Definition of "trigonal" [trig•o•nal]

• Symmetrically triangular (adjective)

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• "Both have identical major ion chemistry CaCO3 but different structures: calcite is trigonal whilst aragonite is orthorhombic."
• "-- Here there is rotation of one portion with respect to the other through 180° about the principal (trigonal) axis, which is perpendicular to the plane c (111); or the same result may be obtained by reflection across this plane."
• "The horns are trigonal, laterally compressed and knotted on the upper edge."