424 Pratt — Crystallography of the Montana Sapphires. 



Aet. XLYI. — On the Crystallography of the Montana Sap- 

 phires ; by J. fl. Pkatt. 



The sapphire crystals from Yogo Gulch, Montana, are 

 etched and striated to such a degree that no crystallographic 

 measurements were possible on the reflecting goniometer ; but 

 sufficiently accurate angles could be obtained with the contact 

 goniometer to allow of the identificatio_n of the faces. 



The prism of the second order a(1120) which is so common 

 on corundum was not observed on any of the crystals from this 

 locality. The only two faces that could be identified were the 

 base c(0001) and the rhombohedron a?(3032) which is a new 

 face for corundum. On one crystal, two very small faces were 

 observed, which were too small to be measured with the con- 

 tact goniometer, but were probably the faces of a pyramid of 

 the second order. 



In determining the rhombohedron, ten or more independent 

 measurements were made of Cs\%. These varied from 66° to 

 68°, but approximated closely to H7°, which agrees verv well 

 with the calculated value, 67° 3', for 0001^3032. 



The crystals are developed as shown in figs. 1, 2 and 3, 

 page 425, the prevailing type being like fig. 3. The crystals 

 vary from those where the base is very largely developed, hav- 

 ing a diameter of 8 mm , while the rhombohedron is only l mm , to 

 those that have the base and rhombohedron equally developed. 

 (Fig. 1). Where the faces are more equally developed, the 

 rhombohedral faces are generally rounded. 



The basal plane often shows characteristic striations which 

 are parallel to the three intersections of the base c, and the 

 rhombohedron x, as shown in fig. 4. These lines are sharp and 

 distinct and on the very flat crystals can easily be measured, 

 when examined under the microscope. The rhombohedral 

 faces are very roughly striated without showing any distinct 

 parallel lines. 



One very common development of these crystals is a repeated 

 growth on the basal plane, of the rhombohedron a?(3032) and 

 the base, <?(0001), as represented in fig. 2. These growths are very 

 varied, as is shown in figs. 11-14 (p. 427), where they are drawn 

 in basal projection. In fig. 11, there is but one secondary rhom- 

 bohedron and base, which has one of its rhombohedron faces a 

 continuation of one of the rhombohedron faces of the crystal. 

 Fig. 12 represents a repeated growth, each face of which is 

 entirely distinct from the faces of the main crystal. In fig. 13 

 there are represented two and in fig. 14 a series of such 

 growths, where a number of the rhombohedral faces coincide. 



