Calcite equant

Not all fibers yield fibers on comminution. Fibrous varieties of quartz (Si02), for example, are formed from tightly bonded, aligned helical fibers that cannot be separated mechanically (Frondel, 1978). Fibrous calcite (CaCOa), when crushed or ground, breaks into equant grains of rhombic shape. The fragments reflect the cleavage characteristics of the mineral. 

The reasons for the equant sparry habit of cements in the meteoric realm are probably more debatable than the fact that these cements should be calcite. There are two principal models in the literature used to explain carbonate cement morphology these models are shown schematically in Figure 7.22. In the Folk (1974) model of Mg-poisoning, the Mg2+/Ca2+ ratio of the water plays an... 

Helderberg carbonates are extensively cemented by clear equant calcite cement that postdates marine cement and fills most of the original pores. These cements are compositionally zoned as shown by cathodoluminescence and trace element analyses. These precipitates include nonluminescent, bright and dull cements. The luminescent cement is the earliest generation of cement followed by... 

The regional distribution of the cathodoluminescent cement zones and variation in their stable isotopic composition (Table 8.7) are shown superimposed on the stratigraphic cross-section of Figure 8.36A in Figures 8.36B and 8.36C. The nonluminescent cement makes up 0 to 60% of the equant calcite cement and... 

Figure 9. Scanning electron micrograph of the underside of a calcite raft that precipitated at the water-air interface in Browns Room. Perimeter has long crystals, whereas the central part of the raft is characterized by mostly equant crystals. Four small rafts coalesced to form this large one. Scale bar is 1 mm.

The final Devils Hole speleothem type is flowstone (Hill and Forti, 1997, p. 70-72), which precipitated from films of calcite-supersaturated water flowing down the exposed walls of Browns Room. Flowstone deposits in Browns Room characteristically are banded in cross section, with bands from about 0.1 to 3 mm thick (Fig. 15, 17). Each band is a depositional unit, with major or minor discontinuities on either side. Outer surfaces of bands commonly are etched and corroded, and may have thin coatings of clay-sized material. The inner, older bases of the bands are composed of large numbers of micron- and decami-cron-sized equant calcite crystals. The equant crystals rapidly grade into fewer elongate, length-fast calcite... 

In the graded storm layers with abundant planktonic foraminifers microcrystalline, moderately fer-roan calcite replaced clayey/marly matrix (Fig. 6C). Some bioclasts display their original composition or show a well-preserved original shell texture, owing to pseudomorphic neomorphism to low-Mg calcite, but others are totally recrystallized/replaced by drusiform or equant mosaic calcite. Blocky calcite and syntaxial overgrowths display non-luminescent... 

Ciyptociystalline, equant high-Mg calcite First, ciyptocrystaUine second, acicular aragonite Similar to previous, but lengths of acicular crystals may be more than 30 fim. Crystals extend to center of pores... 

Shortite [Named after the American mineralogist, Maxwell Naylor Short (1889-1952)1 (ICSD 16495 and PDF 21-1348) Na3Ca,(C03)3 M= 306.16 15.02 wt.%Na 26.18 wt.% Ca 11.77 wt.% C 47.03 wt.% 0 (Nitrates, carbonates, and borates) Orthorhombic a = 496.1 pm b = 1103 pm c= 712pm (Z = 2) P.G.mm2 S.G.Amm2 Biaxial a= 1.531 1.555 r= 1.570 27=75" 3 2600 (2610) Habit wedge-shaped, equant, or short prismatic crystals. Color colorless to pale yellow. Diaphaneity transparent. Luster vitreous. Streak n.a. Geavage (010). Fracture conchoidaL Decon oses in water releasing an insoluble residue of calcium carbonate. Fluorescent under UV radiation. Occurrence with calcite and pyrite. 

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