Anhedral crystals

The U-Th minerals are large (>mm) anhedral crystals of Th-rich uraninite or uranothorianite with lobate contours. High Y2O3 contents (0.56 0.1 wt%) are also distinctive of the U-Th mineralization. 

XENOTOPIC. The fabric of a crystalline sedimentary rock in which the majority of the constituent crystals are anhedral. Fabric found in evaporites. chemically deposited cement, and recrystallized limestone or dolomite. 

Pyrite and marcasite are the major minerals forming the sulfide cement, as identified by XRD and optical microscopy. These sulfides occur as both well-formed cubes and anhedral masses. Arsenic-rich areas (up to 1% by weight as estimated by EDS) occur in the pyrite and marcasite crystals as well as in iron hydroxides, but no separate arsenopyrite phase has been identified. Colloidal size (10-20 nm) iron hydroxide phases were identified using TEM. TEM-EDS analysis showed qualitative differences in arsenic, nickel, and zinc in the iron hydroxides on a nanometer scale. 

Granular Aggregates of anhedral crystals in matrix Scheelite... 

Calcite occurs as cement zones 10-150 cm thick. Pore-filling calcite crystals are anhedral blocky spar crystals (0.1-0.5 mm). In general, the petrographic appearance of basin margin calcite crystals is no different from calcite cements occurring in the basin centre. Some samples exhibit moderate to strong luminescence, whereas others are uniformly non-luminescent. 

Color/Luster Colorless to grayish white, also yellow to brown or black due to iron or foreign inclusions. Its luster is vitreous and transparent. Crystal System/Habit Isometric. Usually occurs as anhedral to subhe-dral crystals in a matrix. 

The severe acid treatments separated the aggregates into single crystals in a size range of 0.5-10/un with an average mean diameter of 3/xm. Twenty per cent of the crystals are euhedral acicular and the remainder are equant and anhedral. 

Some rocks yield only fractured and/or anhedral crystals. This seems especially common in gabbros, where apatites are often seen to be space-filling rather than euhedral. Such grains are difficult to use both because they make the a-ejection correction very difficult and because it is not obvious how to determine the original grain size. As a result the diffusion characteristics, which scale with grain size, are not well known. 

Photograph 7-82 Free lime in clinker thin section. Crystals exhibit prominent cubic cleavage characteristic of free lime. An abundance of free-lime nests (some showing original limestone particle outline), anhedral alite, and extremely nonuniform distribution of silicates suggests poorly mixed coarse feed. Dry-process kiln, coal fired. (S A6700)... 

Facial development on crystals—euhedral, subhedral, anhedral areas of preferential development in a single clinker. 

Anhedral adj. Anhedral crystals are those whose growth has been impeded by adjacent crystals growing simultaneously, so that the development of plane faces is inhibited. Also crystals eroded, partly dissolved, or mechanically deformed to the point where nearly all traces of crystal faces have been removed. All anhedral crystals are irregularly shaped and do not have plane faces, cf, euhedral. 

Euhedral adj. Euhedral crystals are those that are bounded by plane faces, cf., anhedral. 

The feldspar minerals have similar physical properties and often occur as prismatic or tabular crystals in igneous rocks, or as more anhedral grains in metamorphic and sedimentary rocks. They are colourless when fresh but are more commonly white due to incipient alteration impurities or inclusions result in coloured varieties, with green-brown alkali feldspars found in some metamorphic rocks, and orthoclase commonly found as pink. The surfaces of feldspar crystals are often iridescent due to twinning on a microscopic scale, with labradorite characterised by blue surface iridescence. Feldspars readily alter under hydrothermal action or chemical weathering to form members of the clay minerals group (. v.). Sodium-rich feldspars commonly decompose to form montmorillonite, in the presence of limited water, or to kaolinite with excess water alkali feldspars typically form illite or kaolinite sub-group (qq.v.) clay minerals (Deer et al, 1992 Rutley, 1988). 

Matraite is a zinc sulfide mineral with composition ZnS. It was first described by Koch in 1958 from Gyongyosoroszi (Matra Mountains, Hungary) and is still only known from this locality. It occurs as aggregates of brown to brown-yellow anhedral or hemimorphic pyramidal crystals. It is a rare mineral in nature the iron-bearing varieties of zinc sulfide, sphalerite and wurtzite (qq.v.), are more commonly encountered. 

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