Discussion on the publication by H.-J. Schneider et al.     

Th. Bechstädt 《Mineralium Deposita》1978,13(2):281-284

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P-T-X data on P21/c-clinopyroxenes and their displacive phase transitions     

Thilo Arlt  Martin Kunz  Jano Stolz  Thomas Armbruster  Ross J. Angel 《Contributions to Mineralogy and Petrology》2000,138(1):35-45

The P2₁/c clinopyroxene kanoite (ideally MnMgSi₂O₆) was studied as a function of pressure and temperature using powder X-ray diffraction, differential scanning calorimetry (DSC) and optical methods. The temperature of the P2₁/c to high-temperature (HT) C2/c transition ranges from 425?°C in endmember MnMgSi₂O₆ to 125?°C in natural samples with an aegirine component. Compiling pigeonite and clinoenstatite–clinoferrosilite literature data, the temperature of the transformation was found to decrease linearly with M2 cation size. A synchrotron powder diffraction study in a heated diamond-anvil cell (DAC) yielded compression and thermal expansion data for low kanoite of composition Mn_1.2Mg_0.4Fe_0.4Si₂O₆. The high-pressure (HP) phase transition from P2₁/c to HPC2/c was reversed at 5.8 GPa at 417?°C. The high-temperature phase transition from P2₁/c to HTC2/c was rather indistinct and occurred at approximately 530?°C and 0.76 GPa. In a separate experiment, the HT transition was observed optically in a hydrothermal DAC between 0.0 and 0.4 GPa. The in-situP-T data of both experiments yielded an increase in transition temperature with increasing pressure (approx. 149?°C/GPa) and suggest a change in character of the transition from first order to continuous with increasing pressure. The data indicate that the HTC2/c and HPC2/c polymorphs are distinct phases with different stability fields. Since the high-temperature and the high-pressure polymorphs of kanoite were shown to be isotypic with other low-Ca clinopyroxenes such as the (Mg,Fe)SiO₃ series, the conclusions we draw from this study are valid for all clinopyroxenes with small (<0.88 Å) M1 and M2 cation sizes. The petrologic implications of these conclusions for the occurrence of “clinoenstatite” in the Alpe Arami peridotite are discussed.  相似文献   

High precision triple oxygen isotope composition of small size urban micrometeorites indicating constant influx composition in the early geologic past     

Fabian Zahnow  Tido Stracke  Tommaso di Rocco  Thilo Hasse  Andreas Pack 《Meteoritics & planetary science》2023,58(11):1567-1579

In this study, we present a method for high precision Δ′¹⁷O (Δ′¹⁷O_RL = ln(δ¹⁷O + 1) – λ_RL ln(δ¹⁸O + 1)) analysis of small mass silicate and oxide materials. The analyses were conducted by laser fluorination in combination with gas chromatography and continuous flow isotope ratio monitoring gas spectrometry. We could analyze the oxygen isotope composition of samples down to 1 μg, which corresponded to about 13 nmol O₂. The analytical error (we report the 1σ external reproducibility of a single analysis) in δ¹⁸O increases with decreasing sample sizes from ~0.2‰ for ~20 μg samples to ~0.9‰ for 1 μg samples. For Δ′¹⁷O, we achieved an external reproducibility of 0.04‰ for a sample mass range between 1 and 27 μg. The uncertainty in Δ′¹⁷O is smaller than the uncertainty in δ¹⁸O due to the correlated errors in δ¹⁷O and δ¹⁸O. We applied the method to urban micrometeorites, that is, small meteorites (<2 mm) that were sampled from a rooftop in Berlin, Germany. A total of 10 melted micrometeorites (S-type cosmic spherules, masses between 11 and 22 μg) were analyzed. The oxygen isotope compositions are comparable to that of modern Antarctic collections, indicating that the urban micrometeorites sample the same population. No indication for terrestrial weathering had been identified in the studied set of urban micrometeorites making them suitable materials for the study of micrometeorite origins.  相似文献