New Mexico Mineral Symposium — Abstracts

Lorenzenite, seariesite, and cancrinite are three of the uncommon minerals that occur as euhedral crystals in the phonolite sill which forms Point of Rocks mesa, Colfax County, N. Mex. These minerals are contained in late-stage gas cavities up to several centimeters in size, which are lined with projecting crystals mainly of alkali feldspar, acmite, and nepheline. The iorenzenite is a niobium-bearing variety, Na₂(Ti,Nb)₂Si₂O₉ it forms brown, transparent prisms less than 1 mm long. Lorenzenite was previously known from several alkalic massifs in the U.S.S.R., from Norway, and from Narsarsuk, Greenland.
Searlesite, a borosilicate, NaBSi₂O₅(OH)₂, forms colorless, prismatic to bladed crystals as much as 2 cm long. It is previously known only from boron-containing evaporite beds in California, Nevada, Wyoming, and the U.S.S.R.

Cancrinite occurs in alkalic, silica-poor igneous rocks, often with nepheline and sodalite. At Point of Rocks, it forms cylindrical or tapering hexagonal prisms as much as 2 cm long and 0.5-1 mm thick. Some of these crystals consist of a core of transparent or violet-tinged cancrinite (about 0.15 mm in - diameter) surrounded by a layer of gray, inclusion-filled natrolite about 0.10 mm thick, and covered by an outer layer of colorless, subhedral analcime about 0.02 mm thick. The cancrinite family is a group of hexagonal alkali aluminosilicate minerals, with the general formula (Na,Ca,K)_6-9(Si,Al)₁₂O₂₄[(S0₄),(CO₃), (Cl,0H)₂]_2-4*nH2O, which differ according to the dominant cations present (Na or Ca), anions present (CO₃, SO₄, Cl, or OH), and details of the crystal lattice (space group and number of atoms per unit cell). Point of Rocks cancrinite belongs to the species cancrinite sensu strictu. The formula inferred from microprobe analysis is near Na₈Al₆Si₆O₂₄(CO₃)_0.8(SO₄); it corresponds to 29-33 mole percent of the sulfatian end-member, vishnevite. The cancrinite is uniaxial (-) with ε = 1.494 and w = 1.500.