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Peculiar bimodal volcanism in the Hegau region (SW Germany) comprises two contrasting SiO 2 -undersaturated rock suites. (I) Primitive olivine melilitites and melilite-bearing olivine nephelinites (12–9 Ma) are characterized by high MgO, CaO, Fe 2 O 3 , TiO 2 , Ni, V, F, moderate alkalis, Al 2 O 3 , P 2 O 5 , Ba, Nb, Zr, and low SiO 2 , Rb, Pb, and U concentrations. The rocks are composed of forsteritic olivine, diopsidic clinopyroxene, melilite, perovskite, Cr-bearing oxyspinel, F- and Ba-rich mica, and fluorapatite. In rare cases, they contain coeval coarse-grained ijolite patches generated by rapid in-situ fractionation in small melt pockets. (II) Evolved nosean phonolites (14–11 Ma) comprise high alkalis, Al 2 O 3 , SiO 2 , Rb, Nb, Zr, U, Pb, S, and low MgO, CaO, Fe 2 O 3 , TiO 2 , P 2 O 5 , Ba, Ni, and V concentrations, and contain abundant Ba-bearing alkali feldspar and nosean-haüyne-sodalite ss macrocrysts, aegirine-augitic clinopyroxene, as well as accessory apatite, titanite, zircon, and pyrochlore. The melilititic–nephelinitic rocks formed by low degrees of partial melting of a carbonated amphibole ± phlogopite-bearing garnet wehrlite in the uppermost asthenosphere or in the thermal boundary layer and occur also in neighbouring regions. However, their coexistence with evolved nosean phonolites is a unique and so far, unexplained feature in the southern Central European Volcanic Province. Thermodynamic modelling implies that removal of 11–19% oxyspinel, 4–10% olivine, 42–57% clinopyroxene, <3% mica, <9% feldspathoids, <8% feldspar from melilititic–nephelinitic parental melts at upper crustal conditions (∼200 MPa) results in significant amounts of phonolitic residues that are compositionally similar to the exposed nosean phonolites. Strongly negative P and Ti anomalies and a trough of MREE in primitive mantle-normalized trace element patterns of phonolites indicate additional fractionation of titanite and apatite, consistent with the mineralogy of coarse-grained (nepheline) syenitic cumulates, which are present as enclaves in both rock suites. The modelling results suggest crystallization of the (nepheline) syenite cumulates between 1050 and 800 °C and ascent and eruption of the phonolite residues at no less than ∼900 °C with complete solidification of the observed assemblage at >750 °C. Significant crustal assimilation during fractionation appears unnecessary to explain the mineralogical, mineral chemical, and geochemical characteristics of the phonolites. Prolonged upper crustal differentiation of the magmas in one case (nosean phonolites) and fast ascent of primitive melts in the other (olivine melilitites and melilite-bearing olivine nephelinites) can be explained by stress field changes from an extensional to a more compressive regime, the magma ascent becoming thereby increasingly structurally controlled and supported by brittle deformation. • Hegau phonolites derived from melilitites/nephelinites by fractional crystallization. • Their geochemistry and mineralogy are largely reproducible by MELTS modelling. • Nepheline syenite enclaves represent entrained cumulate rocks of upper crustal origin. • Ijolitic patches in melilites/nephelinites are generated by in-situ differentiation. • Ascent of both volcanic suites is controlled by faulting and basement unit competence. [ABSTRACT FROM AUTHOR]

Abstract: Black heavy mineral (HM) aggregates of metallic luster and composed of ilmenite and rutile were named “nigrine” (amount of rutile>ilmenite) and “antinigrine” (amount of ilmenite>rutile). They contain inclusions of, e.g., columbite-(Fe), pyrochlore group minerals, wolframite solid solution series (=s.s.s.), monazite, zircon, Fe oxides and sulfides as well as alteration minerals such as pseudorutile and Fe–Ti–Nb–Ta–Al–P compounds, whose precise chemical composition and mineralogical affiliation cannot be determined. These titaniferous HM aggregates are of widespread occurrence in gneisses and shear zones cutting through them as well as alluvial, fluvial and colluvial deposits at different distances from rare-element phosphate pegmatites within the crystalline basement in SE Germany. “Nigrine” and “antinigrine” may be subdivided into three types which formed at different periods relative to the Variscan tectonic disturbances and which show different qualities as markers for the origin and presence of Nb-bearing pegmatites along the western edge of the Bohemian Massif, SE Germany. Type A developed pre- to synkinematically within or near deep-seated shear zones which formed below 730°C as early as 321 to 329Ma. These HM aggregates are poor in Nb and impoverished in accessory minerals. The HM aggregates having developed in shear zones mark the “kitchen” where friction and heating contributed to the formation of felsic intrusive bodies, such as pegmatites and aplites. This type of Ti-bearing HM aggregates may be held as distal proximity indicator. Type B is early postkinematic and enriched in niobian rutile, rife with lots of inclusions, especially columbite-(Fe). It precipitated in the crystalline country rocks at temperatures around 600°C concomitant with the nearby rare-element pegmatites between 302 and 311Ma. It is the most proximal member of this group of HM aggregates. Type C “nigrine” is enriched in W and late postkinematic relative to the shearing processes in the crystalline basement. It formed around 470°C during the same period of time as type C. It does, however, not qualify as a marker for rare-element pegmatites, as it is unrelated to these felsic intrusives. During the late Neogene the Variscan basement was strongly uplifted and many pegmatites were stripped off their roof rocks. As a consequence of this, “nigrine” and “antinigrine” were released from their host rocks and became part of the terrigenous sediments laid down in the drainage systems which upstream cut into the crystalline basement and the pegmatites/aplites. Due to their high resistance to chemical weathering these titaniferous HM aggregates acted as “armored relics” for less resistant minerals such as columbite and pyrochlore. When these titaniferous HM aggregates got decomposed by attrition these mineral inclusions of lesser stability appear in the fluvial sediments further afield from their source than expected, considering the low stability to weathering of single crystals of columbite- and pyrochlore group minerals. “Antinigrine” and “nigrine” may be spotted in the HM suites of drainage systems around pegmatites at a distance of ≤10km with correlation coefficient of RNb–Ti =+0.42, while in the range 2–5km RNb–Ti increases to +0.77, in the range 1–2km RNb–Ti is +0.85 and around 1km from the rare-element pegmatite RNb–Ti stands at +0.92. “Nigrine” and “antinigrine” are no ideal mineral aggregates to form placer-type deposits of their own due to their variegated mineralogy, excluding some Ta-enriched subtypes in Colombia and Sierra Leone. The variable mineralogy detrimental to its use as a source of Ti, is the strong point as an exploration tool for rare-element pegmatites. [Copyright &y& Elsevier]

The article discusses the discovery of hydroxymanganopyrochlore, a new mineral found in the Eifel Volcanic Region of Germany. It notes that the new mineral is a manganese-dominant mineral that belongs to the pyrochlore supergroup and was approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA). It indicates a sample of the new mineral represented by a fragment of vesicular sanidinite lava that was collected by one of the authors in the pumice quarry in den Dellen near the city of Mendig in Germany. It states that the mineral was discovered in the composition of a late paragenesis including jacobsite, nosean and sanidine. It presents the chemical composition of the mineral as well as the X-ray powder diffraction data.

Lead-bearing phyllotungstite from the Clara mine in the central Black Forest, Germany has a formula (Cs0.41)Na0.14K0.05Pb2+2.01Ca 0.26[W6+10.87Fe3+3.13O 35.75(OH)6.25](O(H2O)3). X-ray diffraction patterns exhibit pseudohexagonal symmetry, but refinement of single-crystal synchrotron data has shown that the true symmetry is orthorhombic, Cmcm, with a = 7.298(1), b= 12.640(2), c= 19.582(4) Å, and that the pseudohexagonal character is due to submicrometre-scale cyclical twinning by rotation about the pseudohexagonal caxis. The structure can be described in terms of an ordered intergrowth, parallel to (001), of (111)pyblocks with pyrochlore-type structures, which are ~6 Å in width, and two-layer wide regions with a hexagonal tungsten bronze (HTB) type structure. Caesium atoms occupy 18-coordinate cavities in the HTB regions, and H2O molecules occupy F sites in the A2B2O6F pyrochlore blocks. The lowering of symmetry from hexagonal to orthorhombic is due to partial ordering of W and Fe in the octahedral Bsites and of Pb and vacancies in the Asites of the pyrochlore blocks. The ideal formula for the intergrowth structure (with no vacancies) is C2A 10[B14(O, OH)42] F4, where Cis the cavity site in the HTB slabs. The mineral has only 21% occupancy of the Csite and 25% occupancy of the Asite, but full occupancy of the F site. There may be some mixing of Cs and H2O between the Cand F sites. [ABSTRACT FROM AUTHOR]