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.
Some experiments pertaining to the magnetite-ulvtispinel miscibility gap
Hydrothermal experiments on the join Feron-FerTioo are consistent with-but do not conclusively prove-the existence of a miscibility gap having a consolute temperature of 565115"C and composition near MtrrUspo, for the pure magnetite-ulvtispinel join. These results appear to conflict with the conclusion of Price (preceding paper) that the consolute point for natural magnetite-ulvcispinel intergrowths lies at or below 490oC. The experirnents can be reconciled if (a) the presence of several percent MgO in the natural specimens depresses the consolute point, or (b) the experiments reported here failed to attain equilibrium-a real possibility since they have not been strictly reversed. Introduction In the preceding paper, Price (1981) argues strongly that the consolute temperature of the magnetite-ulvdspinel (Mt-Usp) miscibility gap lies below 490oC, that is, more than 100'C below the generally accepted value of -600'C (Vincent et al.,1957). In 196'l-68,I made a series of experiments on the MtUsp join. The results, while not conclusive, seemed generally compatible with those of Vincent et al., so I never completed or published them. Price's manuscript prompted a careful review of these old experiments; they do nol disprove Price's low consolute temperature, but they do provide permissive evidence for a consolute point at a higher temperature (565+l5oc) and a slightly asymmetric composition (probably near MtrrUsp4s, mole percent) for the pure FerOo-FerTiOo join. The discrepancy between our results may reflect the effect of minor constituents such as Mg in Price's natural samples or it may reflect a failure of my experiments to reach equilibrium-a very real possibility since they are not reversed in a strict sense. Experimental details The starting materials for the experiments were single-phase powders of synthetic IJspro, U.prr, Llspor, IJspuo, and Uspro; for most runs, two of these spinels were ground together (under ethanol in an 0003-004x/8 I /0708-0759$02.00 agate mortar for one hour) to provide a two-phase mechanical mixture of the desired bulk composition. Other runs were made on single phases of intermediate composition. Approximately 100 mg of powder was sealed with 5-10 wLVo H2O in AgroPdro capsules. Most of these capsules were then sealed with wustite-magnetite (WM) or iron-magnetite (IM) buffer * H,O in Au capsules. The WM buffer (IM below 570'C) was chosen because Mt-Usp solutions ranging from Uspo to lJspro are stable at the oxygen fugacity it imposes (Lindsley, 1962).I hoped that the hydrothermal environment would enhance reaction rates. The external pressure medium for most experiments was methane (rather than HrO) so as to extend the life to the buffers. Some buffers still did not last for the duration of the experiment, and minor amounts of ilmenite formed in them (Table l). Two capsules without buffer were nrn in an Ar-H, mixture at I kbar;'the H, content was maintained at 30G500 bars through a Pt membrane. Compositions of run products were estimated from the positions of the (40), (620), and (622) peaks on X-ray diffraction patterns, which were obtained either at a scan rate of 0.125o20/min. or through stepscans at 0.01/20 per 100 sec. The calibration curves were based on the unit-cell values (Lindsley, 1965) as modified by some additional points. The rather large uncertainties reported for many compositions (Table l) reflect the lack of sharpness in the corresponding X-ray peaks. LINDSLEY: MAGNETITE.T]LTOSPINEL MISCIBILITY GAP Table l. Experiments pertaining to the Mt-Usp miscibility gap Ru # Starting reterial Bulk Phases Temp. oc Buffersr turation Products mediw davs
Cholesterol synthesis from acetate in the diabetic liver.
Brady and Gurin (1) recently compared the ability of liver slices prepaped from both normal and diabetic rats to incorporate added acetate into cholesterol. They drew the conclusion that the conversion of acetate to cholesterol in the diabetic liver is “apparently unimpaired, although this process may be inhibited when diabetes is sufficiently prolonged and severe.” In view of our interest in the relation of cholesterol synthesis to arteriosclerosis, the development of which is generally regarded as being accelerated in diabetes (2), we have reinvestigated cholesterol synthesis from acetate in diabetes. A definite increase in cholesterol formation in the diabetic liver is demonstrated here. It is further shown that, as a result of insulin injections in a diabetic rat, slices subsequently prepared from its liver have a reduced capacity to synthesize cholesterol.