oolicast (o-ol'-i-cast). One of the small, subspherical openings found in an oolitic rock, produced by the selective solution of ooliths without destruction of the matrix. The term is inappropriate unless the opening is subsequently filled. See also: oomold. Syn: oocast.

pyrite (py'-rite). (a) A common, pale-bronze or brass-yellow, cubic mineral: FeS₂ . It is dimorphous with marcasite, and often contains small amounts of other metals. Pyrite has a brilliant metallic luster and an absence of cleavage, and has been mistaken for gold (which is softer and heavier). It commonly crystallizes in cubes (whose faces are usually striated), octahedrons, or pyritohedrons, and it also occurs in shapeless grains and masses. Pyrite is the most widespread and abundant of the sulfide minerals and occurs in all kinds of rocks, such as in nodules in sedimentary rocks and coal seams or as a common vein material associated with many different minerals. Pyrite is an important ore of sulfur, less so of iron, and is burned in making sulfur dioxide and sulfuric acid; it is sometimes mined for the associated gold and copper. Cf: pyrites. Syn: iron pyrites; fool's gold; mundic; common pyrites. (b) A group name for minerals isomorphous with pyrite, with the general formula AX₂ , where A = Fe, Co, Ni, Mn, Cu, Au, Ru, Os, Ir or Pt, and X = S and, more rarely, Se, Te, Sb or As.

shandite (shand'-ite). A metallic white rhombohedral mineral: Pb₂Ni₃S₂ .

triangular facet . As a physiographic feature, a triangular face having a broad base and an apex pointing upward; specif. the face on the end of a faceted spur, usually a remnant of a fault plane at the base of a block mountain. A triangular facet may also form by wave erosion of a mountain front or by glacial truncation of a spur. Syn: spur-end facet.