Section III, 1892. [ 2S ] Trans. Roy. Soc. Canada. 



III. — On the llexican Type inthe Grystallization of the lopaz. 

 By E. J. Chapman, Ph.D., LL.D., Professor in the University of Toronto. 



(Read June 1st, 1892.) 



In the crystallization of many substances, the crystals present certain distinct types 

 or form-combinations, mostly connected with locality, or with geological conditions of 

 occurrence — as seen in Orthoclase, Amphibole, Pyroxene, Barite, and other minerals. 



In the crystallization of the Topaz, three types of this kind — the Brazilian, Siberian, 

 and Saxon types — have long been recognized.' The object of the present communica- 

 tion is to point out a fourth, or Mexican, type, as exhibited by crystals from the State of 

 San Luis Potosi in Central Mexico. 



A brief, preliminary notice of the leading points involved in the crystallization of 

 the Topaz generally, may serve to bring out more appreciably the distinctive character of 

 this Mexican type. 



In all topaz crystals certain vertical or prismatic forms are invariably present. As a 

 general rule, these comprise two well-marked rhombic prisms : V, with front or obtuse 

 angle of 124' 11'; and V2, with front inclination (i.e., that over axis .■) of 86' 49'. A third 

 prism V3, with angle in front of 64" 28', is also frequently present ; and very commonly 

 intermediate forms are indicated by coarse or fine vertical striae, causing these prisms to 

 merge into one another or to lose their sharpness of definition. The Front-vertical or 

 Macro-pinakoid Y is of exceedingly rare occurrence. In many scores of topaz crystals 

 that have passed through my hands I have never seen it ; but the Side or Brachy-vertical 

 V occasionally appears, although its two planes when present are usually very narrow, 

 or little more than mere lines. It seems to be best developed in some of the Trumbull 

 crystals, as figured by Dana. 



The basal form, B, is practically unknown in Brazilian crystals, or is merely indicated 

 in some rare examples of these ; and although frequently present in crystals of the 

 Siberian type, it is mostly of small size, and often little more than linear in its dimen- 

 sions. In the Saxon type, on the other hand, it is always largely developed, forming the 

 characteristic feature of this type. 



The polar forms in Topaz, or those which incline upon the vertical axis, are chiefly 

 rhombic pyramids, = m P, and side-polars [i.e., brachy-polars or brachydomes) = m P. 

 Front-polars or macrodomes are very rarely present and are then only of small size. 



The rhombic pyramids or octahedrons in the vertical zone of V, are chiefly, if not 

 only, three in number ; but in many crystals only one of these is present, This is the 

 form P, with front (or macro-) and side (or brachy-) angles equal respectively to 141' and 



' The colourless, compaiativel.v large crystals obtained of late years in Japan belong to the Siberian type. To 

 this type, also, the Aberdeen crystals essentially belong. 



Sec. III., 1802. 4. 



