Criteria for determining sequence (1a)

Plane-polarized light : Crossed polars

Consider the microstructure above. The moderate relief mineral on the right is andalusite, the needles on the left (and within andalusite) are sillimanite. Which of these explanations would you prefer to believe?

1. A small mat of sillimanite needles has been partly enveloped by an advancing poikiloblast of andalusite.
2. A sillimanite cluster has nucleated near the edge of a pre-existing andalusite, and some sillimanite needles have penetrated into the andalusite.

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Created on ... October 4, 2003