1,721,008 research outputs found
Orthopyroxene as a Geospeedometer: Thermal History of Kapoeta, Old Homestead 001 and Hughes 002 Howardites
No full textSingle crystals of orthopyroxene from small fragments of the Kapoeta, Old Homestead 001, and
Hughes 002 howardites were studied by x-ray diffraction and microprobe analyses. The Fe-Mg equilibrium
distribution coefficients k~ of the crystals were used to calculate the closure temperatures (T,) using the
calibration by Stimpfl et al. (1 999). The compositions, the presence of exsolved augite lamellae, and the T,
values (from 365 to 385 "C) obtained for Kapoeta orthopyroxenes suggest that our fragment comes from a
diogenitic cumulate clast. The more Fe-rich composition, the absence of exsolved lamellae, and the higher
T, values (from 583 to 605 "C) measured in the Old Homestead 001 orthopyroxenes suggest that this
fragment comes from a cumulitic clast affected by fast cooling at high temperature. For the Hughes 002
orthopyroxenes, close in composition to Old Homestead 001, the different T, values (339, 358, and 607 "C)
recorded by the various crystals and the presence of augite lamellae in the crystals with the lowest T, support
the hypothesis that this howardite sample is an unheated breccia containing a mixture of cumulitic
orthopyroxenes with different thermal histories
The high temperature P21/c-C2/c phase transition in Fe-free pyroxene (Ca0.15Mg1.85Si2O6): structural and thermodynamic behavior.
No full textThe compressional behaviour of the columbite-group minerals along the FeNb2O6-MnNb2O6 solid solution
No full textAbstract 3.3.e
The effect of composition and cation ordering on the compressibility of columbites up to 7 GPa
No full textThe high temperature P21/c-C2/c phase transition in Fe-free pyroxenes: structural and thermodynamic behavior
No full text
The high temperature P21/c-C2/c phase transition in the clinopyroxenes of the CMS system (CaO-MgO-SiO2)
No full textCarbon polymorphs in Frontier Mountain ureilitic meteorites: A correlation with increasing the degree of shock?
No full textUreilites meteorite fragments present different levels of shock classified on the basis of optical observations of shock features in silicates. We have investigated by scanning electron microscopy (SEM), micro X-Ray Diffraction (XRD) and Micro-Raman Spectroscopy (MRS) five ureilitic fragments (FRO 95028, FRO 01089, FRO 97013, FRO 01088 and FRO 01012) with increasing degree of shock (from S2 to S6) with the purpose to observe if there is any correlation between the level of shock recorded by silicates and the presence of the different carbon polymorphs formed by shock. XRD patterns showed that all investigated samples contain nano-graphite. Besides this phase, sample FRO 95028, with S2 degree of shock, contains nanodiamond, while samples from level of shock S3 to S6 contain both nano- and micro-diamond. XRD results support the shock formation of micrometer-diamonds found in FRO 01089, FRO 97013, FRO 01088 and FRO 01012 with the assistance of (Fe, Ni)-alloys as catalysts at pressure >10 GPa (S3 shock level recorded by silicates). The formation of polycrystalline diamond is already allowed at pressure between 5-10 GPa. Temperature estimated by a graphite-thermometer based on MRS data, provided values in the range of 1291 to 1398 °C ±120 °C, revealing that there is not a considerable variation of the graphite temperature with the increasing degree of shock
Thermal history of martian meteorites ALH 84001 and MIL 03346: a study of Fe2+-Mg ordering in pyroxene by X-ray single-crystal diffraction
No full textRiassunto presentato al V Forum italiano di Scienze della Terra, Geoitalia 2005, Spoleto, 21-23 settembre 2005
HP-phase transition of natural P21/c pigeonite: spontaneous strain and structure evolution
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