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¶Rocks containing braunite from the Ossa-Morena central belt (Iberian Massif, SW Spain) have been studied; these include nodules and layers of braunite (association I), Mn-slates (association II) and Mn-metatuffs (associations III and IV). Geochemical features of braunite nodules such as Mn/Fe ratios around 2, positive Ce-anomalies and good correlations among Mn, Fe, Co, Cu and REE contents indicate that the protolith of the braunite-nodules was precipitated from oxidising sea water. Greenschist facies Hercynian metamorphism reduced initial Mn4+ to Mn3+ and Mn2+. High initial fO2 of oxide beds (association I) limited reduction to the formation of braunite. Reduction continued until the formation of garnet + piemontite (associations II and III), and pyroxmangite + pyrophanite (association IV). Ti-rich braunites (up to 6.8% of TiO2) occur in slates and metatuffs in which the (Mn + Fe)/Ti ratio of the whole rock is lower than 30, while braunites have lower Ti contents in slates and metatuffs with (Mn + Fe)/Ti ratios around 90. Fe-rich braunite crystallized in rocks with Mn2+ oxide and silicate where low Mn3+/Mn2+ in the whole rock facilitated substitution of Fe3+ for Mn3+.

11 páginas, 9 figuras, 3 tablas., Experimental illitization of smectite was studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Experiments were performed on the, Financial support was supplied by Research Project nº BTE 2000-0582 of the Spanish Ministry of Science and Technology, Research Group RNM-0179 of the Junta de Andalucia and the REN2001-3868 and REN 2000-0798 projects.

Principal component analysis (PCA) and correspondence analysis (CA) have been used to identify several critical mineral crystallization factors during metamorphism of Mn-rich lithologies from the Iberian Massif, Spain. Three types of variables have been considered in the system: mineralogical variables, bulk composition variables and one physical variable (oxygen fugacity). PCA was applied to the compositional variables to obtain four components, which were used as new compositional variables. These, together with the mineralogical and physical variables, were treated in the CA. The plot of the variables on the factor 1-factor 2 plane reveals that tephroite crystallization is controlled by a compositional variable representing low values of ratio Si/(Mn+Fe) in the rocks. Moreover, it is possible to deduce from this plot the importance of high oxygen fugacity conditions for the crystallization of piemontite. The third factor shows that the crystallization of spessartine does not require very restrictive physical-chemical conditions.

13 páginas, In order to obtain an indication of metamorphic conditions we have studied the mineralogy of the clastic rocks, in particular the phyllosilicates, from three Precambrian series in the Central-Iberian and Ossa-Morena zones of the Southern Hesperian Massif: the Esquisto Grauváquico Complex; the so-called Serie Negra; and the Azuaga Formation. For this purpose we have used the crystal-chemical parameters obtained via X-ray diffraction (d001, b0, ratios of the basal intensity of the white micas and chlorites, and the crystallinity index of the mica) together with electron microprobe data from selected areas, obtained via backscattered electron images. The mineral assemblages, commonly quartz+muscovite +albite +chlorite ±biotite, and the values obtained for the illite crystallinity index (between 0.18° and 0.20° 2Θ) in rocks with non-diagnostic mineral assemblages point to at least greenschist-facies metamorphic conditons. The phengite content of the mica indicates an overall pressure regime near the limits between low and intermediate pressure, but some significant differences exist between the series studied. The Azuaga Formation shows the lowest pressure (about 4 kbar), while the Serie Negra indicates a pressure of at least 1 kbar more. In the Esquisto Grauváquico Complex, when the samples are considered overall, the results are similar to those of the Serie Negra, but there is inhomogeneity in the rocks from the two sections studied. The various types of chlorite, both textural and optical, are homogeneous in composition in each rock specimen and their chemical composition depends primarily on the composition of the host-rock.