Plane-polarized light : Crossed polars
It's tempting to assume that a mineral that appears to envelop and enclose another one must have formed later. Nevertheless, when I ask a group of students for their opinion, the result is usually close to 50% sillimanite first, 50% andalusite first. Why is this so ambiguous?
The problem is that there is no difference between parts of the andalusite that enclose sillimanite and parts that don't, nor is there any very distinctive contrast between the sillimanite needles outside andalusite and those within it. So, simple contacts between minerals, even when one mineral appears to partially enclose another, don't provide unambiguous criteria for sequence of growth.
[ Actually there's another possibility we might at first not wish to consider, since it appears to violate the concept of equilibrium: that both Al-silicates were growing simultaneously. ]
Created on ... October 4, 2003