Euhedral crystals

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

In the Taupo volcanic zone of New Zealand, the 26.5 ka Oruanui eruption was studied by Charlier and Zellmer (2000). Three fractions of zircons (sub 63 pm 63-125 pm 125-250 pm) were extracted from the rhyolitic pumice, which together with the whole rock respectively define three ages from 5.5 to 12.3 ka before eruption (Fig. 12b). Microscopic observation of the zircons showed that they are composed of a core surrounded by euhedral rims, and the preferred explanation of the authors is that zircons represent mixtures in variable proportions of old crystal cores crystallized 27 ka before eruption and crystal rims crystallized just before eruption. 

The outer 1.5 mm is a mantle of polycrystalline melilite whose Mg content increases radially inwards, as does the abundance of included spinel. The interior of the inclusion is predominantly coarsely crystalline Ti-Al-pyroxene, melilite, and anorthite, all containing euhedral crystals of spinel. The entire inclusion is bounded by a fine-grained rim of complex mineralogy. The inclusion is 1.5 cm in diameter. 

Prismatic A term commonly used in descriptions of minerals for crystals exhibiting aspect ratios usually below 3 and grading into equant (aspect ratio = 1). The term may refer to crystals embedded in a matrix, but is more commonly used to describe free-standing, euhedral crystals, whether micro- or macroscopic. 

The magnetite crystals are well developed (euhedral), and this ensures that they act as single magnetic domains (SD) and produce remanent magnetization in sediments. The average number of magnetite crystals/cell in 220 cells of the microaero-... 

A record of morphology classes for each pyrite occurrence was kept during petrographic analyses. Monocrystalline pyrite includes euhedral and subhedral pyrite crystals. This morphology class is always more prevalent than framboidal pyrite except at the top of core 1 and the bottom of core 3. 

Euhedral a crystal that is completely bounded by well-developed crystal faces whereby its growth is not restrained by adjacent crystals. 

As [ai] decreases, morphology evolves from spheroidal to euhedral hexagonal passing through intermediate cylinder-shaped crystals. The growth rate of the (001) face presents an activation energy of 23 kcal/mol and is proportional to [ai]. For the growth of the (hkO), surface the activation ener y is 30 kcal/mol and the rate is proportional to [AI ]. These differences are... 

A better picture of the crystallization phenomena was obtained from the microscope examination of the crystals at different stages of their development. The first crystals of zeolite omega appeared as spheres with diameters around 0.5-1 pm (Fig.2). With the appearance of (001) faces, this form evolved into barrel-shaped particles (Fig.3). At a later stage of the syntheses, the habit evolved to euhedral hexagonal with the development of (100) faces (Fig.4). 

Figure 17 Backscattered electron images of three chondrules in Tieschitz (H/L3.6) chondrite. (a) Type lA porphyritic chondrule composed largely of forsteritic olivine (ol), mesostasis (mes), metallic Fe,Ni droplets (white). Dusty olivines are relict, FeO-rich olivines that crystallized elsewhere and formed tiny metallic Fe,Ni particles when heated in the chondrule melt, (b) Type IIA porphyritic chondrule containing large euhedral, FeO-bearing, olivine phenocrysts, dark mesostasis (mes), and white sulfide droplets (sf). (c) Nonporphyritic chondrule containing fine pyroxene crystals that appear to radiate from the upper edge of the chondrule.

Kakangari matrix contains mostly euhedral to subhedral crystals with grain sizes of 200 nm to... 

Figure 7 Replacement driven by force-of-crystal-Uzation is characterized by authigenic phases that develop euhedral faces that are not plausibly constmed as crystal growth within pore spaces (a) sphalerite replaces albitized detrital feldspar and adjacent portions of clay-rich matrix, Frio Formation, Oligocene, South Texas and (b) siderite crystal (s) attacks a detrital K-feldspar (K) in sandstone, Breathitt Formation, Pennsylvanian, eastern Kentucky.

Quartz occurs in dolomite crusts in Carlsbad Cavern, Lechuguilla Cave, and probably other nearby caves, and within unusual iron oxide-rich stalactites (the Rusticles ) in Lechuguilla Cave. Most of the crystals of quartz were euhedral and 10-200 pm in diameter however some exceeded 200 pm in diameter (Fig. 3). In the Rusticles of Lechuguilla Cave, quartz crystals, along with dolomite and calcite, formed cement in the central canal of these stalactites. Within the quartz were filaments of iron-... 

Chafetz, H.S. and Zhang, J. 1998, Authigenic euhedral megaquartz crystals in a Quaternary dolomite, J Sediment Res, 68 994-1000. 

Fig. 3.1.9. Phosphatic concretion from the valley of the Dniester River, Uk.R.S.S.R. showing euhedral crystals of a variety of apatite — presumably francolite — within a matrix of quartz. The concretions also contain glauconite, pyrite, etc. Magnification 67x. Reproduced with permis.sion (McConnell, 1950).

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