Interpretation of the composition of trioctahedral micas

Structural formulas calculated for more than 200 published analyses of phlogopites, biotites, siderophyllites, and lepidomelanes indicate that the additional positive charges carried by trivalent cations proxying for bivalent Mg in the octahedral group of trioctahedral micas are accommodated in two different ways, (1) by a positive charge on the octahedral layer, which is neutralized by an equivalent increase in the negative tetrahedral charge (increased replacement of Si by Al), and all the octahedral sites are occupied, or (2) are neutralized by negative charges associated with unoccupied octahedral sites. In almost all the structural formulas the octahedral group had a positive charge and octahedral occupancy was less than 3.00, indicating some degree of accommodation by both methods. The degree to which accommodation is made by (1) or (2) varies greatly. In general, however, there is a greater accommodation by (2) than by (1) the greater the octahedral trivalent cation content. As a result, there is also a general decrease in octahedral occupancy with increase in octahedral trivalent cation content. In most biotites, siderophyllites, and lepidomelanes octahedral occupancy is significantly less than 3.00 sites hence they are not truly trioctahedral, nor are they octaphyllites. The few formulas in which trivalent cations occupy more than one octahedral site suggests that this is the essential limit of replacement of R+2 by R+3 in these micas. It is also strong evidence against the existence of a complete series between phlogopite and muscovite. Coincident with replacement of Mg by E+3, there is also replacement of Mg by Fe+2, ion for ion. With few exceptions the calculated formulas show both types of octahedral replacement. But the two types, although coincident, are independent; there is no relation between the amount of E+3 present and the amount of Fe+2 present. Neither type of replacement forms a separate series, as a series of Fe+2 only replacing Mg, of which Winchell's annite is the theoretical end member. No representative of this end member was found among the more than 200 analyses collected. On the evidence of the analyses and their calculated formulas, an octrahedral occupancy of more than 2.20 positions by Fe+2 is not to be expected. The only trioctahedral micas in which more than 90 percent of the octahedral positions are occupied by one species of cation are some phlogopites. From pure phlogopite as the prototype, the composition of all other trioctahedral micas may be derived by replacement of Mg by, most commonly, Fe+2 and R+3 (Al and Fe+3). In phlogopites the proxying of such cations for Mg is minor, and Mg occupies more than 70 percent of the occupied positions. Progressively greater proxying of these other cations for Mg leads successively to Mg biotites, in which Mg is still the dominant octahedral cation but in which Fe+2 is present in significant amounts, Fe+2 biotites, in which Fe+2 is the dominant octahedral cation, with Mg present in subordinate but significant amounts, and siderophyllites and lepidomelanes, in which Mg is essentially absent, with Fe+2 the greatly dominant bivalent octahedral cation and with significant amounts of aluminum and (or) ferric iron. These relations are expressed in the following formulas, which show the range in composition of the different groups: