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Natural Diamond
Published in Mark A. Prelas, Galina Popovici, Louis K. Bigelow, Handbook of Industrial Diamonds and Diamond Films, 2018
Henry O. A. Meyer, Michael Seal
Kimberlite is a hybrid igneous volcanic volatile-rich potassic ultramafic rock. The major constituent minerals (some of which may not be present) include olivine, phlogopite mica, calcite, monticellite, apatite, ilmenite and spinel. These minerals usually form a fine grained matrix in which larger minerals (referred to as megacrysts) such as olivine, garnet, ilmenite, pyroxene, mica and diamond occur. Some of these megacryst minerals are foreign to kimberlite and represent minerals from disaggregated deep mantle rocks probably torn from the sides of the conduit through which the kimberlite passed on its way to the surface. These specific minerals are known as xenocrysts and include garnet, ilmenite, pyroxene and diamond. Thus it is important to realize that diamond does not form in kimberlite but rather that kimberlite is the transporting medium that carries diamond from its place of formation (about 200km depth) to the Earth’s surface. Lamproite also has a similar role as a transporting agent for diamond. The other xenocryst minerals (garnet, ilmenite, pyroxene) are important as they are used extensively as diamond indicator minerals in geochemical exploration for kimberlites and diamond.
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
Plutonic rocks, which have cooled slowly under a cover perhaps several kilometres thick, are coarsely crystalline or phaneritic; their component crystals are large (2 to 5 mm or more) and can easily be distinguished by the naked eye (Fig. 5.16a and b). Rocks of medium grain often have crystals between about 1 and 2 mm, and in fine-grained rocks crystals may be considerably less than 1 mm across. When the texture is so fine that individual crystals cannot be distinguished without the aid of a microscope it is called aphanitic, or microcrystalline. For extremely fine-grained rocks, when their crystalline character is only revealed by viewing a rock slice through crossed polars, which enables the birefringent colours of each embryo crystal to be displayed, the term cryptocrystalline is used. These textures are all even-grained, or equigranular, i.e. having crystals of much the same size (Fig. 5.16a); but some rocks show the porphyritic texture (Fig. 5.16b), in which a number of larger crystals are set in a uniformly finer base (or groundmass). The large, conspicuous crystals are called porphyritic crystals or megacrysts. Porphyritic feldspars in some granites, for instance, may be 5 to 10 cm long.
North wall stability analysis of Vasilkovskoye open pit Kazakhstan
Published in Reginald E. Hammah, Thamer E. Yacoub, Alison McQuillan, John Curran, The Evolution of Geotech - 25 Years of Innovation, 2021
The main observed rock types are as follow: Megacrystic granodiorite with abundant K-feldspar megacrysts. This is the main host rock for the mineralization.Quartz diorite and gabbro-diorite. These rock types contribute to a lesser extent to the mineralization.Mineralization is hosted mainly by megacrystic granodiorite and it is associated with hydrothermal quartz and quartz-arsenopyrite veins.
Petrogenesis of amphibole megacrysts in lamprophyric intraplate magmatism in southern New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2020
Simon H. Serre, Quinten H. A. van der Meer, Tod E. Waight, James M. Scott, Carsten Münker, Tonny B. Thomsen, Petrus J. le Roux
The lamprophyre model. Considering tentative geothermobarometric estimations, megacrystic amphibole may have crystallised at lower crustal depths. Thus, low-Mg# amphibole megacrysts likely fractionated from low-Mg# lamprophyric melts in lower crustal magma-plumbing systems. Repeated melting of an isotopically moderately heterogeneous source region initiated the injection of more primitive lamprophyric host melts and promoted repeated magma mingling in lower crustal settings, resulting in both the variable Mg# and isotopic spread in megacryst cores as well as the high Mg# in megacryst rims and microphenocrysts. Alternatively, QH12 and KST-2 amphibole megacrysts may have fractionated in magma reservoirs from precursor low-Mg# lamprophyric melts that remained emplaced in deeper settings than the investigated WDS lamprophyric host rocks. Subsequent ascent of WDS host melts may have reactivated such reservoirs and collected xenocrystic cumulate amphibole megacrysts. Models involving different generations of lamprophyric melts in the WDS are further supported by the fact that the KST-2 lamprophyre host rock is part of a composite dike showing diffuse contacts with an associated amphibole megacryst-bearing phonolitic unit (Waight et al. 1998a; van der Meer et al. 2013), indicating the simultaneous presence of magmas of contrasting compositions and indicating that some degree of magma mixing and/or magma reservoir reactivation could have occurred in the WDS. This is also consistent with the presence of oscillatory zoning and sieve textures in plagioclase phenocrysts in doleritic members of the Hohonu Dike Swarm (Waight 1995)
Cretaceous tungsten-tin mineralisation in the Tin Range, Stewart Island, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2022
Hamish C. Lilley, James M. Scott, Josh J. Schwartz, Rose E. Turnbull, Andy J. Tulloch
The Tin Range granodiorite includes both equigranular (3–5 mm) and megacrystic areas (1–3 cm) with a dominant mineralogy of K-feldspar, plagioclase, quartz, biotite, white mica, garnet and apatite (Figure 4D,E) (Henley and Higgins 1977). The megacrystic parts include phenocrysts of either euhedral to subhedral almandine-spessartine garnet (semi-quantitatively: Fe80Mn17Ca3) ranging from ∼0.5 to 1 cm in size or 1 to 3 cm megacrystic aggregates of subhedral to anhedral K-feldspar and plagioclase. The feldspars are generally near endmember orthoclase and albite, but perthitic feldspar with ∼10 µm lamellae is also present in minor quantities.