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Euhedral (also known as idiomorphic or automorphic) crystals are those that are well-formed, with sharp, easily recognised faces. The opposite is anhedral (also known as xenomorphic or allotriomorphic ), which describes rock with a microstructure composed of mineral grains that have no well-formed crystal faces or cross-section shape in thin ...
Anhedral crystals do not, usually because the crystal is one grain in a polycrystalline solid. The flat faces (also called facets) of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: they are planes of relatively low Miller index. [10]
Crystal shape is also an important factor in the texture of an igneous rock. Crystals may be euhedral, subeuhedral or anhedral: Euhedral or automorphic, if the crystallographic shape is preserved. Subeuhedral or Subhedral, if only part is preserved. Anhedral or xenomorphic, if the crystals present no recognizable crystallographic forms.
Carlosruizite occurs as thin, colorless to pale yellow, transparent, euhedral, platy crystals with hexagonal outline, generally <200 μm in diameter and 20 μm thick. The crystals have a pseudorhombohedral habit, flattened on {0001} with beveled edges {1012}.
Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape. The most common porphyroblasts in metapelites (metamorphosed mudstones and siltstones ) are garnets and staurolites , which stand out in well- foliated metapelites (such as schists ) against the platy mica matrix.
Euhedral crystal of augite from Teide (4.4 x 3.0 x 2.3 cm). Augite is a solid solution in the pyroxene group. Diopside and hedenbergite are important endmembers in augite, but augite can also contain significant aluminium, titanium, and sodium and other elements.
Crystal habit: As euhedral crystals or columnar aggregates having nearly square cross sections; fibrous compact to massive: Twinning: On {101}, rare: Cleavage: Good on {110}, poor on {100} Fracture: uneven to subconchoidal: Mohs scale hardness: 6.5–7.5: Luster: Vitreous: Streak: White: Diaphaneity: Transparent to nearly opaque with inclusions ...
Phenocrysts often have euhedral forms, either due to early growth within a magma, or by post-emplacement recrystallization. Normally the term phenocryst is not used unless the crystals are directly observable, which is sometimes stated as greater than 0.5 mm (0.020 in) in diameter. [1]