MINERALOGY; WITH A CLASSIFICATION OF SILICATES. 65 



each other, not only as the related silicates, albite, labradorite and anorthite, or as zoisite 

 and jadeite, but also, at the same time, as anorthite difTers irom meiouite, or as lime- 

 garnet from idocrase or epidote. In other words, not only tlie relations of the protoxyds 

 to the sesquioxyds, but the relations of both of these to the silica are subject to notable varia- 

 tions in species of tourmaline which so closely resemble each other that it is difficult, if not 

 impossible, to distinguish them by physical characters aloue. The studios of Eammels- 

 berg, which first clearly showed the varying composition of the toui'maliues, enabled him 

 to divide them into five species, each of which is the the type of a genus, distinguished by 

 the ratios of protoxyd to sesquioxyd. We follow him in regarding the boric oxyd, which 

 is not constant in amount, as replacing silica, and recognize the fact that the tourmalines 

 are oxyfluorids containing a small and variable amount of fluorine. 



For the brown magnesia-tourmaline, the most highly protobasic species, with the 

 atomic ratios of 1 : 3 : 5, a trivial name was needed, and we have ventured to suggest that 

 of " coronite," from Crown Point, in New York, a well-known locality of this species ; while 

 to the black magnesia-iron tourmaline, with the ratios 1 : 4 : 6, we have given the name of 

 " schorlite," from the ancient trivial designation of schorl, under which the tourmalines 

 were formerly included with many other species. The names of "aphrizite" for the black 

 ferrous tourmaline, 1 : 6 : 8 ; of "indicolite" for the blue and green tourmalines, 1 : 9 : 12, 

 holding both ferrous and manganous oxyds ; and of " rubellite " for the red and colorless 

 tourmalines, 1 : 12 : 15, containing manganese but no iron, are familiar to mineralogists. 

 It is worthy of notice that we pass, in this family, from compounds having the ratio 

 of protoxyds to sesquioxyds found in the feldspars (coronite having the general atomic 

 formula of iolite,) to the ratios of the muscovitic micas, and that with the diminution 

 in the proportions of protoxyd bases, the relative amount of alkalies is augmented ; 

 while indicolite and rubellite become, like beryl, lithia-bearing. The atomic ratio of 

 the sesquioxyds (in which alumina is sometimes partially replaced by ferric and appar- 

 ently by mangauic-oxyd) to the silica, with its included and varying amount of boric 

 oxyd, is, moreover, not constant, but differs in the different species of tourmaline. There 

 are thus many chemical variables to be taken into account in the study of a group of 

 minerals which, from their similarity in external characters were, previous to the careful 

 researches of chemists, united in one species, the supposed varieties of which were distin- 

 guished only by differences in color. 



§ 86. We have given in the preceding paragraph, and also in table No. IX, the ratios 

 deduced by Eammelsberg for the five species named and defined, to which it should be 

 added that he found for a red tourmaline from Rozena, with a density of 2.998, the ratios 

 1 : 15 : 21. This, the least protobasic silicate of the order, should constitute a new species. 

 The mean densities of these species, as deduced by him from a large number of examples, 

 show but small variations, and, with the exception of the highly ferriferous aphrizite, 

 which is 3.20, range from 3.10 to 3.04.^ These figures are adopted in the table, save that 

 for rubellite the density has been placed at 3.00. The ec[uivalent volumes, as calculated 

 by Eammelsberg from his own arbitrary formulas for these five species, respectively gave 



' For the original memoir of Rammelsberg, see Pogg. Ann., 1850, Ixxx. 449 ; and for a summary of his results, 

 Amer. .Tour. Science, xi. 257; also a farther discus.sion thereof by the pre.sent writer, Ilm!., xvi. 211. Fur later 

 studies of the tourmalines by Rammelsberg, see Annal. Phys. Cliem., 1870, cxxxis. 379 and 547. 



Sec. III., 1S85. y. 



