![]() ![]() (zircon, vesuvianite, cassiter ite, rutile) 1 mm m vertical axis of fourfold Ditetragonal pyramidal (no 7. THE CRYSTAL SYSTEMS Di tetragonal bipyramidal 6. When only one of a ditetragonal pair is preserved, as in Class 8 (or Class 15 in the hexagonal system), it is known as a third-order form (see Fig. Eight-faced prisms are known as ditetragonal prisms, when the adjoining angles in agCross section are unlike, which helps to distinguish them from a combination of first- and second-order prisms. Intermediate slanted and truncating faces are bipyramids (in the classes that are alike at top and bottom with a center of symmetry,- otherwise simply pyramids), and they too may be first or second order. The top and bottom faces are known as the basal pinacoids. 17 Tetragonal ones the prisms which are systems prisms vertical and are said to be first or second order, depending on whether a face cuts one horizontal axis and is parallel to the other or whether each face cuts both horizontal axes at an equal distance from the center (Fig. The definitive forms are the upright Fig. ^4r This change in the axes means that the vertical direction is now fixed and the crystal may not be turned equally well to bring a side up, as in the cubic system. ![]() MARIN COUNTY FREE LIBRARY lELD GUIDES 989 0i mm 1 ocks and men \\is I^^mA m»\ v^-?^ I I I I (orient) the crystal, is longer or shorter than the other two. ![]()
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