It was not common carbonate of lime, for the crystals were rhombohedral and did not show the cleavage of calcite.
Siderite, or spathic iron ore, FeCO3, crystallizes in the rhombohedral system and contains 48.28% of iron.
The second species, or rhombohedral calamine, is a carbonate of zinc.
A stereographic projection of a rhombohedral crystal is given in fig. 72.
The habit of the crystals may be rhombohedral, pyramidal or tabular, rarely prismatic.
Hexagonal axes of reference are used: if rhombohedral axes be used many of the simple forms will have two sets of indices.
The crystals belong to the hexagonal system, and are generally of rhombohedral habit, sometimes twinned.
In composition it differs entirely from sapphire, or the rhombohedral corundum.
Haematite crystallizes in the rhombohedral system, and is isomorphous with corundum (Al2O3).
There are two divisions of the hexagonal system; the hexagonal system properly so-called, and its rhombohedral division.