amphibole mineral

42 Amphiboles - (Mg,Fe,Ca,Na)2(Mg,Fe,Al)5(Si,Al)8O22(OH)2 Structure: - Double chain silicates with (Si,Al)O4 tetrahedra Fig 53 - The OH group is an essential part of the structure -there are both orthorhombic and monoclinic amphiboles - The complex structure allows a large number of different ion substitutions, therefore, you can get amphiboles occurring a very diverse number of rock types Eg hornblende, there is no simple composition and it occurs in many rock types, hence there are no simple charts that can be developed to describe properties Chemistry There are several amphibole groups (see handout) Anthophyllite-Cummingtonite (Ca+Na≅0) Calcium Amphiboles (Ca>Na) Alkali Amphiboles (Na>Ca) Within these groups there are several continuous series 43 Chemistry cont A typical series is: Tremolite Ca2Mg5Si8O22(OH)2 → Actinolite → Ca2(Mg,Fe)5Si8O22(OH)2 Ferro-Actinolite Ca2Fe5Si8O22(OH)2 These are essentially metamorphic minerals and are found in all types of metamorphic regimes They are usually found as fibrous, radiating clusters There are many such series for the amphiboles There are also mixtures to Hornblende: (Ca,Na,K)2-3(Mg,Fe+2,Fe+3,Al)5[Si6(Si,Al)2O22](OH,F)2 Higher temperatures allow Al to enter the amphibole structure and hornblende often occurs However, depending on Fe/Mg ration and pressure, you can get an miscibility gap between actinolite/tremolite and hornblende 44 Alterations Anthophyllite (orthorhombic), Tremolite/Actinolite, and Nephrite alter to Talc Riebeckite alters to an iron-stained fibrous quartz known as “tigers-eye” Optical and Physical Properties Orthoamphibole (Eg: Anthophyllite) Colour: colourless Form: Long prismatic crystals and columnar to fibrous Relief: High Birefringence: Moderate Cleavage: directions at ~ 60° and 120° Extinction: Parallel Clinoamphibole Eg: Cummingtonite → Grunerite series Tremolite → Ferrotremolite series Hornblende → Basaltic Hornblende series Glaucophane → Riebeckite series Colour: Colourless (cummingtonite-grunerite), colourless to pale green (trem-act), Green to brown to yellowish brown (hbl- basaltic hbl) and blue to violet (glaucrieb) 45 Optical and Physical Properties cont Clinoamphibole Form: Long prismatic crystals (pseudohexagonal crosssections and columnar to fibrous aggregates Relief: High to Fairly High Birefringence: Moderate to Strong (except for Riebeckite which is very weak) Cleavage: directions at ~ 60° and 120° Extinction: Inclined Changes in the optical properties (optical angle, extinction, birefringence) are governed by replacement of Mg by Fe The Refractive increases with Fe content Al also substitutes and the is OH replacement by F Paragenesis is very important for identifying the amphiboles Amphibole Paragenesis 1) Thermal metamorphism of sediments (common reaction) 5CaMg(CO3)2 + 8SiO2 + H2O → Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2 (dolomite) (silica) (tremolite) (calcite) If you increase the grade of metamorphism, tremolite breaks down to diopside and at very high temperatures forsterite can form All such reactions liberate CO2 and H2O and are therefore pressure dependent (Figure 65 on handout) 46 Amphibole Paragenesis 1) Thermal metamorphism of sediments (common reaction) 5CaMg(CO3)2 + 8SiO2 + H2O → Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2 (dolomite) (silica) (tremolite) (calcite) If you increase the grade of metamorphism, tremolite breaks down to diopside and at very high temperatures forsterite can form All such reactions liberate CO2 and H2O and are therefore pressure dependent (Figure 65 on handout) 2) Regional Metamorphism - tremolite occurs where calcareous sediments dominate 3) Regional metamorphism of Ultrabasic rocks - both tremolite and actinolite (common) + talk, carbonates and chlorite 47 Amphibole Paragenesis cont Other Amphiboles: 4) Hornblende - occurs in a wide variety of T-P conditions in both igneous and metamorphic rocks - stable in most silicious rocks and very common in intermediate plutonic rocks Gabbros Mg rich (Mg:Fe = 3:1) Syenites and granites Diorites Fe rich (Mg:Fe=5:95 Mg:F=1:1 5) Basaltic Hornblende - only in igneous rocks - occur as phenocrysts in andesites 6) Kaersutite (Ti bearing amphibole) - occur as large phenocrysts (glomoporhyritic) in alkalic volcanic rocks such as trachyte 7) Alkali Amphiboles - Ca (Na,K) Glaucophane - occur under high pressure, low temperature conditions such as subduction zones (also large amounts of Na from seawater) Eg: Glaucophane schists Riebeckite - occurs in alkali igneous rocks (plutonic) such as quartz syenites, granites, trachyte and rhyolite  ... many such series for the amphiboles There are also mixtures to Hornblende: (Ca,Na,K)2-3(Mg,Fe+2,Fe+3,Al)5[Si6(Si,Al)2O22](OH,F)2 Higher temperatures allow Al to enter the amphibole structure and... substitutes and the is OH replacement by F Paragenesis is very important for identifying the amphiboles Amphibole Paragenesis 1) Thermal metamorphism of sediments (common reaction) 5CaMg(CO3)2 +... rocks - both tremolite and actinolite (common) + talk, carbonates and chlorite 47 Amphibole Paragenesis cont Other Amphiboles: 4) Hornblende - occurs in a wide variety of T-P conditions in both