MrNBRALOGY; WITH A CLASSIFICATION OP SILICATES. 61 



farther stiidy. Lapis lazuli, a snijjhatic and sulphuretted speciefs, the compositiou of 

 which is not accurately determined, is apparently related to the sodalite group. Not- 

 withstanding the resemblance in compositiou between these silicates and the scapolites, 

 they differ very widely from the latter in degree of condensation ; for, while the scapolites 

 approach the feldspars in volume, the species of the sodalite group give a higher value 

 for V than leucite, or indeed than any other known natural silicates. 



§ *78. The relations between anorthite, meionite and the silicates, sodalite, nosite and 

 hauyne, which we include under the general name of the sodalite group, help us to 

 understand the nature of cancrinite, and its relation to nephelite. This latter species, 

 which has an atomic volume near that of orthoclase, is a little more siliceous than anor- 

 thite, and its atomic formula, (na,.|,alj.„si,.,,)os.,r„ may be multipled by six, making it (na^al,^ 

 si2;)o5i, to show more simply its relation to cancrinite. This mineral which was formerly 

 imagined to be an admixtu.re of car1)onate of lime with a hydrated nephelite, appears 

 from recent studies to be an integral carbosilicate, comparable wiih the sulphatosilicates 

 and the chlorosilicates of the sodalite and scapolite groups. The cancrinite of Miask, 

 with a specific gravity of 2-45, as analyzed by Rautf, is represented by (na^ali^si,.)©,,! 

 + SCiCaiO^ + 4|aq (c = 6), while the cancrinite of Ditro, according to Koch, contains a por- 

 tion of potash, and offers slight variations from the above formula in the amounts of lime 

 and silica. This hydrous carbosilicate, like the hydrous sulphatosilicate ittnerite, will 

 find a place among zeolitoids rather than among spathoids. 



§ "79. Coming next to more highly protobasic spathoids, we have the remarkable barytic 

 species, barylite, presenting the anomaly of a highly basic silicate having the ratios, 

 2 : 3 : *7, and the volume of the feldspars and scapolites, which is said to resist the action of 

 acid. Milarite, sarcolite and gehlenite present an interesting group in which, the ratio of 

 protoxyd to alumina being 1 : 1, there is a great variation in the proportion of silica from, 1 : 

 1 : 8 in milarite, to 1 : 1 : 2 in sarcolite, and 1 : 1 : li in gehlenite. In the native gehlenite a 

 small portion of alumina appears to be replaced by ferric oxyd, but the artificial gehlenite 

 from furnace-slags, analyzed by Percy, is without iron, and is an oxysulphid containing 

 1.50 per cent, of sulphur. Melilite, a spathoid silicate, is also found as a furnace-product, 

 and, according to Percy, contains a variable amount of sulphirr, ecjual in one case to 1.62 

 per cent., while the native melilite is destitute of sulphur. Under this name are, perhaps, 

 confounded two distinct species. The artificial melilite, which approaches the so-called 

 humboldtilite in composition, has au atomic formula near to (ca.aLsiJo,,,, and a volume 

 almost identical with gehlenite, while the native melilite is more nearly (ca.,al|Si.,)o,;. 

 Similar atomic ratios to the last, as regards the bases, are presented by eudialyte, a zirconic 

 spathoid, the composition of which is nearly represented by (m^zrosi,o)0|.i, and which 

 contains much lime and soda, with a little chlorine. Wohlerite, another zirconic spathoid, 

 containing some niobic acid replacing silica, has the atomic formula (m.,zr2si.^)0[,„ the 

 the ratio between the protoxyd-bases and zirconia being, as in the artificial melilite, o : 2. 

 In these two species we have examples of the comijlete replacement of alumina by zirconia. 

 In melilite as analysed by Damour, mid also in gehlenite, a partial replacement of alumina 

 by ferric oxyd is shown, and a complete substitution of this kind appears in ilvaite, a 

 spathoid species having the ratios of wohlerite, with a density of 3.71. The higher specific 

 gravity of 4.00 observed for some examples of ilvaite, may show a related species, or more 

 probably, as suggested by Dana, may be due to an admixture of gothite or other iron-oxyd. 



