Equant

Over the last seventeen year s the Analytical center at our Institute amassed the actual material on the application of XRF method to the quantitative determination of some major (Mg, Al, P, S, Cl, K, Ti, Mn, Fe) and trace (V, Cr, Co, Ni, Zn, Rb, Sr, Y, Zr, Nb, Mo, Ba, La, Ce, Pb, Th, U) element contents [1, 2]. This paper presents the specific features of developed techniques for the determination of 25 element contents in different types of rocks using new Biaiker Pioneer automated spectrometer connected to Intel Pentium IV. The special features of X-ray fluorescence analysis application to the determination of analyzed elements in various types of rocks are presented. The softwai e of this new X-ray spectrometer allows to choose optimal calibration equations and the coefficients for accounting for line overlaps by Equant program and to make a mathematic processing of the calibration ai ray of CRMs measured by the Loader program. 

Equant Particles of similar length, width, and thickness. 

When the aspect ratio of a particle is 10 or greater, mineralogists use the term fiber. Below 10, terms such as columnar, prismatic, blocky, and—at an aspect ratio of 1—equant are applied. Aspect ratios, ordinarily much greater than 20 for the fibers of asbestos samples, may be as high as 5000. 

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. 

Over the past several decades six different silicate minerals have been mined as asbestos and processed for industrial and commercial applications. The most commonly encountered asbestos mineral today is chrysotile. The five other minerals are tremolite, actinolite, anthophyllite, grunerite, and rie-beckite. All five are members of the amphibole group of minerals, and each can occur as chunky, acicular, or equant crystals, as well as in fibrous form. When found as fine fibrous aggregates, in quantities appropriate for mining, they are usually distinguished as a special variety—for example, tremolite-asbestos. 

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. 

Particle morphology refers to the external features or form of a powder s primary particles. This includes descriptions of shape, including aspect ratio, or crystal habit (plate, needle, lath, equant, etc.). Particles are not always observed as discrete entities. Rather, they are often associated with other particles, sometimes loosely held... 

Characteristics Equant, column Flake Equant, lath ... 

Characteristics Equant, plate Plate, column Equant... 

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... 

Coarse Avg. >2.0 mm Straight to smoothly curving Equant, tabular, protogranular, granular ... 

Granulohlastic Avg. <2.0 mm Straight or smooth, polygonal Equant, tabular... 

Equant, blocky, prismatic (large crystal faces)... 

Rapid accumulation of silt and clay on mammillary surfaces stops crystal growth. Large numbers of very small, equant to columnar crystals that grade quickly to a few large columnar crystals (Fig. 6) mark the reinitiation of crystal growth. 

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... 

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