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. 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
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. 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,. i^^H^--^^ k- -? ^ ^* PINK FLUORITE Near Gascliencn, Svvitz. PINK FLUORITE ON QUARTZ Grimsel, Svvitz. Bement Collection, American Muscimi of Natural History DEFINITION OF TERMS r3 rhombohedron and scalenohedron; Fig. 193, Two scalcnohedrons;Fig. 194, pyramid of the first order; Fig. 195, pyramid of thesecond order; Fig. 196, Two rhombohedrons, trigonal prism andhexagonal prism; Fig. 197, Two rhombohedrons, and pyramid ofthe second order; Fig. 198, Basal pinacoid and rhombohedron;Fig. 199, Rhombohedron, and two scalcnohedrons; Fig. 200, Basalpinacoid, rhombohedron, two hexagonal pyramids, hexagonalprism; Fig. 201, Basal pinacoid, rhombohedron, hexagonal pyra-mid, hexagonal prism; Fig. 202, Four rhombohedrons, two scalen- ^\
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmineral, bookyear1912
