Crystal chemistry and petrology of coexisting galaxite and jacobsite and other spinel solutions and solvi '
Galaxite (Gx:MnAlzO+) and jacobsite ("ID:MnFezO+)-rich spinels occur together in the metamorphosed manganese deposit near Bald Knob, North Carolina. They show limited mutual solubility with lz-lgVa Jb in Gx and 19-22% Gx in Jb. The implied solvus is consistent with syntheses oflshida et al. (1977) along the Gx-"ID pseudobinary. Refinement of the structure of one galaxite (Gxg5JbrcSpaGhr) reveals that Mn is located on tetrahedral sites and Al on octahedral sites and that galaxite is therefore a normal spinel. All available data suggest that jacobsite is also a normal spinel at low temperatures, and the solvus between galaxite and jacobsite does not seem to be caused by differences in normality/inverseness. Chemical and experimental data available for (Mg,Zn,Mg,Fe2*XFe3*,Al)zOa spinels suggest hat the ferrites (acobsite, franklinite, magnesioferrite and magnetite) show extensive solid solution and are separated from aluminate solid solutions (galaxite, gahnite, spinel (s.s.) and hercynite) by wide solvi at metamorphic temperatures.
Intra Site Cation Ordering and Clustering in Natural Mn–Zn Ferrites
Cooling rates of 10−3 K/yr. permit naturally occurring Mn–Zn ferrites (franklinites) to be in thermodynamic equilibrium at ambient temperatures with respect to both electrostatic and elastic energies. The elastic strain energy resulting from the occupancy of the A sites by ions of differing sizes is minimized by clustering of Mn and Zn in different regions of a crystal. Using 57Fe NGR, these regions have been observed directly in franklinite with low Mn3+ contents and their structures elucidated. The Mn rich region has a local crystal chemistry very similar to that of MnFe2O4 and the Zn rich region has a local structure similar to that of ZnFe2O4. The two regions are crystallographically coherent. Laboratory heat treatments destroy the Mn–Zn ordering and alter the magnetic properties. The Fe spin configurations are also observed to be collinear. The discovery of the intrasite ordering in these materials proves that Good‐enough's criteria for square B‐H loops in ferrites can be realized. For samples contai...
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’.