Apophyllite (#31)

Lustrous gemmy translucent green crystals to 1".
Asia Poona, Maharashtra State, India 6 ½" x 4 ¼" x 2"

 

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Apophyllite belongs to a class of silicates (that is, minerals based on SiO. Atomic groups) known as phyllosilicates, or equivalently, sheet silicates. The name phyllosilicate comes from Greek, meaning "leaf," and this very large class of minerals is so called because its members cleave easily into very thin plates or sheets. The chemical formula of apophyllite is not straightforward, and here is why:

Many minerals can occur in a specific range of compositions. The extremes of such ranges are called the "end-members." Between those end-members, the compositions occur in what is called a "solid solution" - some of the chemical elements can substitute freely for one another to form compositions in between those of the endmembers. The end-members of the apophyllite solid solution series are:

KCa4Si8O20(F) · 8H2O called fluorapophyllite-(K)
KCa4Si8O20(OH) · 8H2O called hydroxyapophyllite-(K)
NaCa4Si8O20(F) · 8H2O called fluorapophyllite-(Na)

The "fluor'' prefix indicates that the mineral contains fluorine, and "hydroxy" indicates that (OH) occurs where the F would have otherwise been. K and Na (potassium and sodium, respectively) are the characteristic large atoms in the crystal structure, but these, too, can substitute for one another. So the general formula expressing the solid solution might be written: (K,Na)Ca4Si8O20(F,OH) · 8H2O.

As someone has said, "Mother Nature is a sloppy chemist." She just works with the elements at hand and mixes and matches them any what they will fit.