TOPAZ 327 



of these gems leave llussia, but some find their way to the Orient (Persia, India, &c.), through 

 the dealers who frequent the Nizhniy Novgorod fairs. 



Phenakite has been found comparatively recently in North America, chiefly in Colorado. 

 One of the localities in this State is Topaz Butte, near Florissant, sixteen miles from Pike's 

 Peak, where it occiu-s as flat rhombohedral crystals, and, as at Miask, associated with 

 topaz and amazon-stone in veins penetrating granite. The other locality is Mount Antero, 

 ni Chaffee County, ten miles north of Salida, where it occurs as prismatic crystals, sometimes 

 an inch in length, on quartz and beryl. These American phenakites are cut as gems, and 

 are valued on account of their national origin. Other American localities, like the European, 

 have no trade importance. In Eui'ope small brown crystals, scarcely suitable for cutting, 

 were formerly found in the iron mines of Framont, in the Vosges Mountains. The small 

 crystals of phenakite, which have been found in recent years in mica-schist in the Canton 

 Valais in Switzerland, are of mineralogical interest only. 



TOPAZ. 



Topaz is the most familiar of yellow stones, and for this reason its name is often applied 

 to other minerals of the same colour. Thus, yellow corundum, as we have seen, is known as 

 " oriental topaz," yellow quartz (citrine) is referred to variously as " occidental topaz," 

 " Bohemian topaz," and " Spanish topaz," while yellow fluor-spar is sometimes known as " false 

 topaz." The mineral species to which mineralogists apply the name topaz includes not only 

 the stones known as precious topaz, or as Brazilian, Saxon, Siberian, or Tauridan topaz, but 

 also blue, red, and colourless stones, which are known to dealers in precious stones by other 

 names. 



Topaz is a fluo-silicate of aluminium with the formula (AlF)2Si04 and the percentage 

 composition of, silica 33'3, alumina 56'5, and fluorine 17'6 ; from this it will be seen thai 

 alumina forms a large part of the mineral, as it does also of most of the precious stones 

 hitherto considered. Besides the constituents already mentioned, other substances, such as 

 ferrous oxide, lime, alkalies, water, &c., are sometimes present in small amount. Until 

 recently the water, which is often present, was considered to be an impurity due in part to 

 the alteration of the material by hydration. Penfield and Minor, however, have shown 

 (1894) by a series of carefully conducted analyses combined with detailed determinations of 

 the optical and other physical constants of the mineral, that water is one of its essential 

 constituents and not a mere impurity. The amount of water present in the specimens 

 which they analysed varied from 0"18 to 2"50 per cent. The specimen which contains only 

 O'lS per cent of water is almost pure fluor-topaz, and its composition is expressed by the 

 formula already given, (AlF)2Si04. Specimens containing more water may be regarded as 

 hydro-fluor-topaz, in which the water is present as hydroxyl (OH), which replaces fluorine 

 isomorphously, so that the formula becomes [Al(F,OH)]2Si04. A mineral in which the 

 whole of the fluorine is replaced by hydroxyl, and which would ha^e the formula (A1.0H)2Si04, 

 has not yet been met with. The isomorphous replacement of fluorine by hydroxyl in 

 this mineral is accompanied by small variations in its physical characters, such as specific 

 gravity, refraction, double refraction, &c. ; these variations are very slight and of purely 

 scientific interest. 



