A Monchiquite from Mount Girnar, Junagarh (Kathiawar)

Abstract

I. Monchiquites and their Isotropic Groundmass The term monchiquite is now recognized as the designation of a rock, consisting mainly of ferromagnesian silicates in an isotropic groundmass, which has approximately the chemical composition and specific gravity of analcime. The name was given in 1890 by Hunter & Rosenbusch [10] 1 to a rock occurring in narrow dykes, near Cabo Frio in the Serra de Tingua and elsewhere in Brazil, and containing pyroxene, soda-hornblende, mica, and olivine-phenocrysts in a colourless—or, rarely, transparent brown—matrix. This material, which they assumed to be a glass, contains numerous microlites of the porphyritic minerals and, when in a fresh condition, is completely isotropic. It has a specific gravity of 2·31. In a few cases nepheline and, rather more frequently, a felspar with twin lamellation occur. The groundmass is very subject to alteration, natrolite and analcime being formed [20] p. 539 & [24] p. 233. In one instance fluidal structure was noticed to be present. The rock was distinguished from camptonite on account of its glassy base, and received the name of monchiquite, as L. van Werveke had described a similar rock from the Serra de Monchique in Southern Portugal [7]. Rocks with a groundmass of the same character, but without olivine, have been reported from Bohemia by Bořicky [4,5] and Hibsch [15]. Nepheline [5] p. 176, and leucite [15] p. 99, are sometimes present. J. R. Williams [11] described similar rocks from Arkansas, in which he believed a glassy base had once existed but had since become