| General Formula: Al2SiO5 |
Sampl: BUPM 17 | System: Orthorhombic |
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| Andalusite, var. Chiastolite Andalusite porphyroblast showing the development of the chiastolite cross with arms radiating out from the centre of the grain. The andalusite is 6 mm wide, plane light view. |
Andalusite, var. Chiastolite The andalusite is altering to fine grained muscovite and chlorite along the edges, and is hosted within a very fine grained pelitic matrix. The andalusite is 6 mm wide, crossed polar view. |
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes. |
Optical Properties
| Colour Pleochroism |
colourless, rarely pink coloured varieties are weakly pleochroic |
Form | usually elongate prisms with nearly square cross sections. var. chiastolite contains dark inclusions that form a cross along the diagonals of the prism. |
| Relief RI |
moderate high positive nalpha= 1.629-1.640 nbeta= 1.633-1.644 ngamma= 1.638-1.650 |
Cleavage | two good prismatic cleavages {110} nearly at 90° |
| Birefringence Interference Colours |
0.009-0.013 firsrt order grey to white |
Twinning | rare |
| Interference Figure Optic Sign 2V |
biaxial negative 71-88° |
Optic Orientation | elongate sections display parallel extinction, cross sections exhibit symmetrical extinction, generally length fast |
| Composition | relatively pure Al2SiO5, but significant amounts of Mn3+ and Fe3+ may substitute for octohedral aluminium | Alteration | to sericite, chlorite or other phyllosilicates, or as reactant in metamorphic reactions |
| Occurrence | common in contact and regional metamorphsim, associated with cordierite, garnet, sillimanite, kyanite, staurolite, muscovite, biotite, chlorite and plagioclase | Distinguishing Features | moderate high relief, large 2V, parallel extinction, and length fast |
