Any of a subgroup of minerals of the pyrochlore group, found in various pegmatites, that are complex oxides of tantalum and niobium with sodium and calcium, often with small amounts of other elements such as uranium and titanium, crystallizing in the cubic system as yellow, brownish, or blackish translucent crystals.

A microscopic acicular crystal with determinable optical properties, found in microcrystalline rocks and groundmasses.