Uvarovite: Stability of uvarovite-andradite solid solutions at low pressure

Uvarovite (Ca₃Cr₂Si₃O₁₂) forms a complete solid solution series with andradite (Ca₃Fe ₂ ⁺³ Si₃O₁₂) below 1,137±5 ° C at a total pressure of 1 atm. Pure uvarovite decomposes to pseudowollastonite (CaSiO₃)+eskolaite (Cr₂O₃) at 1,385 ± 10 ° C. The incorporation of Ca₃Fe ₂ ⁺³ Si₃O₁₂ component in the uvarovite structure lowers the thermal stability of the garnet. The breakdown assemblage is garnet_ss (Ca₃(Cr,Fe⁺³ ₂)Si₃O₁₂)+pseudowollastonite (CaSiO₃)+hemeskolaite_ss(Cr,Fe⁺³O₃). Pure andradite decomposes to pseudowollastonite (CaSiO₃)+hematite (Fe₂O₃) at 1,137±5 °C. Andradite thermal stability is increased by incorporation of Ca₃Cr₂Si₃O₁₂ component by 248 °C.At 1,264±5 °C pseudowollastonite+hematite react to liquid defining a thermal minimum of the CaSiO₃-Cr₂O₃-Fe₂O₃ ternary system. This minimum is located at about 64.5 wt.-% CaSiO₃, 0.5 wt.-% Cr₂O₃, and 35.0 wt.-% Fe₂O₃. Uvarovite and andradite bulk compositions start to melt at 1,420 °C and 1,265 ±5 °C, respectively.The unit-cell parameter for uvarovite is 11.999 (2) Å, the refractive index 1.866 (2). The substitution of Cr⁺³ by Fe⁺³ increases a and n almost linearly toward the andradite end member which displays a unit-cell parameter of 12.059 (3) Å and a refractive index of 1.887 (2).     

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Der petrogenetische Werdegang der Klinopyroxene in den tertiären Vulkaniten der Hocheifel

The course of crystallization of basalt forming clinopyroxenes in the olivine-basalt-hawaiite-mugearite-trachyte-and basanitoid (= basanite)-series from the Hocheifel area (Western Germany) has been presented by the author in two earlier papers (Hucken-holz, 1965 a, 1965 b). The present paper deals with the evolution of clinopyroxenes from the ankaramites in the same area. The ankaramites are richer in olivines and clinopyroxenes than the normal olivine basalts and may be a product of accumulation of olivines and clinopyroxenes in an olivine basalt magma. The first clinopyroxene formed in the ankaramites is a greenish chromian salite Ca_47.1 Mg_40.9 Fe_12.0 which is preserved in the cores of the phenocrysts. Strongly zoned brown titansalite Ca₄₅ Mg₄₂Fe₁₃ surrounds the chromian salite (HF 5) or appears in independent microphenocrysts (HF53). The groundmass clinopyroxene is titansalite Ca₄₅Mg₃₇Fe₁₈ with a small optic axial angle. Phenocrysts of nickel-rich olivine Fe_12–14 and chrome spinell were formed together with chromium clinopyroxene. The same minerals together with orthopyroxenes have been observed in the olivine basalts, hawaiites and basanitoids (= basanites). This paragenesis indicates high pressure and high temperature in the alkali basalts of the Hocheifel during an initial stage of magmatic evolution. With the decrease of pressure, mineral reaction occurred with the alkali basalt melt. The high pressure clinopyroxenes of the ankaramites and basanitoids (= basanites) were changed to a larger extent than the clinopyroxenes of the olivine basalts and hawaiites or were completely destroyed (HF 53) because they are in contact with the hot magma for a longer period of time. This favours the reaction between the solids and the melt, and the high pressure garnet and/or enstatite components in the clinopyroxenes are substituted by Ca-Tschermak’s and titanaugite molecules.      

Uvarovite: Stability of uvarovite-grossularite solid solution at low pressure

Uvarovite (Ca₃Cr₂Si₃O₁₂) forms a complete solid solution series with grossularite (Ca₃Al₂Si₃O₁₂) below 855 ± 5 ° C at a total pressure of 1 atm. Pure uvarovite decomposes to pwo (CaSiO₃) + esk (Cr₂O₃) at 1385 ± 10 ° C. The incorporation of about 5 wt-% of Ca₃Al₂ Si₃O₁₂ component in the uvarovite structure raises the thermal stability of the garnet_ss to 1410 ± 5 ° C, and uvarovite₉₅ grossularite₀₅ melts incongruently to pwo (CaSiO₃) + coresk_ss ((Al, Cr)₂O₃) + L. Pure grossularite decomposes to wo (CaSi0₃) + geh (Ca₂Al₂SiO₇) + and (CaAl₂Si₂O₈) at 855 ± 5 ° C, grossularite thermal stability is increased by incorporation of Ca₃Cr₂Si₃O₁₂ component by 530 ° C. At 1280±5 ° C coresk_ss + L react to gar_ss + geh + an defining an invariant tequilibrium of the CaO-Cr₂O₃-Al₂O₃-SiO₂ quaternary system. Liquid reacts to gar_ss + pwo + geh + an at 1263 ±5 ° C terminating univariant and divariant liquid relations occurring along the join Ca₃Cr₂Si₃O₁₂-Ca₃Al₂Si₃O₁₂. The unit-cell parameter for uvarovite is 11.996(2) Å, the refractive index 1.865(3). The substitution of Cr by Al decreases a and n almost linearly toward the grossularite end member which displays a unit-cell parameter of 11.848(2) Å and a refractive index of 1.732 (1).     

The origin of fassaitic augite in the alkali basalt suite of the Hocheifel area,Western Germany

Fassaitic augite (augite 3) occurs in clinopyroxenite fragments with cumulus textures or as anhedral crystals in alkali basalts and nepheline basanites of the Hocheifel Area. Rimming of augite 3 by phenocrystic augite (augite 2) followed by groundmass augite (augite 4) defines the sequence of the clinopyroxene crystallization. Fassaitic augites from other alkali-basalt series reveal clinopyroxene crystallization trends of increasing ferri Tschermak's molecule and concomitant acmite as fractionation proceeds. This trend appears to be much more common than previously assumed.Dedicated to K. Jasmund in honor of his sixtieth birthday.     

Crystal chemistry of a natural schorlomite and Ti-andradites synthesized at different oxygen fugacities

Ti-andradites were synthesized at a pressure of P(H₂O)=3 kbar and temperatures of 700–800° C. Oxygen fugacities were controlled by solid state buffers (Ni/NiO; SiO₂ + Fe/Fe₂SiO₄). The Fe²⁺-and Fe³⁺-distribution was determined by low temperature Mössbauer spectroscopy. The water content was measured by a solid's moisture analyzer. The chemical composition of the synthetic and the natural sample has been determined by electron microprobe. Ti-andradites from runs at high oxygen fugacities have Fe³⁺ on octahedral and tetrahedral sites; Ti-andradites from runs at low oxygen fugacities have tetrahedrally and octahedrally coordinated Fe²⁺ as well. These “reduced” garnets must also contain Ti³⁺ on octahedral sites. Charge balance is maintained due to substitution of O^2? by (OH)^? by two mechanisms: (SiO₄)^4? ? (O₄H₄)^4? and (Fe³⁺O₆)^9? ? (Fe²⁺O₅OH)^9?. FTIR spectra of the synthetic samples do show the presence of structurally bound (OH)^?. In a natural sample tetrahedrally and octahedrally coordinated Fe³⁺ are observed together with Fe²⁺ on all three cation sites of the garnet structure.     

Der petrogenetische Werdegang der Klinopyroxene in den tertiären Vulkaniten der Hocheifel II. Die Klinopyroxene der Basanitoide

The course of crystallization of basalt forming clinopyroxenes in the olivine basalt-hawaiite-mugearite-trachyte series from the Hocheifel area (Western Germany) has been presented by the author in, an earlier paper (Huckenholz 1964, part I). The present paper deals with the evolution of clinopyroxenes from the basanitoid rocks in the same area. The basanitoids are richer in clinopyroxenes than the normal olivine basalts and may be a product of accumulation of clinopyroxenes in an olivine basalt magma.Mineral separation is very troublesome in fine-grained volcanic rocks but special procedures (as outlined in part I) allow chemical, optical and X-ray analyses of three generations of clinopyroxenes and of other basalt forming minerals. The first clinopyroxenes formed in the basanitoids is a greenish chromian salite Ca₄₅Mg₄₃Fe₁₂ which is preserved in the cores of the phenocrysts. Strongly zoned brown titansalite Ca₄₅Mg₄₁Fe₁₄ surrounds the chromian salite or appears in independent microphenocrysts. The groundmass clinopyroxene is a sodian titansalite Ca₄₆Mg₃₇Fe₁₇ with a small optic axial angle. In contrast to the olivine basalt-trachyte series the clinopyroxenes of the basanitoids are richer in titanium and aluminum.Phenocrysts of nickel-rich olivine Fe_14–16, relics of orthopyroxene Fs₁₄ and chrome spinell were formed together with the chromian clinopyroxene. The same minerals have been observed in the olivine basalts and hawaiites. This paragenesis indicates high pressure and high temperature in the alkali basalts of the Hocheifel during an initial stage of magmatic evolution. With the decrease of pressure mineral reactions occurred with the alkali basalt melt. The high pressure clinopyroxenes of the basanitoids were changed to a larger extent than the clinopyroxenes of the olivine basalts and hawaiites because they are in contact with the magma for a longer period of time. This favours the reaction between the solids and the melt, and the high pressure garnet and/or enstatite components in the elinopyroxenes are substituted by Ca-Tschermak's and titanaugite molecule.     

Sedimentpetrographische Untersuchungen an Gesteinen der Tanner Grauwacke

Zusammenfassung Gesteine der Tanner Grauwacke bei Straßberg (Unterharz) wurden optisch, chemisch-analytisch und röntgenographisch untersucht. Sie sind feldspatreiche tonige Grob- bis Feinsandsteine mit wechselnden Gehalten an Gesteinsbruch stücken sowie an Glimmer-Chlorit und als Grauwacken zu bezeichnen. Die Gesteinsbruchstücke setzen sich aus magmatischen und metamorphen Materialien zusammen, sedimentäre Reste sind nur gering vertreten. Typische Gefügeeigen-schaften sind der Wechsel des mittleren Korndurchmessers von grob nach fein, das Besserwerden von Sortierung, die Abnahme der Kornrundung sowie die Zunahme von anisometrischen Kornformen und der Porosität zum hangenden Teil der einzelnen Bänke. Geosynklinale Sedimentation am Beginn der variszischen Faltungsära im Vorfeld der aufsteigenden Mitteldeutschen Schwelle bedingte die Entstehung der Tanner Grauwacke.     

Petrographie und Mineralfazies einer konglomeratischen Tanner Grauwacke aus dem Unterharz

A very coarse grained graywacke from Strassberg in the Lower Harz Mountains (Germany) consists of rock-fragments, feldspars, quartz, chlorites, micas, accessories and ores. Low grade metamorphic mineral reactions are indicated by the formation of 2M-muscovite +chlorite+sphene+albite in the matrix of the graywacke, by prehnite+chlorite+ sphene+calcite+sericite+albite (±quartz) in volcanic rock-fragments, and by prehnite +calcite+quartz along fine fissures. The newly formed minerals in the coarse grained graywacke belong to the laumontite-prehnite-quartz-facies of the burial metmorphism. However, the formation of the low grade minerals is caused by thermal metamorphism in outer contact zones of the Ramberg granite intrusion in the Strassberg area.