Octahedral occupancy

The most important character of all of the berthierine compositions is their low silica content. The variation of compositions found for pellets from the recent sediments appears to be the result of the crystallization of a chlorite structure with full octahedral occupancy. The meta-berthierines fall within the limits deduced for synthetic magnesian 7 X chlorites, limits which are also near full octahedral occupancy. 

Analysis No. 1 represents the only truly tetrasilic celadonite. The average value for Si in the 17 analyses is 3.83. Except for Nos. 17-19 variations are between 3.73 and 4.00. Octahedral occupancy is close to 2 for all celadonites, with an average of 2.05. Interlayer cations often number substantially less than 1.0 and average 0.81 with a low value of 0.38 (No. 14). This, of course, is typical also for glauconites and hydrous micas. ... 

Celadonite, with a low trivalent ion octahedral occupancy, differs from the other clays in having, on an average, twice as much Mg. A plot of octahedral Mg versus Al/(A1+Fe3+) (Fig. 11) shows that as the relative percentage of Al in the octahedral position decreases, the percentage of octahedral Mg increases. Thus, as the relatively... 

There is a complete isomorphous series from Si4.0Al0.o to Si3.soAl0.So. Values larger than Al0.so have been reported (Ross and Hendricks, 1945 Sawhney and Jack-son, 1958) but in these samples, the sum of the octahedral cations is excessive, usually being on the order of 2.2, and gives a positive charge to the octahedral sheet. Whether such high octahedral occupancy is real, or not, has not been satisfactorily determined. 

Fig.28. The relation of percent octahedral occupancy to RJ+/(R3+ +R2 +) for layer structure and chain structure clays, = saponite = attapulgite x = sepiolite (nine octahedral positions) o = sepiolite (eight octahedral positions).

Attapulgite-palygorskite clays lie between the Mg-rich and R3+-rich sheet clays. Thus, a continuous series exists between Mg and R3+. The Mg/R3+ ratio in this series is related to the relative amount of filled octahedral positions per total positions available. The dioctahedral clays theoretically have 67% of their total octahedral positions filled the attapulgites 80% the sepiolites 90% and the trioctahedral clays approximately 100%. Fig.28 shows a plot of percent octahedral occupancy versus R2+/r3+ + r2+ inustrating this relation. As R2 + is replaced by R3 + there is a gradual increase in unoccupied positions to satisfy charge requirements. 

In the dioctahedral 2 1 sheet-structure silicate with the occupied sites more than 85% occupied by Al, the structure seems to be able to compensate for the internal strain and can grow to a considerable size. The Al octahedral occupancy values of muscovite (>1.7) and the 2 1 dioctahedral clays (1.3—1.7) indicate that there is little overlap. It is likely that the decreased amount of tetrahedral twist induced by increasing the size of the octahedral cations and octahedral charge (decreasing Al) determines that a clay-size rather than a larger mineral will form. The R3+ occupancy value can be less than 1.3 when the larger Fe3+ is substituted for Al. When Al occupancy values are less than 1.3 (65%), in the absence of appreciable iron, the internal strain is such that growth is in only one direction. The width of the layer is restricted to five octahedral sites. Sufficient layer strain accumulates within this five-site interval such that the silica tetrahedral sheet is forced to invert to accommodate the strain. 

The octahedral occupancy distinguishes these verdine facies or odinite minerals from other sedimentary and in fact all metamorphic phyllo-silicate minerals. These relations are clearly demonstrated by Hornibrook and Longstaffe (1996). 

HiUier S. and Velde B. (1991) Octahedral occupancy and the chemical composition of diagenetic (low temperature) chlorites. Clay Min. 26, 149-168. 

Group name Layer type Layer charge per unit cell Octahedral occupancy Subgroup Species... 

Several micas were described with total octahedral occupancy midway between dioctahedral and trioctahedral. However, in many cases, intermediate compositions may represent interstratified mixtures of dioctahedral and trioctahedral layers (or species). Li-rich muscovite crystals (Brigatti et al. 2001b) have a total octahedral occupancy of 2.24 apfu. The volume of the M(2) site increases sharply with an increase in Li. The M(l) cation site is partially occupied. These results seem to indicate a partial dioctahedral-trioctahedral solid solution. 

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