Andalusite



Occurrence and Compositon
Andalusite is a low pressure metamorphic mineral and is a polymorph of kyanite and sillimanite. Its formula is Al2SiO5.

Identification
Keys to identification are high relief, low birefringence and parallel extinction.

Important properties
 ·Appearance and habit - often euhedral crystals with square outlines or coarse columnar aggergates
 ·Color - generally colorless, rarely reddish. Often cloudy due to inclusions.
 ·Interference colors - maximum colors are first order yellow
 ·Twinning - Although rarely seen in thin section, some andalusite shows distinctive penetration twins, forming a cross, that helps identify andalusite.
 ·Inclusions - often contains inclusion of dark organic material; "chiastolite" is a variety of andalusite with inclusions form dark crosses on the ends of prisms
 ·Interference figure - biaxial (-) with large 2V

Similar minerals:
 ·Andalusite is distinguished from sillimanite by being length fast, having lower birefringence, and having a large 2V
 ·Kyanite has oblique extinction
 ·Some andalusite, especially if weakly colored, resembles orthopyroxene but orthopyroxene is length slow.


Andalusite Slate

These views show square end views of a few small andalusite crystals in a slate from Mariposa County, California. Just a hint of zoning, caused by carbonaceous inclusions, can be seen in the crystals. When the zoning is well deveoped and appears as a cross it is termed chiastolite. The field of view is about 2.5 mm.


Twinned Andalusite

These views show a twinned andalusite crystal, partially altered to sericite. Andalusite that shows a cross in end view is termed chiastolite. The cross appears, in part, because of dark included organic matter. The field of view is about 3.5 mm.