Euhedral

Pyrite is the most abundant ore mineral. It occurs as euhedral, framboidal, and colloform forms. Abundance of framboidal pyrite increases stratigraphically upwards. Colloform pyrite contains appreciable amounts of As and Cu (Nakata and Shikazono, unpublished), whereas these contents of euhedral and framboidal pyrite are less than the detection limit of an electron microprobe analyzer. Ishizuka and Imai (1998) found that the As content increases toward outer rim and reaches up to 5 wt% in the rim of colloform pyrite from the Fukazawa deposit. 

Dominant gangue minerals in Kuroko deposits are quartz, barite, anhydrite, gypsum, chlorite, sericite, and sericite/smectite. Morphology of quartz changes from euhedral in the centre to the irregular in the margin of the deposits (Urabe, 1978). No amorphous silica and cristobalite have been found. 

Present in euhedral to anhedral forms (E-A), colloform (Collo.), pelletal (Pell.), columnar (Col.), dendritic (Den.). 

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. 

The PGM study reveals that sulfarsenide minerals are very common in the ores. They account for the Ir, Rh and Pt missing from our calculated sulfide mass balance. These sulfarsenide minerals are euhedral and zoned in PGE (Fig. 2). They have an irarsite (IrAsS) surrounded by hollingworthite (RhAsS) core and a Ni-cobaltite (CoAsS) rim that contains some Rh (3 wt. %). The PGM cores also contain minor amounts of Pt (6 wt. %) and Os-Ru (1 wt%). 

Fig. 2. Back-scattered electron image of a euhedral, zoned sulfarsenide hosted in pentlandite (Pn). Inner core (white) is irarsite (IrAsS), outer core (pale grey) is hollingworthite (RhAsS), rim (dark grey) is Rh-bearing Ni-cobaltite.

Fig. 2. SEM backscattered electron image of a Type II vein containing euhedral to cataclastically brecciated arsenopyrite (asp), pyrite (py), quartz, and calcite. A thin Type 1 quartz vein showed a SEM-EDAX analysis of a very fine-grained mineral mass rich in Hg-Au-As vein (arrow).

In Type II veins arsenopyrite is situated along the walls of the vein, with pyrite overgrowths and as vein fill. Wider veins contain angular to subrounded host rock fragments. These veins contain euhedral arsenopyrite (<4 mm) and pyrite (< 3 mm) (Fig. 3). Both minerals have Au enriched rims and cores with an intermediate zone devoid of detectable Au. 

One sample of euhedral pyrite collected from a zone of disseminated sulfide coincident with a high-grade gold zone gave a S S value of -3.3 %o (Fig. 3). Similar S isotope data have been reported from the BB Formation, Caribou mine, and other base-metal deposits in the area (Fig. 3). 

Hydrothermal euhedral quartz (blue-green/red CL colors), of Type II veins exhibit growth textures and are related to sulfide mineralization during early Salinic Orogeny. 

Evidence from drill core and microstructure studies indicate that euhedral, zoned arsenopyrite grains tend to be clustered, and mantled by pyrite (Fig. 4), and their distribution is structurally controlled. These textures are interpreted to represent pressure solution as the main deformation mechanism during Di. This interpretation is supported by serrated pyrite boundaries (Fig. 4) and pyrite-bearing veins. However, locally unstrained euhedral pyrite porphyroblasts overprint Di and D2 structures, implying a late-stage post-D2 growth (Fig. 4). 

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. 

Fig. 15.8 Well ciystalline euhedral platy hematite (a), poorly crystalline spherical Si-containing hematite together with ring-like layer Fe-silicates ( ) (b), and akaganeite (c) from the Atlantis Deep, Red Sea, (Photo H.-Ch. Bartscherer) (Schwertmann etal., unpubl.)...

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-... 

The largest intrusion is represented by the Monte Capanne monzogran-itic stock (about 10 km in diameter), located in the western side of the island. It exhibit a porphyritic texture with centimetre- to decimetre-sized euhedral megacrysts of K-feldspar that are set in a medium- to coarsegrained matrix formed by variable amounts of plagioclase, quartz, K-feldspar and biotite with accessory apatite, zircon, monazite, ilmenite, and tourmaline. The intrusion contains abundant microgranular calc-alkaline... 

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. 

There are also a small proportion of the chondrules which contain euhedral grains with red CL, and some chondrules appear a uniform dull... 

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... 

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