184 H. B. PATTON TOPAZ-BEARING RHYOLITE, UTAH 



Transparent topaz. — These are the crystals that are already well 

 known to mineral collectors and that occur invariably in small cavities 

 of the rhyolite. As already mentioned by A. H. Jones/ the topaz crys- 

 tals of this locality are all originally of a wine color, which color fades 

 on exposure to the sunlight. It invariably happens, therefore, that all 

 the crystals found on the surface are perfectly colorless and all have a 

 wine color when found in a freshly opened cavity. This wine color has a 

 characteristic brownish tint and is very beautiful in its deeper shades. 

 The depth of color varies considerably, but is always pronoimced. The 

 larger crystals have naturally a deeper shade, owing to their greater 

 thickness; but, aside from the thickness of the crystal, some are darker 

 than others. It is unfortunate that the color, as is so often the case with 

 topaz, is not permanent. The color disappears quickly with very gentle 

 heating. After exposure to bright sunlight for thirty hours a marked 

 fading of the color was to be noticed. After forty-eight hours' exposure 

 to direct sunlight the smaller and lighter colored crystals lose their color, 

 and after about seventy hours the deeper colored ones become nearly 

 colorless (see plate 13, figure 1). 



It occasionally happens that one finds a crystal or cluster of crystals 

 lying partly buried beneath the surface and partly exposed, so that the 

 buried part is wine colored and the exposed portion perfectly colorless. 



These crystals are mostly small, say one-quarter to one-half inch, occa- 

 sionally one inch in length. In breadth they average about one-half the 

 length, although at times they are much more slender than this. Un- 

 doubtedly larger crystals do occur, and when the locality was first visited 

 must have been found on the surface, but naturally the larger ones are 

 the first to be picked up. 



There is no doubt that these topaz crystals were formed after the litho- 

 physae were practically complete, as they grow upon the walls of the 

 cavities and inclose to a considerable extent the quartz material of these 

 walls. The crystals are attached sometimes at one end only, sometimes 

 along the whole side, and again both ends may be attached, while the 

 center is free and clear. Doubly terminated crystals are not altogether 

 rare, but they are not often found free on the surface of the ground. 

 Once in a while it happens that a crystal is attached to the very thin 

 wall separating two adjacent lithophysse and grows freely into each cav- 

 ity, thus producing double terminations. Clusters of crystals produced 

 by the growing of one crystal upon another are by no means uncommon. 



The habit and crystal forms of these topazes are already well known 



» Loc. cit., p. 176. 



