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Igneous Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
A central eruption builds a volcano that has a cone with a summit crater connected to the volcanic ‘pipe’, through which are ejected lava, gases, and fragments of exploded lava (ash) and broken rock. Vesuvius, Etna, and Strom-boli in the Mediterranean region, Popocatapetl, and Cotopaxi in the Andes, the Mounts St Helens, Rainier, Crater Lake, Shaster and Lassen Peak in the Cascades of N. America, and other active volcanoes of the present day belong to this type. When the dissected volcanic cones of central eruptions that were active in past geological ages are now exposed to view after long denudation, their structure can be studied in detail; Arthur’s Seat at Edinburgh and Largo Law in Fife are examples of such ‘fossil volcanoes’.
Identifying critical parameters in the settling of African kimberlites
Published in Mineral Processing and Extractive Metallurgy Review, 2018
E. T. Boshoff, J. Morkel, N. Naude
Kimberlite is the host rock from which diamonds are mined. There are to date up to 4000 different kimberlites and lamproites intrusions identified, and more are identified each year due to the systematic exploration program carried out by the various diamond exploration companies. Various attempts have been made to classify kimberlites according to the different mineralogical compositions. Mitchell (1986) classifies three groups of kimberlites according to the amount of olivine, phlogopite and calcite, kimberlite (equivalent to basaltic kimberlite), micaceous kimberlite (equivalent to lamprophyric kimberlite), and calcite or calcareous kimberlite. Skinner and Clement (1979) classify kimberlites into five groups according to the predominance of diopside, monticellite, phlogopite, calcite, and serpentine present. Serpentine is formed over centuries during the alteration process of olivine and is one of the main minerals in blue ground. Blue ground is unweathered kimberlite found deeper in the volcanic pipe. Serpentine can account for 20–50% of the groundmass in the mineral and chemically breaks down into the clay minerals of smectite, vermiculite, calcite, chlorite, and talc during the weathering process (Hopwood et al., 1975).