Compositional Variation in Component Layers in Natural Illite/Smectite
Published chemical data for suites of mixed-layer minerals from diagenetic sedimentary rocks, hydrothermally altered tufts, and a metasomatic bentonite bed indicate that the layer charge and composition of the different components of illite/smectite (I/S) differ from one geological environment to another. It appears that the composition of the elemental smectite and illite layers in the I/S is more or less constant for samples within each geologic setting. In the examples considered, the smectite layers are predominantly montmorillonitic in character (i.e., the charge is in the octahedral site), whereas the illite layers show different types of charge sites, depending upon the suite studied. Illite layers appear to have about the same charge in all three suites studied, slightly more than 0.7 per O10(OH)2 unit, whereas the smectite layers in the different suites range in charge from about 0.3 to 0.7 per O10(OH)2 unit. Cation-exchange capacities reflect these differences in charge, although not ideally. The differences in the composition of the component layers in each geologic suite of mixed-layer clays are probably due either to differences in the bulk chemistry of the rocks in the different suites or to differences in intensive variables, such as temperature and pressure, of the regime under which they have formed.
Solid solutions in I / S mixedJayer minerals and illite
Chemographic analysis of suites of I/S minerals using M*-4Si-R'z* coordinates reveals the existence of independent mineral series. The interlayered smectite is a montmorillonite whose composition and charge depends upon the P, T, and chemical conditions of formation. The charge can vary from near 0.33 to 0.66 depending upon the series. In each of the I/S series the montmorillonite is interlayered with an illite that has nearly the same composition for all I/S phases. The illite layer has a charge of 0.87. It appears that the members of the I/S mineral series are interlayered from a fully smectitic composition to a bulk composition with a charge of 0.75. Thus the interlayering does not reach the illite end-member with a charge of 0.87. This series can be treated as an incomplete solidsolution. I/S mixedJayer minerals can be considered as a partial solid solution with three components: high-charge illorr, Iow-charge mnto33, and high-charge mnto66. It is probable that the high-charge illo ' component cannot replace the montmorillonite in all proportions from 0 to l00o/o because of crystallographic incompatibilities.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.