In a recent contribution to the American Mineralogist. Dr. A. N. Winchell has included a very interesting comparison between a revision of his theory of the micas and the theory I ventured to suggest some time ago in the Mineralogical Magazine. The two theories yield in general very similar values for the composition of the principal minerals concerned, but Winchell's present assumption that Al2O3 is replaced by MgO · SiO2 necessitates a steady dimunition in the ratio K2O:SiO2 as the MgO increases, while my own suggestion that Al2O3 is replaced by MgO leaves this ratio unaltered. Dealing with the composition of muscovite, he remarks that a number of ‘probably inaccurate’ analyses (of sericite etc.) contain an average ratio K2O:SiO2 of only 69:600. I had disregarded these because I felt that the materials were of too doubtful a nature: if they are to be accepted we must both seek a new end member for this section of the group, since Winchell's earlier formula contains 8SiO2 (75:600) and his present formula 7 SiO2(86:600). But I still think the best course is to leave the old analyses of sericite for better mineralogical identification: there remains a sufficient number of analyses of clean muscovites. Winchell points out that pure muscovite would not serve to test the substitution theories. But very few of the published analyses represent pure muscovite; out of the 46 analyses I quoted, no fewer than 25 contain over 30MgO: 6OOSiO2, representing over 30 per cent of the phengite molecule; they are, therefore, well adapted for testing his theory. Referring to my diagram showing the incidence of potash ratios in these micas, he considers that ‘the relative frequency of the potash ratios is not the question at issue.’ But it is unfortunately the case that his theory requires a susbstantial diminution of this ratio, to below 96, in more than half the analyses plotted, and of this there is little trace in the muscovite diagram or in the diagrams for the other types of mica where similar considerations would obtain.

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