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Megacrystic clinopyroxenes from Victoria (Australia): crystal chemical comparisons of pyroxenes from high and low pressure regimes.
No full textCrystallographic and crystal chemical analysis of a suite of Ca-rich clinopyroxene megacrysts occurring
in alkaline rocks from Victoria (Australia) showed the following important features. Cell volumes of
clinopyroxene megacrysts are similar to those of clinopyroxenes from spine I peridotite nodules, and are
significantly lower than those of phenocryst and microphenocryst clinopyroxenes crystallized at low pressure in
alkaline to tholeiitic basalts. The small celt volume of clinopyroxene megacrysts is achieved essentialty through
a large reduction of MI and M2 polyhedral volumes. On the contrary , the tetrahedral volume is larger than
that of clinopyroxenes from spinel peridotite nodules, but it is comparable with tetrahedral volume of clinopyroxenes
of low pressures crystaltized from alkaline magmas. A notable feature is that ctinopyroxene megacrysts
are characterized by Mg/(Mg+ FeH) ratios distinctly lower than those of ctinopyroxenes from spinel peridotite
nodules but similar to those of ctinopyroxenes of low pressure . Low Mg/(Mg+ Fe2+) ratio values, large tetrahedral
volumes (i. e. high Apv content) and small cell volume , are consistent with the ctinopyroxene megacrysts
having crystallized (from alkaline magmas) at high pressures (e.g. in the spinel peridotite field)
Crystal-chemistry and evolution of the clinopyroxene in a suite of high pressure ultramafic nodules from the Newer Volcanics of Victoria, Australia
No full textCrystal chemistry of aegirine as an indicator of P-T conditions
No full textOne metamorphic andfour magmatic aegirines, andtwo end-member aegirines synthesizedat
atmospheric pressure and different temperatures, were investigated by single-crystal X-ray diffraction.
The limited compositional differences allow the polyhedral volumes to be almost constant in all the
aegirines investigated( VM1 & 11.0 A ¢a 3; VM2 & 26.3 A ¢a 3; VT & 2.21 A ¢a 3). However, differences in
polyhedral distortions are responsible for the cell-volume variations, reflected mainly in change of a
and b cell parameters. Cell volume is relatedonly partly to the composition of these aegirines: with
increasing temperature of formation, an increase in the unit-cell volume of about 1.2 A ¢a 3 is observed,
while a significant contraction of the cell volume takes place at high-pressure conditions of formation.
As the difference in cell volume between the two synthetic aegirines is ascribed to the different
conditions of synthesis temperature, the same interpretation could be adopted for the differences
observedin natural aegirines
The effect of non-stoichiometry on high-temperature behavior of MgAl2O4. Mineralogical Magazine, 73, 301-306
No full textThe effect of cation vacancies upon the thermal expansion and crystal structure of a synthetic defect
spinel with composition Mg0.4Al2.4&0.2O4 was investigated by X-ray diffraction, in situ, at
temperatures up to 1273 K. No evidence of symmetry violations from the Fd3 ̄m evenat the highest
temperature were noted. The volume thermal expansion is markedly less than that of stoichiometric
MgAl2O4 spinel, regardless of the degree of inversion. The u oxygen atomic coordinate remains
constant throughout the temperature range investigated, with the MO an dTO bond lengths showing
identical rates of expansivities. An analysis of the evolution of polyhedral volumes with temperature
indicates that at 1273 K the octahedron inflates by 0.099 A ̊ 3 and the tetrahedron by 0.056 A ̊ 3. The
expansion of the octahedron is significantly greater than in stoichiometric MgAl2O4 spinel, whereas the
expansion of the tetrahedron is similar. The results demonstrate that an excess of Al in the spinel
structure accompanied by the formation of cation vacancies strongly affect an important
thermodynamic property, in this case, thermal expansion. Such an effect must be considered for
those phases stable inthe Earth’s mantle where 45 wt.% Al2O3 is thought to be present
Crystal chemistry of pyroxenes from basalts and rhyodacites of the Paranà basin (Brazil)
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