Fluorite with Calcite and Sphalerite (#65)

Lustrous gem to gemmy colorless cubes of fluorite to 7/8" on edge with flattened calcite rhombs to ½" and shiny black opaque twinned sphalerites. A very aesthetic sample.
Asia Second Sovietski mine, 135 meter Level, Dal'Negorsk, Primorsky Krai, Russia 4 ¼" x 4 ¼" x 3"

 

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First, an explanation of some terminology found in the label for this specimen:

1. Fluorite (CaF2) and calcite (CaCO3) are minerals we've seen in numerically earlier specimens. Sphalerite is ZnS, a sulfide mineral and the major ore of zinc.

2. "Rhomb" is short for rhombohedron, a common crystal form for calcite. Think of a cube and imagine that it is flexible. Now imagine taking that cube between your thumb and index finger by opposite corners - that is, two corners that are as far apart as two corners can be - and squeezing or stretching the cube. It then becomes a rhombohedron, and if you continue to squeeze, it becomes a “flattened rhomb."

3. What about the word "twinned"? Twinning is quite common among minerals. As a crystal is growing (i.e., atoms are being added to it), a mistake may be made that causes the remainder of the crystal to take off in a different direction. The original crystal and its twin are related by a reflection - as if a mirror were placed between them - or by some rotation.

Sphalerite, ZnS, has the same crystal structure (arrangement of atoms) as diamond, with carbon atoms in diamond being alternately replaced by zinc and sulfur atoms, but otherwise, they could hardly be more different. Iron is common impurity element in sphalerite, substituting for zinc. As the amount of iron increases, the color goes from pale yellow to dark, metallic near-black, as in this specimen.