Aragonite is a polymorph of calcite, a mineral that has the same chemical composition as calcite, but has a slightly different crystal structure. For most purposes aragonite and calcite can be considered to be the same mineral. In its crystal form, aragonite tends to have needlelike crystals and lacks calciteâs rhombohedral cleavage. In massive form, it is difficult to distinguish the two minerals in the field, as both react vigorously with cool dilute acid.