Monday, March 12, 2018

Andalusite, a mineral with an unusual feature

Andalusite is a metamorphic rock mineral (aluminum-neosilicate mineral = Al2Si05). Because its formation involves contact metamorphism (i.e., heating of rocks near the intrusion of igneous magma), andalusite is resistant to high heat and can be used in making spark plugs, furnaces, and kilns.


Cross-section of a crystal of andalusite (variety chiastolite);
 33 mm width
.
The variety chiastolite contains dark inclusions of graphite (carbon), which form a very distinctive and sharply delineated cruciform pattern, when shown in cross section. The graphite is pushed aside by crystal growth during metamorphism. Specimens can be of gemstone quality. Chiastolite is also used to make amulets and charms.



Oblique view of same specimen as above; 30 mm height.

Notice how the black band of graphite on the side of the crystal is coincident with one of the rays of the cruciform pattern. Each  of the rays is coincident with a black band. This crystal occurs in a mica schist, which is the most common occurrence of andalusite. 

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