Washington — Kaersutite from Linosa and Greenland. 205 



Penfield and Stanley, as actinolite or common hornblende, 

 are less deeply colored, are never blue, and are markedly less 

 pleochroic ; and the same is true of the ordinary pyroxenes and 

 augites. 



It is commonly supposed* that the blue color of these 

 amphiboles is connected with the presence of abundant iron, 

 and Pirssonf has recently suggested that the blue color is due 

 to the presence of ferrous-ferric molecules, the analogy of 

 Prussian blue and altered vivianite being cited. Since, how- 

 ever, a similar color is characteristic of glaucophane and gas- 

 taldite, in which the trivalent element is practically entirely 

 aluminum, ferric iron being either absent or present in small 

 amount, it would seem to be necessary to amend this hypothesis 

 by assuming aluminum to replace the ferric iron either wholly 

 or partially. Similarly the deep browns and reds are supposed 

 to be connected with the presence of titanium and, as pointed 

 out by Brogger,;}; the intensity of the color increases with 

 increasing content in this element, as is shown by the series 

 barkevikite, basaltic hornblende, and kaersutite and aenigmatite. 



We have seen above that (Al, Fe) /// may enter the amphi- 

 bole molecule either in a bivalent radical, such as those sug- 

 gested by Penfield and Stanley, the radical as a whole acting 

 as a base, or it may itself act as a base, replacing three atoms 

 of hydrogen in the amphibole acid. It would thus occupy 

 different positions and perform very distinct functions in the 

 molecular arrangement. In these different positions, there- 

 fore, the trivalent element may reasonably be supposed to 

 affect differently the optical and other physical properties of 

 the minerals into which it enters, in analogy with the well- 

 established fact in the chemistry of the carbon compounds. 



Following out this line of thought, it may be suggested 

 that this basic (Al, Fey", and not that which forms part of 

 bivalent radicals, acts as a chromophore, as such color-giving 

 radicals are known in organic chemistry, where they are 

 especially notable among the aromatic compounds ; and that, 

 furthermore, the property of pleochroism may be connected 

 with its presence, this either causing a mineral variety to be 

 absolutely pleochroic, when varieties in which the basic triva- 

 lent element is not present are not so, or intensifying the 

 pleochroism of otherwise weakly-pleochroic complex mineral 

 molecules. That this chromophoric radical does not consist 

 solely of (Al, F)'", but contains Na as well, probably in the 

 ratio 1 : 1, is indicated by the constant presence of much soda 

 in the j)eculiarly colored and pleochroic amphiboles and pyrox- 



*Cf. W. C. Brogger, Grorudit-Tinguait Serie, p. 35, 1894. 

 fL. V. Pirsson, this Journal, vol. xxiii, p. 440, 1907. 

 \ Brogger, loc. cit. 



