. Bulletin - New York State Museum. Science. GUIDE TO THE MINERALOGIC COLLECTIONS 37 all of which are inclined to one another. The axes are desig- nated as in the orthorhombic system. The angle between b and c is called a, that between '/ and c, ,' and that between a and 6, y. These angles are distinct for every triclinic species (fig. 149). The similarity in molecular structure between minerals of the orthorhombic, monoclinic and triclinic systems indicated by their crystallization is further accentu- ated by their optical properties, errs- I tals of all three systems being opti- ^W^"' c
. Bulletin - New York State Museum. Science. GUIDE TO THE MINERALOGIC COLLECTIONS 37 all of which are inclined to one another. The axes are desig- nated as in the orthorhombic system. The angle between b and c is called a, that between '/ and c, ,' and that between a and 6, y. These angles are distinct for every triclinic species (fig. 149). The similarity in molecular structure between minerals of the orthorhombic, monoclinic and triclinic systems indicated by their crystallization is further accentu- ated by their optical properties, errs- I tals of all three systems being opti- ^W^"' cally biaxial; that is, there are two v-—-"" ^""M directions in which polarized light is ic transmitted through them without double Fig- 149 refraction. Lines bisecting the angle between these optic axes bear a close relation to the symmetry and outward form of the crystal. Normal group Crystals occurring in this group are symmetric only to a center, which is the point of intersection of the crystallographic axes. This symmetry admits of forms occurring only in the pairs of faces;1 thus all prismatic and dome forms which in the orthorhombic system are represented by four faces here occur as hemiprisms and hemidomes. two faces alone being required to satisfy the symmetry of the class. Similarly, pyramidal. Fig. 160 Fig. 151 forms which in the orthorhombic system consisted of eighl fares are replaced by four complementary forms each consisl ing of two parallel planes. Compare mode] 11, which shows a triclinic or doubly inclined rhombic prism. With the above exceptions See p. 10, fig. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original New York State Museum; New York State Museum. Albany : New York State Education Dept
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