Druses

Druses

Minerals

Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017

Quartz is an essential constituent of granites, and can be recognized in the rock as hard, glassy grains of irregular shape and without cleavage. It occurs in smaller amount in granodiorite and quartz-diorite, and is present as well-shaped porphyritic crystals in acid dyke-rocks and lavas. Vein quartz is an aggregate of interlocking crystals of glassy or milky appearance, filling fractures in rocks; the boundaries of the crystals may be coated with brown iron oxide. Well-shaped quartz crystals are found in cavities (druses) in both veins and granitic rocks. Most sands and sandstones have quartz as their main constituent; the grains have a high resistance to abrasion and thus persist over long periods during erosion and transport. The mineral is also found abundantly in gneisses, quartzites, and in some schists and other metamorphic rocks.

Gibbsite as low-temperature hydrothermal overprint on lateritised Cenozoic sediments, Quadrilátero Ferrífero of Minas Gerais, Brazil

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Journal Information

Published in Applied Earth Science, 2020

Alexandre Raphael Cabral, Matheus Luís de Sales Oliveira, Thaís Keuffer Mendonça

The veins differ from the host rock by the presence of coarse-grained gibbsite, which was identified by powder XRD, BSE imaging and EDS. The mineral distinctively occurs as euhedral, pseudohexagonal, coarse-grained crystals, generally between 50 and 150 µm across, decorating cavities that resemble druses (Figure 3). The druse-like cavities are disseminated in the vein clayey matrix. They are abundant in the veins, but absent or rarely observed in the host red diamictite. Clay-sized gibbsite is a main component of the vein matrix and the wall-rock matrix.

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Minerals

Gibbsite as low-temperature hydrothermal overprint on lateritised Cenozoic sediments, Quadrilátero Ferrífero of Minas Gerais, Brazil