FLUOR-SPAR 529 



octahedron, which truncates the corners of the cube. It is brittle, and is the mineral 

 which stands fourth on Mohs' scale of hardness ; it is scratched, therefore, by ordinary 

 window-glass. The specific gravity varies between S'l and 3"2, so that the mineral sinks 

 in test liquid No. 3 but floats in No. 2. When heated before the blowpipe, fluor-spar 

 generally decrepitates violently and falls into small splinters, which are scattered about 

 with some violence. This is explained by the fact that it usually contains a number of 

 small cavities which are either vacuous or filled with liquid, and this on being exposed to 

 heat expands and I'educes the stone to splintei's. Fluor-.spar alone is not very fusible, but 

 heated with other minerals it melts readily, and for this reason is used as a flux in the 

 reduction of metallic ores, hence its name. Some specimens are remarkable in that when 

 raised to a temperature below red-heat they phosphoresce, that is to say, they emit a 

 beautiful bluish or greenish light. Fluor-spar is completely decomposed by sulphuric acid 

 with the formation of hydrofluoric acid, a gas much used for etching glass and certain 

 precious stones. 



The behaviour of fluor-spar with respect to light has an important connection with 

 the uses of the mineral as an ornamental stone. It has a characteristic moist-looking 

 vitreous lustre, and ranges from perfect transparency through all degrees of translucency, 

 to complete opacity. The most important character of the mineral is its colour, which is 

 extremely variable ; no other mineral, in fact, displays such a wide range of beautiful colours, 

 and no colour represented in the mineral kingdom is absent. Perfectly pure fluor-spar, 

 which is rare, is limpid and colourless. By the mechanical intermixture of very small 

 amounts of foreign substances, for the most part of organic matter, colour is produced, and 

 this is either destroyed or altered when the fluor-spar is heated. The colour of fluor-spar 

 may be very pale and delicate in shade, or, on the other hand, it may be so deep and 

 intense as to be only recognisable in thin sections of the stone, thick pieces appearing 

 almost black. The powder or, in other words, the streak of the mineral is always white, 

 or at least very pale. Owing to the irregular distribution of the colouring matter fluor-spar 

 often has a patchy appearance. Specimens showing several colours alternating with each 

 other in regular bands arc common, this being especially the case in inassive, crystalline 

 ■aggregates. Crystals may have a nucleus which differs in colour from the external 

 portions, for example, the central portion may be yellow and the outer violet. 



The fluor-spar used for ornamental purposes must be transparent and of a fine uniform 

 colour. Material of this description is known in the trade by the name of the precious 

 stone it most resembles in colour with the prefix false, " false topaz,^' " false ruby," " false 

 emerald," " false sapphire," " false amethyst," &c., being some of the terms used for stones 

 of different colour. Coloured fluor-spar may be passed off for stones other than those just 

 mentioned : thus yellow fluor-spar may resemble yellow quartz (citrine) as well as yellow 

 topaz, and red fluor-spar either red tourmaline (rubellite) or red corundum (ruby). It is to 

 be noted that the inaccurate term "false topaz" is applied to yellow quartz as well as 

 to yellow fluor-spar. 



Yellow fluor-spar is very common ; it is found at Freiberg, Gersdorf, and other places 

 in the Saxon Erzgebirge, in various shades of yellow, including wine-yellow, honey-yellow, 

 and brownish-yellow. 



Red octahedra, usually more or less corroded on the surface, occur in crevices in 

 gneiss in the Swiss Alps, but are not abundant. They are found, for example, on the 

 St. Gotthard, where Goschenen is often named as a locality ; on the Zinkenstock, near the 

 Grimsel ; in the Tavetsch valley in Graubiindeii ; in Wallis ; Tessin, &c. The crystals are 

 usually of a light rose-red colour, darker shades being rare. 



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