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Review of Basic Chemistry and Geology
Published in Arthur W. Hounslow, Water Quality Data, 2018
Shape. Euhedral—having well-developed crystal faces.This indicates cooling and crystallization at widely separated centers in the rock mass.Anhedral—minerals crystallized without having definite crystal faces.This indicates crystallization of minerals in close proximity to each other, thus interfering with each other as they cool.
Mineral Crystals
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
As described in the previous chapter, mineral crystals may be euhedral, subhedral, or anhedral. Euhedral crystalline materials have visible crystal shapes with flat crystal faces oriented at specific angles to each other. The properties and orientations of the faces are controlled by atomic arrangement in unit cells and the way unit cells stack together to make the entire crystal. Some mineral crystals are subhedral—they show a few, generally imperfect crystal faces. Most mineral crystals, however, are anhedral and do not show crystal faces at all. Nonetheless, they must contain unit cells of fixed composition with ordered atomic arrangements.
Optical mineralogy
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
Crystals whose outlines in thin section show well defined straight edges, which are slices through the faces of the crystal, are described as euhedral crystals (Figure 1, Figure 2); crystals which have no recognizable straight edges are anhedral and crystals with some straight edges and others curved are subhedral.
Rocky relationships: the petroglyphs of the Murujuga (Burrup Peninsula and Dampier Archipelago) in Western Australia
Published in Australian Journal of Earth Sciences, 2019
E. R. Ramanaidou, L. C. Fonteneau
The elongated, prismatic and lamellar clinopyroxenes crystals (augite) belong to the diopside–hedenbergite series with an Mg/Fe ratio of 1.25 diagnostic of tholeiites. The observed lamellae along the cleavages are chloritic with rare actinolite. Quartz crystals (mean size 300 µm) crystallise between augite and labradorite (Figure 8). Hickman (2001) interprets the interstitial quartz as owing to inclusions of remelted granitoids at the base of the gabbroic unit. Sub-euhedral ilmenites (Table 2), replaced by sphene (Table 2) and rutile along fractures, are commonly associated with euhedral apatite (Table 2) disseminated in the rock. In summary, all minerals described here are igneous, or metasomatic equivalents, and were formed by fractional crystallisation from an enriched tholeiitic magma. This gabbro also suffered hydrothermal alteration, with some slight clastic deformation. Secondary minerals (Table 2) include dominant chlorite and amphibole and minor epidote, phengite, rutile and barite. Chlorite, as ripidolite, replaces augite and labradorite along the cleavages and around sphene (Figure 8).
Geology and field relations of the Wilsons Promontory batholith, Victoria: multiple, shallow-dipping, S-type, granitic sheets
Published in Australian Journal of Earth Sciences, 2018
Concentrations of K-feldspar phenocrysts, and less abundant plagioclase crystals, occur in the NPBM, but are not associated with the enclave concentrations. The feldspar crystals in these concentrations are larger than in the typical NPBM—K-feldspar ∼5 cm and plagioclase ∼3 cm long. These concentrations contain up to 80 vol% feldspar crystals and form steeply dipping lenticular masses 1 to 2 m wide and ≥7 m long. The granitic matrices are otherwise mineralogically similar to normal massive NPBM. Such concentrations of K-feldspar certainly arose through mechanical accumulation during constricted magma flow in pipes or dykes (e.g. Paterson, Vernon, & Zak, 2005). Much of the liquid magma was probably withdrawn, leaving behind these feldspar ‘log jams.’ The feldspar crystals are euhedral, with microperthitic K-feldspar commonly rimmed by plagioclase in a rapakivi texture. Phenocryst preferred orientations are parallel to the margins of the concentrations but weak in the larger lenses. Despite their high concentrations, there appears to be minimal interlocking of adjacent feldspar crystals, at least in two dimensions.
Mineralogy and character of the Liikavaara Östra Cu-(W-Au) deposit, northern Sweden
Published in GFF, 2020
Mathis Warlo, Christina Wanhainen, Olof Martinsson, Peter Karlsson
Findings of this study support an igneous origin of the wall rocks suggested by both Zweifel (1976) and Estholm (2014). Geochemical data clearly show an andesitic-basaltic composition of the footwall rocks and an andesitic composition of the hanging wall rocks (Fig. 6A, C, D). A volcaniclastic character of the wall rocks is favoured based on the dominant porphyritic texture with partly euhedral phenocrysts and varying abundance of several centimetre large clasts with mineralogy similar to that of the surrounding rock. Apparent sedimentary features like graded bedding could be caused by a variation in the intensity of eruption and pyroclastic flow but may also be the result of sub-aqueous re-working.