Allomorph

Part of speech: noun (n.)

1. Any one of two or more distinct crystalline forms of the same substance; or the substance having such forms; -- as, carbonate of lime occurs in the allomorphs calcite and aragonite.

2. A variety of pseudomorph which has undergone partial or complete change or substitution of material; -- thus limonite is frequently an allomorph after pyrite. G. H. Williams.