replaced by actinolite as intrusions cool • Meta-basalts have actinolite at low grade & hornblende at intermediate to high grade. Granulites have enstatite that forms as hornblende begins to break down

  8. Amphibolites and granulites at Broken Hill • Zone 1 Blue-green hornblende • Zone 2 Green/brown hornblende • Zone 3 Brown hornblende and cpx + opx • Changes involve increases in Na, Al and Ti and a decrease in ferric iron.

  9. MICAS X2Y4-6Z8O20(OH,F)4 (If Y=4:dioctohedral; 6:trioctohedral) X=K, Na, Ca; Y=Al, Fe, Mg ,Li; Z=Si, Al Common micas are: Muscovite (Na - paragonite) K2Al4Si6Al2O20(OH,F)4 Phlogopite (end-member) K2Mg6Si6Al2O20(OH,F)4 Biotite K2(Mg,Fe,Al)6Si5Al3O20(OH,F)4 Rare micas include: Margarite Ca2Al4Si4Al4O20(OH,F)4 Lepidolite K2(Li,Al)5-6Si6-5Al2-3O20(OH,F)4

  10. Optical properties • Perfect cleavage • Parallel extinction • Birefingence .04-.08 • Mottled extinction (soft)

  11. Mica structure:Sheets of (Si,Al)O4 tetrahedra • 2 sheets with opposite facing are connected by octohedrally coordinated atoms to form a sandwich. The sandwiches are weakly bonded with K atoms. Monoclinic but pseudo-hexagonal

  12. Biotite compositions MgO

  13. Micas in igneous rocks • Biotite is common in K-rich plutonic rocks from gabbro to granite. Biotite coexists with hornblende in many I-type granites and with cordierite in many S-type granites. Phlogopite is a minor mineral in parts of the mantle. Biotite also occurs in many potassic andesites, dacites and rhyolites • Muscovite occurs in a small number of granites as a primary mineral but occurs in many as a minor alteration minerals formed as the pluton cools. A couple of very rare rhyolites have muscovite phenocrysts

  14. Micas in metamorphic rocks. • Meta-mudstones and meta-granites contain biotite at all except the lowest and highest grades. At low grade muscovite and chlorite are present and at the highest enstatite and K-feldspar • Muscovite is present in many meta-mudstones except at the highest grades when it is replaced by andalusite/sillimanite plus K-feldspar • K-rich meta-basalts/andesites will also contain biotite • Metalimestones with K and Al can have phlogopite

  15. Micas in rocks continued… • Micas are very common in pegmatites which were mined for large books of muscovite that were used as an electrical and thermal insulator. (manufactured products have replaced mica in this role) • The clay mineral illite is very similar to muscovite in composition (a little more Si and less K). On deep burial this changes to muscovite • Detrital muscovite is not uncommon in sandstones but biotite is common only in pyroclastic sedimentary rocks and their redeposited equivalents

  16. Biotite is easily deformedand perhaps surprisingly, so is quartz . S-type granites (lots of quartz & mica)are significantly more likely to show deformation than I-type granites

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