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The exsolution game... Willemite, calcite and tephroite.

The labels of specimens like this nearly always read "Calcite & willemite". They never mention the non-fluorescent massive brown stuff, the tephroite. Exsolution of willemite from zincian tephroite happens during cooling. The expelled willemite is deposited in cracks that form in the shrinking mass, following the surface of the tephroite crystallographic planes. Hence the neatly ordered fine lines. (Thanks to Earl Verbeek for the explanation.) The calcite just adds to the fun...

Locality: Franklin, New Jersey, USA.

Perpendicular exsolution sheets of willemite in tephroite

The fine lines are visible in white light too

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