Orthoamphibole There are two common forms of orthoamphibole: Anthophyllite: (Mg,Fe)2(Mg,Fe)5SiO22(OH)2 Gedrite: (Mg,Fe)2(Mg,Fe)3Al2[Al2Si6O 22 (OH)2] At high temperatures the two form a continuous solid solution The general formula for amphibole: W 0-1X 2Y 5Z 8O 22(OH)2 For orthoamphible: W X Y Z 8O 22(OH)2 - (Mg,Fe)2 (Mg,Fe)5 Si 8O 22(OH)2 - (Mg,Fe)2 (Mg,Fe)3Al2 Al2Si 6O 22(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 greenish- black 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 1 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= 65-122 o 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 2 . 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. 3 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 break- down of hydrous minerals such as micas. 4.