204 Washington — Kaersutite from Linosa and Greenland. 



paper their objection to the introduction of the molecule under 

 discussion lay in the fact that, according to Tschermak's theory, 

 " a definite basic alumo-silicate molecule is regarded as isomor- 

 phous with Ca (Fe, Mg) 3 Si 4 12 ."* As has been shown above, 

 however, such a riebeckite-glaucophane molecule may be 

 regarded as a metasilicate and may be written structurally as 

 a ring formula, exactly analogous to those suggested by the 

 authors named. It is clear, therefore, that the presence of a 

 riebeckitic molecule need not be regarded as a case of isomor- 

 phism of two chemically and structurally unlike molecules, or 

 as inconsistent with the views of Penfield as to the structure 

 of the amphibole acid and the mass effect of complex mineral 

 molecules. On the contrary, they are in complete accord, as 

 the matter reduces itself, in the last analysis, to the simulta- 

 neous replacement of one hydrogen atom by Na and, three by 

 Fe //; , just as two atoms are replaced by Ca", (Mg, Fe)", or by 

 one of Penfield and Stanley's bivalent radicals. 



These authors noted the highly interesting and probably 

 significant fact that the CaO formed " very closely 25 per 

 cent of the various radicals and bases, or in other words 

 replaces one-fourth of the hydrogen atoms of the amphibole 

 acid." Without giving all the percentage figures, in the 

 Linosa hornblende, as calculated above, CaO forms 33*8 per 

 cent, and in that of Kaersut 26*3 per cent. The latter approx- 

 imates to one-quarter, while the former is about one-third of 

 the radicals and bases. If this last is substantiated by analyses 

 of other hornblendes, and found to be characteristic of certain 

 kinds, it might be held to indicate that the amphibole acid 

 contains a number of hydrogen atoms which is divisible both 

 by 4 and by 3, such as H 12 Si 6 18 , or a multiple of this. But 

 our data are at present far too insufficient for more than a 

 speculative suggestion. 



As is well known, the amphiboles which are high in soda 

 and in alumina or ferric oxide, and which there is good reason 

 to believe contain the riebeckite-glaucophane molecule or 

 basic (A^Fe)'", such as riebeckite, glaucophane, arfvedsonite, 

 crossite, hastingsite, barkevikite, aenigmatite, and those we 

 have been describing, are all intensely pleochroic and show 

 either very distinctive blue colors or very intense reds and 

 browns. Similarly, the pyroxenes which contain the acmite 

 molecule, or basic ]STa' and (Al,Fe) //; , as aegirite, aegirite-aug- 

 ite, babingtonite, etc., are deeply colored and are characteristi- 

 cally pleochroic, in contrast with the common, generally 

 non-pleochroic pyroxenes, which do not contain the acmite 

 molecule. On the other hand, the amphiboles which contain 

 trivalent Al and Fe only in bivalent radicals, on the theory of 

 * Penfield and Stanley, op. cit., p. 49. 



