A Preliminary Report on the Mineralogy of the Norton County, Kansas‐Furnas County, Nebraska, Achondrite (1000,400)

Abstract

ABSTRACTThis paper presents a preliminary report on the friable enstatite achondrite, with nickel‐iron inclusions, more than a ton of which fell in Norton County, Kansas, and Furnas County, Nebraska, on 1948 February 18 (C.N. = 1000,400). Megascopically, the achondrite resembles a rhyolite porphyry, the fine‐grained groundmass of grayish‐white color containing phenocrysts of 2 types, the first, a light‐gray mineral exhibiting cleavage at angles of 88° and 92°, the second, a clear, glassy mineral which does not cleave or part, but rather shatters with a conchoidal fracture. Under closer examination, both the groundmass and the phenocrysts of the stone are found to consist chiefly or solely of a very pure enstatite. In addition to inclusions of nickel‐iron, the achondrite contains abundant small flakes of graphite, and a brown, platy mineral, identified tentatively as diallage. For both the cleavage enstatite and the glassy enstatite, α = 1.652, β = 1.654, γ = 1.660; and, furthermore, the other optical constants are the same, except that the axial angle found for the glassy enstatite (80° to 85°) is more than twice that found for the cleavage enstatite (40°). Primary, secondary, and tertiary fusion crusts, ranging in thickness from at least 17.7 mm. down to less than 0.2 mm., have been found on the achondrite. Those portions of the fusion rind so far examined microscopically in oil immersions were found to be extremely fine‐crystalline, and not glassy, and they had an average index of refraction of 1.656.