A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . Fig. 184 Fig. 185 Fig. 186 divided hexagonal scalenohedron; Figs. 181-182; {quartz). The skezv trigonohedron is a tetartohedral form, being thehalved tritopyramid; Figs. 183, 184. 42 A POPULAR GUIDE TO MINERALS The skew rhombohedron is a tetartohedral form, being a halvedhexagonal scalenohedron; Figs. 185-186, the faces above and belowthe equator being differently eliminated from the case of the trigonaltrapezohedron; which see. All of
A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . Fig. 184 Fig. 185 Fig. 186 divided hexagonal scalenohedron; Figs. 181-182; {quartz). The skezv trigonohedron is a tetartohedral form, being thehalved tritopyramid; Figs. 183, 184. 42 A POPULAR GUIDE TO MINERALS The skew rhombohedron is a tetartohedral form, being a halvedhexagonal scalenohedron; Figs. 185-186, the faces above and belowthe equator being differently eliminated from the case of the trigonaltrapezohedron; which see. All of the forms above enumerated are not found in nature. The symbols of the hemihedral forms are not always simply. Fig. 187 the symbols of the holohedral forms divided by 2, as, for the rhom- bohedron As, from many considerations if is desirable to give to the rhombohedron an especial symbol and to designate the rhom-bohedron, deduced from the primitive pyramid P by R, the rhom-bohedron derived from m P becomes m R, and is positive (-j- mR)or negative (— m R) as the front edge of the rhombohedron isin the first or second sectant^of the six upper and lower sectantsformed by the intersection of three planes at right angles to eachother. Figs. 187, 188, 189. Fig. 187 shows the intersection ofthree planes and the six upper and lower sectants, Fig. 188 is apositive rhombohedron, Fig. 189 is a negative rhombohedron. Thesymbol of the scalenohedron is w R w + or —, that of the trapez- fftPtiohedron right and left. COMBINATIONS OF HOLO AND HEMIHEDRALFORMS. PLATE 9. Fig. 190, Hexagonal prism and rhombohedron; Fig. 191, Tworhombohedrons, right and left, positive and negative; Fig. 192,
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmineral, bookyear1912
