Download Orthoamphibole

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
(Mg,Fe)2
(Mg,Fe)2
Y
(Mg,Fe)5
(Mg,Fe)3Al2
Z8O22(OH)2
Si8O22(OH)2
Al2Si6O22(OH)2
The W cation fills a ~10-fold coordination site in the amphiboles. It
Contains 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 greenishblack color
H: 5.5-6
G=2.85-3.57
Cleavage: perfect {210} cleavage like all amphiboles. Also distinct
Cleavage on {100} and {010}.
Luster: vitreous
Optical Properties
a=1.587-1.694
b=1.602-1.710
g=1.613-1.722
d=0.013-0.028
Conclusion: moderate to high relief and second and third-order
Colors under polarized light.
Pleochroism: weak; usually with Z>Y>X
Since orthoamphibole is orthorhombic, it is a biaxial mineral with
2Vx anthophyllite= 65-122o
2Vx gedrite= 72-133o
Both minerals may either be optically positive or negative. Either
Addition 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
.
O3A and O3B anion positions may be occupied by OH-, F- or O2-
The bases of the double chains face each other
There are two crystallographically distinct double chains, the A
chain and the B chain. Both have mirror symmetry, but the B-chain
is more kinked.
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 because
Considerable 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 from
this 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 anhydrous
environments.
They are not found in mafic and ultramafic igneous rocks because these
magmas have little dissolved water. The amphiboles will break down
to 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 breakdown of hydrous minerals such as micas.