DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Prehnite mineral. The mineral prehnite is a hydrated calcium aluminium silicate (Ca2.Al.(Al.Si3.O10).(OH)2). Its crystal system is orthorhombic, but prehnite rarely displays well-developed crystals. It usually occurs as softly green globular or stalactitic precipitates that are formed inside fractures and voids in igneous and metamorphic rocks. It often occurs associated with zeolites. This sample, from Patterson in New Jersey, USA, is 40 centimetres across.
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