DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Apophyllite crystals. Apophyllite is a general term for the three separate minerals fluorapophyllite, hydroxyapophyllite, and natroapophyllite. All apophyllites consist of hydrated calcium, potassium and sodium silicates, fluorides and hydroxides. The most common form has a tetragonal structure (shown centre). Apophyllite is a hydrothermal mineral found filling cavities in igneous rocks such as basalt.
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