Scalenohedral calcite

A priori considerations. An extensive review of the form and mineralogy of the secondary cave deposits is provided by Hill and Forti (1997), but also see Ford and Williams (1989). By far the most useful speleothems for geological purposes are the most common forms stalactites, stalagmites and flowstones composed of calcite, and occasionally aragonite. Other deposits that have been used to derive useful information include travertines, gypsum crusts, calcite veins and scalenohedral calcite (dog-tooth spar). 

Figure 2.14. Different crystalline forms of calcium carbonate. Courtesy of Omya/Pliiss-Staufer AG (micrographs of crystals), Solvay, GmbH, Rheinberg, Germany (crystal stmcture and micrographs of Socal trigonal-scalenohedral calcite), and ECC International Ltd., St. Austell, UK (rhombohedral calcite and aragonite).

The [hkil] faces, which determine the dog-tooth (scalenohedral) Habitus of calcite crystals, and the prismatic [hkil] face of tourmaline crystals show striations only, never step patterns. These faces are S faces, by PBC analysis, and they appear due to a pile up of steps developing on the neighboring F faces. Yet they develop as large as those that determine the Habitus. 

Figure 11.5. Three examples showing different Habitus of later-grown calcite crystals preferentially nucleated on the edges and corners of earlier-formed calcite crystal. Changes in Habitus depending on crystallization stages or growth temperatures are indicated [3]. (a) Earlier-formed hexagonal prism (A) and later-formed scalenohedral crystal (B). (b) Earlier-formed hexagonal prism (b) and later-formed thicker crystal (a), (c) The shaded area shows an earlier-formed rhombohedral crystal, and the remaining area represents later-formed scalenohedral crystals.

In the [0111] direction formed by rhombohedral (1010) and scalenohedral (1231) faces of calcite... 

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