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[Peter Perkins]

This specimen of calcite came from the Trepca mine. I have many examples of calcite in my collection - rhombs, scalenohedra, Nail Head Spar, etc - but nothing like this. Can someone tell me what form this is showing. Crystals are hexagonal with a flat end. Where damaged it shows typical rhombic cleavage.

[Pete Richards]

These crystals appear to be hexagonal prisms modified by (basal) pinacoids. There are two hexagonal prisms of calcite; which one is present here cannot be determined without looking at where the cleavage lies relative to the prism. See the drawing below, in which the position of the cleavage facets is indicated by the yellow faces.

The two crystals are drawn with the same orientation of their crystallographic axes, so the cleavage is in the same orientation in both. The prism {100} is oriented at 30° rotation about the c-axis relative to the prism {110}, so while they both have hexagonal cross-sections, they appear in different positions relative to the drawings and relative to the cleavage.

[Peter Perkins]

To Pete Richards
Many thanks for your reply. Examination of cleavage shows that my crystals are comparable to the left-hand diagram. P.J.P.