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Calcite, (w. Duftite)


Mineral: Calcite, Duftite
Chemistry: CaCO3
Size: 108 mm.
Owner: Geoff Krasnov
Photographer: Geoff Krasnov

An interesting example of orientated inclusions of duftite(?) in colourless calcite crystals. Three stages of crystallisation appear to have been involved:

Calcite crystallised initially as flattened rhombohedra, stacked in parallel growth to form elongated composite crystals with an overall columnar habit. A vertex of the uppermost rhombohedron, with three convergent crystal edges, formed the termination of the crystal at this time.

The second stage of the process was the crystallisation of duftite(?), which formed preferentially over the terminations of these columnar stacks of flattened calcite rhombs. In this specimen the duftite(?) appears to be intimately mixed with a second generation of calcite, which takes the form of very small sugary rhombohedral crystals and which, like the duftite(?), formed preferentially over the first generation terminations, resulting in the characteristic "Mercedes star" pattern (Photograph 2; FOV=35mm).

Finally, a late phase of rhombohedral calcite overgrew the encrusted terminations, so that the duftite(?) and sugary calcite combination appear as a crystallographically orientated three-point star, through the colourless third generation calcite crystals and therefore inside of the crystal terminations. Interestingly, there are some sparsely distributed inclusions of a brownish mineral (unidentified) within this final generation of otherwise colourless calcite (Photograph 3; FOV = c. 10mm).

In fairness, the zones of inclusions and their symmetry are less than perfectly developed in this particular specimen, but the habit of the crystals serves to illustrate the crystallisation sequence rather well.  Please note that the duftite(?) on this specimen has not been verified by analysis.