amphibole mineral

Chemistry cont.A typical series is: Tremolite → Actinolite → Ferro-Actinolite Ca2Mg5Si8O22OH2 Ca2Mg,Fe5Si8O22OH2 Ca2Fe5Si8O22OH2 These are essentially metamorphic minerals and are found

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

Chemistry cont.

A typical series is:

Tremolite → Actinolite → Ferro-Actinolite

Ca2Mg5Si8O22(OH)2 Ca2(Mg,Fe)5Si8O22(OH)2 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

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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: 2 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 (glauc-rieb)

45 Optical and Physical Properties cont

Clinoamphibole

Form: Long prismatic crystals (pseudohexagonal

cross-sections and columnar to fibrous aggregates

Relief: High to Fairly High

Birefringence: Moderate to Strong (except for Riebeckite

which is very weak)

Cleavage: 2 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)

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

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 Fe rich (Mg:Fe=5:95 granites

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

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