Orthoamphibole

There are two common forms of orthoamphibole:

Anthophyllite: (Mg,Fe)2(Mg,Fe)5SiO22(OH)2

Gedrite: (Mg,Fe)2(Mg,Fe)3Al2[Al2Si6O22(OH)2]

At high temperatures the two form a continuous solid solution

The general formula for amphibole:

W0-1X2Y5Z8O22(OH)2

For orthoamphible:W X Y Z8O22(OH)2 - (Mg,Fe)2 (Mg,Fe)5 Si8O22(OH)2

- (Mg,Fe)2 (Mg,Fe)3Al2 Al2Si6O22(OH)2

The W cation fills a ~10-fold coordination site in the amphiboles. ItContains large cations like Na+. In anthophyllite this site is vacant. However, in Gedrite it may sometimes contain Na+ .

Physical Properties

Color: brown

note: the brown color of orthoamphibole may be useful in distinguishing it from hornblende, which usually has a greenish-black color

H: 5.5-6

G=2.85-3.57

Cleavage: perfect {210} cleavage like all amphiboles. Also distinctCleavage on {100} and {010}.

Luster: vitreous

Optical Properties

=1.587-1.694=1.602-1.710=1.613-1.722

=0.013-0.028

Conclusion: moderate to high relief and second and third-orderColors under polarized light.

Pleochroism: weak; usually with Z>Y>X

Since orthoamphibole is orthorhombic, it is a biaxial mineral with2Vx anthophyllite= 65-122o

2Vx gedrite= 72-133o

Both minerals may either be optically positive or negative. EitherAddition or subtraction may happen to the slow ray of the plane.

Ways to tell gedrite and anthophyllite apart: gedrite has a slightly higher index of refraction and is more likely to have a negative optic sign than anthophyllite

Crystallography:

Orthorhombic

Point group: 2/m 2/m 2/m

Space group: Pnma

.

There are two crystallographically distinct double chains, the Achain and the B chain. Both have mirror symmetry, but the B-chainis more kinked.

The bases of the double chains face each other

O3A and O3B anion positions may be occupied by OH-, F- or O2-

Stacking along the a axis occurs for alternating tetrahedral layers.Stagger for amphiboles is +c/3, +c/3, -c/3, -c/3

The sites are pseudo-tetrahedral and pseudo-octahedral. This is becauseConsiderable variation in local enviroments causes stereochemical Differences amoung tetrahedra and octahedra.

Pauling’s second rule: “In a stable ionic structure, the valence of each anion, with the sign changed, is equal to the sum of the strengths of the electrostatic bonds to it from the adjacent cations”. The electrostatic Bond strength is defined as the formal valence of the cation divided by its coordination number.

However, many minerals, including the amphiboles may deviate fromthis rule. Models have been developed which explain the relationship between bond strength and bond length. The models Explain how the bond lengths are adjusted and distorted in order to minimize the difference between the bond strength and the bond-valence sums around the anions

Occurrences

Orthoamphibole occurs in metamorphic rock of high grade, but not in igneous rocks.

Amphiboes are hydrous minerals which break down in anhydrousenvironments.

They are not found in mafic and ultramafic igneous rocks because thesemagmas have little dissolved water. The amphiboles will break downto form pyroxenes instead.

However, some amphiboles can form in felsic igneous rocks, but not Orthoamphibole.

Metamorphic conditions have high amounts of water due to the break-down of hydrous minerals such as micas.