WHKRRY AND ADAMvS: PINAVERDOL 40I 



domes, 5, T, U, V, W, X, Y, and Z, for the unit pyramids, and 

 A and B, regarded as Greek capitals, for two of the orthopyra- 

 mids; and finally, two letters left over, Q and R, are applied to a 

 steep orthodome and the remaining orthopyramid, respectively. 



A habit rather frequently assumed, especially by material 

 received from England, is shown in plan and perspective in figure 

 3. Its peculiar tabular aspect is due to the prominent develop- 

 ment of the minus orthodome D (Toi). The prism m (no) is 

 usually present around the edge of the plates, although these 

 sometimes become so thin that the prism is practically crowded 

 off. The clinopinacoid, h (010), is often better developed on 

 crystals of this habit than in those of type i, and a small plus 

 orthodome, d (loi), is also usually present as a bounding form. 

 Instead of a plane face where the base, c (001), should lie, a curved 

 surface is usually present, which extends from about the position 

 of c back as far as that of the minus orthodome I (106). 



Still another distinct habit is illustrated in figure 4. This 

 appears most frequently in a preparation obtained from Germany 

 before the war, which was crystallized from an unknown solvent. 

 In this there is marked elongation along axis h, making the 

 crystals pseudo-prismatic on the orthodomes d (loi) and D (Toi). 

 The only other unusual feature shown by these crystals is the 

 rather prominent development of the clinodome 5 (on). 



COI^OR PHENOMENA 



As noted in the introductory paragraph under the heading 

 Crystallography, the crystals of pinaverdol exhibit a striking 

 and brilliant reflection pleochroism. This phenomenon being a 

 rather uncommon one, it will now be described in greater detail. 



In any biaxial crystal there are three directions at right angles 

 to each other in which properties connected with light are ex- 

 hibited. In the rhombic system these directions coincide with 

 the crystallographic axes; in the monoclinic, one coincides with 

 axis h, while the other two are limited to the plane of symmetry, 

 but do not in general coincide with either of the crystallographic 

 axes lying in that plane. In the present instance, however, the 

 properties of the crystals are peri-rhombic, as we have termed it. 



