Prehnite casts of Anhydrite (#54)

Lustrous translucent pale green hollow casts in jackstraw form.
Asia Khandiuvali Bombay Area, Maharashtra State, India 5 ¼" x 3 ¾"x 2 ½"

 

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The label for this specimen names two minerals: prehnite [Ca2Al(AISi3O10)(OH)2] and anhydrite (CaSO4). The composit ions are not very much alike. So which is it, prehnite or anhydrite? Anhydrite crystallizes in long, slender crystals. Prehnite tends to form granular or rounded aggregates. So this looks like anhydrite, right? Correct, it does. But it is prehnite.

Note the word "casts" in the label. A cast, geologically speaking, is what is formed by a mold. Think of a Jello mold. The container is the mold, and the Jello is the cast. The cast has the shape of the mold. Most casts are formed inside a mold, but not this one. Technically, this is what is called an epimorph - from the Greek επι, "upon." One mineral grew over another, and the first dissolved away, leaving a hollow cast that is the shape of the original mineral. In this case, prehnite coated crystals of anhydrite after they had formed, and the anhydrite was dissolved. So the mold, in this case, is an interior mold, rather than an exterior mold (as for Jello). Something similar is seen in Specimen #38, but in that case the original mineral is not known.