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They used optical techniques to examine each flake, looking specifically for five-layer flakes, arranged naturally in a rhombohedral pattern.

More specifically, lead titanate changes shape from tetragonal to rhombohedral through monoclinic intermediates under pressure.

Those with a triad axis are grouped together in the rhombohedral or trigonal division, and those with a hexad axis in the hexagonal division.

The habit of the crystals may be rhombohedral, pyramidal or tabular, rarely prismatic.

GRAPHITE, a mineral species consisting of the element carbon crystallized in the rhombohedral system.