Mineral Set #6

actinolite formula

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

Tremolite formula

Ca2Mg5Si8O22(OH)2 (asbestoform)

Hornblende formula

(Na,K)(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5 (Si,Al)8O22(OH)2

Ca causes

high birefringence colors

asbestoform

natural manifestation of amphibole rock

Fe causes

pleichroism

Anthophyllite formula

(Mg,Fe)7Si8O22(OH)2

Cummingtonite- Grunerite formula

(Mg,Fe,Mn minimal Ca)7Si8O22(OH)2

Cummingtonite- Grunerite

Monoclinic Biaxial + or -, w/ inc. Fe Common amphibole in low Ca metamorphic rocks - fibrous in occurrence - may show twinning. Hard to resolve from hornblende in HS. Solid solution low calcium version of Hornblende

Tremolite

Monoclinic Biaxial negative Found in Mg-rich metamorphic rocks (ultramafics and calc-silicates (dirty methamorphosized limestone) Not pleichroic due to lack of Fe Significant amounts of Ca means high birefringence colors

Glaucophane

Monoclinic Biaxial negative: Pleochroic purple in TS Common mineral in subduction-complex metamorphic associations (high P, low T metamorphism) Asbestiform called Crocidolite.

Edenite

Monoclinic Biaxial negative: often green in TS Occurs in mafic/ultramafic metamorphic rocks - grades into hornblende Distinguished from tremolite and actinolite because it picks up Ca and Al Most similar to hornblende in structure

actinolite

Monoclinic Biaxial negative; pleochroic green Common in Fe,Mg - rich metamorphic rocks. In range with tremolite when Fe is added

Riebeckite

Monoclinic Biaxial positive or negative; Pleochroic; blue in TS!! Alkaline rocks igneous occurence

Hornblende

Monoclinic Biaxial negative, strongly pleichroic Common mineral in mafic to intermediate igneous and metamorphic rocks Catch all term for amphibole Pleichroic and birefringence

Riebeckite formula

Na2Fe5Si8O22(OH)2

Glaucophane formula

Na2Mg3Al2(Si8O22)(OH)2

Edenite formula

NaCa2(Mg,Fe)5Si7AlO22(OH)2

Properties of amphiboles

One of the most common minerals A little bit less hard than pyroxenes Significant compositional spread All prismatic in their crystal forms (long and thin) Can grade into fibrous forms (asbestoform) 60-120 cleavage perfect cleavage, distinctive stair stepping More amphiboles than any other group are pleichroic (due to iron) Also birefringent Hydrated with (OH)

Anthophyllite

Orthorhombic Biaxial negative Amphibolite facies metamorphic mineral in ultramafic rocks: often asbestiform. Assoc. with olivine