Roger H. Mitchell - Academia.edu (original) (raw)

Papers by Roger H. Mitchell

Research paper thumbnail of What is a Kimberlite? Petrology and Mineralogy of Hypabyssal Kimberlites

Elements

Hypabyssal kimberlites are subvolcanic intrusive rocks crystallised from mantle-derived magmas po... more Hypabyssal kimberlites are subvolcanic intrusive rocks crystallised from mantle-derived magmas poor in SiO2 and rich in CO2 and H2O. They are complex, hybrid rocks containing significant amounts of mantle-derived fragments, primarily olivine with rare diamonds, set in a matrix of essentially magmatic origin. Unambiguous identification of kimberlites requires careful petrographic examination combined with mineral compositional analyses. Melt inclusion studies have shown that kimberlite melts contain higher alkali concentrations than previously thought but have not clarified the ultimate origin of these melts. Because of the hybrid nature of kimberlites and their common hydrothermal alteration by fluids of controversial origin (magmatic and/or crustal), the composition of primary kimberlite melts remains unknown.

Research paper thumbnail of Mineralogy and genesis of pyrochlore-apatitite from The Good Hope Carbonatite, Ontario: A potential Nb deposit

Mineralogical Magazine

The Good Hope carbonatite is located adjacent to the Prairie Lake alkaline rock and carbonatite c... more The Good Hope carbonatite is located adjacent to the Prairie Lake alkaline rock and carbonatite complex in northwestern Ontario. The occurrence is a heterolithic breccia consisting of diverse calcite, dolomite and ferrodolomite carbonatites containing clasts of magnesio-arfvedsonite + potassium feldspar, phlogopite + potassium feldspar together with pyrochlore-bearing apatitite clasts. The apatitite occurs as angular, boudinaged and schlieren clasts up to 5 cm in maximum dimensions. In these pyrochlore occurs principally as euhedral single crystals (0.1–1.5 cm) and can comprise up to 25 vol.% of the clasts. Individual clasts contain compositionally- and texturally-distinct suites of pyrochlore. The pyrochlores are hosted by small prismatic crystals of apatite (~100–500 μm × 10–25 μm) that are commonly flow-aligned and in some instances occur as folds. Allotriogranular cumulate textures are not evident in the apatitites. The fluorapatite does not exhibit compositional zonation under ...

Research paper thumbnail of Mineralogy of the baotite-bearing Gundrapalli lamproite, Nalgonda district, Telangana, India

Mineralogical Magazine

We describe the mineralogy of a lamproite dyke from Gundrapalli village (Nalgonda district), Tela... more We describe the mineralogy of a lamproite dyke from Gundrapalli village (Nalgonda district), Telangana, India. The dyke consists of a mineral assemblage characteristic of lamproites in terms of the presence of amphiboles (mainly potassic-richterite together with potassic-arfvedsonite, magnesio-riebeckite, Ti-rich potassic-magnesio-arfvedsonite, potassic-magnesio-arfvedsonite, katophorite and potassic-ferri-katophorite), Al-poor pyroxene, phlogopite (Ti-rich, Al-poor), pseudomorphed leucite, spinel (chromite-magnesiochromite), fluorapatite, baryte, titanite, rutile, barytocalcite, calcite, ilmenite, hydro-zircon, baotite, strontianite, allanite, quartz and pyrite. The absence of wadeite and priderite have been compensated for by the presence of baotite, rutile, titanite, baryte and hydro-zircons. The presence of the secondary phases: allanite, hydro-zircon, chlorite, quartz and cryptocrystalline silica, implies that the dyke has undergone deuteric alteration. On the basis of its typo...

Research paper thumbnail of Theoretical aspects of gaseous and isotopic equilibria in the system C-H-O-S with application to kimberlites

International Kimberlite Conference: Extended Abstracts

Research paper thumbnail of Mineralogy of the Tunraq Kimberlite Somerset Island, N.W.T., Canada

Kimberlites, Diatremes, and Diamonds: Their Geology, Petrology, and Geochemistry, 1979

Research paper thumbnail of A structural study of the perovskite series Na0.75Ln0.25Ti0.5Nb0.5O3

J Solid State Chem, 2005

The ternary stoichiometric perovskite compounds, Na 0.

Research paper thumbnail of Rare Earth Element Geochemistry of Potassie Lavas from the Birunga and Toro-Ankole Regions of Uganda, Africa

Contrib Mineral Petrol, 1976

Research paper thumbnail of A structural study of the perovskite series Ca1-xNaxTi1-xTaxO3

J Solid State Chem, 2004

Compounds in the solid solution series Ca 1Àx Na x Ti 1Àx Ta x O 3 were synthesized at 1300 1C, f... more Compounds in the solid solution series Ca 1Àx Na x Ti 1Àx Ta x O 3 were synthesized at 1300 1C, followed by annealing at 850 1C or 800 1C with quenching and/or slow cooling to room temperature. Rietveld refinement of their powder X-ray diffraction patterns show that all compounds are single-phase ternary perovskites which adopt the space group Pbnm (aEbEO2a p ; cE2a p ; Z=4) at ambient conditions. The unit cell parameters and cell volumes of the compounds increase regularly with increasing values of x. The coordination of the A-site cations changes throughout the series from eight for CaTiO 3 to nine for NaTaO 3. Compounds with 0px p0.4 have A-site cations in eight fold coordination, whereas the coordination of those with 0.4oxo0.9 is ambiguous. Analysis of the crystal chemistry of the compounds shows that the change in coordination at x=0.4 is related to the departure of the B-site cations from the second coordination sphere of the A-site cations, as in compounds with x40.4 the A-II O distances become less than the A-B intercation distances. Contemporaneous with these coordination changes, the tilt angles of the BO 6 polyhedra decrease with increasing values of x. This solid solution series is unusual in that these structural and coordination changes occur regardless that Goldschmidt tolerance factors remain essentially constant at approximately 0.89, and observed tolerance factors, assuming eight fold coordination of the A-site cations, range only from 0.91 to 0.93 (0pxp0.8).

Research paper thumbnail of Petrology of hypabyssal kimberlites: Relevance to primary magma compositions

Journal of Volcanology and Geothermal Research, Jun 1, 2008

Hypabyssal kimberlites are similar in their mineralogy and geochemistry across the world, and thu... more Hypabyssal kimberlites are similar in their mineralogy and geochemistry across the world, and thus form a distinct magma type that is produced repeatedly in time and space. This small volume magma is derived from the asthenosphere (200–?650 km), and as it nears the surface consists of olivine macrocrysts, i.e. xenocrysts (~25 vol.%), olivine phenocrysts (~25 vol.%), entrained in a Ba–Ti–Mg-rich, Al-poor carbonate-silicate liquid

Research paper thumbnail of Shcherbakovite in Leucite Phlogopite Lamproites from the Leucite Hills, Wyoming

Mineralogical Magazine, Dec 1, 1990

Shcherbakovite in leucite phlogopite lamproites from the Leucite Hills, Wyoming POTASSIUM barium ... more Shcherbakovite in leucite phlogopite lamproites from the Leucite Hills, Wyoming POTASSIUM barium titanosilicates belonging to the shcherbakovite-batisite series are relatively rare Flo. 1. Back-scattered electron image of shcherbakovite crystal(s) occurring in a vesicle associated with apatite, amphibole and sanidine. Emmons Mesa,

Research paper thumbnail of The Diamond-Bearing Kokchetav Massif, Kazakhstan: Petrochemistry and Tectonic Evolution of a Unique Ultrahigh-Pressure Metamorphic Terrain.: C.D. Parkinson, I. Katayama, J. Liou & S. Maruyama, eds. Frontiers Science Series No. 38, Universal Academy Press Inc., Tokyo, Japan. US$70. ISBN 4-946443-78-9

Research paper thumbnail of The Mineralogy of the diamond : by . Wiley-Interscience. 235 pp., 84 figs. $32.00, 1978

Geochim Cosmochim Acta, 1978

Research paper thumbnail of Crystal structures of CsFe2S3 and RbFe2S3: synthetic analogs of rasvumite KFe2S3

Journal of Solid State Chemistry, 2004

The isostructural alkali thioferrate compounds CsFe2S3, RbFe2S3 and KFe2S3 have been synthesized ... more The isostructural alkali thioferrate compounds CsFe2S3, RbFe2S3 and KFe2S3 have been synthesized by reacting Fe and S with their corresponding AFeS2 (A=K, Rb, Cs) precursors. The crystal structures of these and binary compounds of intermediate composition were determined by Rietveld analysis of laboratory powder X-ray diffraction patterns. All of the synthesized compounds adopt the space group Cmcm (#63), Z=4 with:

Research paper thumbnail of Diamonds and their Source rocks in India

Gsi Publications, Sep 4, 2012

Research paper thumbnail of A structural study of the perovskite series Ca{sub 1-x}Na�Ti{sub 1-x}Ta�O�

Research paper thumbnail of Second International Kimberlite Conference

Geoscience Canada, Mar 3, 1978

Research paper thumbnail of Rb-Sr geochronology of the Coldwell Complex, northwestern Ontario, Canada: Reply

Can J Earth Sci, 1984

The Coldwell Complex of Northwestern Ontario is North America's largest structurally and petrolog... more The Coldwell Complex of Northwestern Ontario is North America's largest structurally and petrologically complex alkaline intrusion. Situated on the north shore of Lake Superior, it consists of at least three intrusive centres and is cross-cut by a diverse suite of coeval-cogenetic dikes. The main intrusive rocks range from gabbros to ferroaugite syenites, nepheline syenites, and quartz syenites. The dikes are predominantly lamprophyric. A seventeen point whole rock Rb-Sr isochron (MSWD 2.22) gives an age of 1044.5 * 6.2 Ma (2 4 and an initial ratio of 0.70354 + 0.00016 (20). The age is late Neohelikian and is younger than the bulk of igneous activity (Keweenawan activity) prevalent in the Lake Superior Basin during the Neohelikian. The low initial ratio indicates an upper mantle origin for the parental magma of the complex. Le complexe de Coldwell du nord-ouest de I'Ontario constitue le plus grand complexe intrusif alcalin en Amtrique du Nord, tant s w le plan structural que pCtrologique. SituC s w la rive nord du Lac Sugrieur, il est form6 d'au moins trois centres intrusifs et il est entrecoup6 par diffkrentes stries de dykes contemporains et cogCnCtiques. Les principales roches formant I'intrusif varient de gabbros B des syCnites & ferroaugite, des syCnites & nCphCline et des syCnites quartzifkres. Les dykes sont en gCnCral lamprophyriques. Une courbe isochrone tracCe avec 17 points pour Rb-Sr sur roche totale fournit un Lge de 1044,5 + 6,2 Ma (2u) et un rapport initial de 0,70354 + 0,00016 (2u). L'Lge correspond au NCohClikien su@rieur et est plus jeune que celui de I'ensemble de I'activitC ign6e (activit.6 keweenavienne) qui a prCvalu dans le bassin du Lac SupCrieur durant le NCohClikien. Le faible rapport initial indique que la source du magma de ce complexe est localisCe dans le manteau supCrieur.

Research paper thumbnail of Latrappite: A re-investigation

The Canadian Mineralogist, 1998

Research paper thumbnail of Crystal structure of a synthetic aluminoan tantalian titanite: a reconnaissance study

Mineralogical Magazine, Feb 1, 2006

A synthetic analogue, Ca(Ti0.6Al0.2Ta0.16Nb0.04)OSiO4, of an aluminoan tantalian titanite previou... more A synthetic analogue, Ca(Ti0.6Al0.2Ta0.16Nb0.04)OSiO4, of an aluminoan tantalian titanite previously described from Craveggia pegmatite (Piemonte, Italy) has been prepared by a ceramic synthesis technique and its crystal structure determined by Rietveld analysis of the powder X-ray diffraction pattern. The synthetic Al-Ta-Nb-rich titanite adopts space group A2/a implying that substitutions at the single octahedral site destroy the coherence of the off-centering of octahedral chains in the titanite structure resulting in a P21/a → A2/a phase transition. Unit-cell dimensions obtained for the Al-Ta-Nb-rich titanite are: a = 7.0649(1) Å; b = 8.7187(1) Å; c = 6.5701(1) Å; β = 113.755(1)°, V = 370.41(1) Å3. The extensive replacement of Ti by Al, Ta and Nb results in a considerable decrease in the distortion of all coordination polyhedra in the structure of this titanite. These structural data suggest that a solid solution CaTi1-x(Alx/2 [Ta,Nb]x/2)OSiO4 (0 ≤ x ≤ 0.4) adopting the titanite structure might exist.

Research paper thumbnail of Instability of Perovskite in a CO2-RICH Environment : Examples from Carbonatite and Kimberlite

Canadian Mineralogist, 1998

Intricate multiphase pseudomorphs after perovskite (Nb-, LREE-poor) from calcite carbonatite (Seb... more Intricate multiphase pseudomorphs after perovskite (Nb-, LREE-poor) from calcite carbonatite (Sebljavr complex, Kola Peninsula, Russia) and serpentine calcite kimberlite (Iron Mountain, Wyoming) are described. In the kimberlite, the major products of perovskite replacement are (in order of crystallization): kassite, anatase and titanite plus calcite, ilmenite, LREE-Ti oxide [? lucasite-(Ce)]. In the carbonatite, perovskite is initially replaced by anatase plus calcite and, subsequently, ilmenite and ancylite-(Ce). In both instances, the development of calcite and Ti-bearing phases after perovskite involved initial progressive leaching of Ca 2+ from the structure followed by crystallization of ilmenite and LREE minerals in the final stages, after the precipitation of groundmass calcite. The formation of kassite and titanite in the pseudomorphs in kimberlite was controlled by a lower Ca leach-rate and higher activity of SiO 2 in this system, compared with the carbonatite. The similarity between the two types of pseudomorphs results from the instability of Nb-LREE-poor perovskite in a CO 2-rich fluid at low temperatures. Perovskite is considered an unsuitable host for radioactive 90 Sr and REE isotopes due to the low resistance of CaTiO 3 to leaching and incompatibility of Sr and the rare earths with the products of perovskite replacement.

Research paper thumbnail of What is a Kimberlite? Petrology and Mineralogy of Hypabyssal Kimberlites

Elements

Hypabyssal kimberlites are subvolcanic intrusive rocks crystallised from mantle-derived magmas po... more Hypabyssal kimberlites are subvolcanic intrusive rocks crystallised from mantle-derived magmas poor in SiO2 and rich in CO2 and H2O. They are complex, hybrid rocks containing significant amounts of mantle-derived fragments, primarily olivine with rare diamonds, set in a matrix of essentially magmatic origin. Unambiguous identification of kimberlites requires careful petrographic examination combined with mineral compositional analyses. Melt inclusion studies have shown that kimberlite melts contain higher alkali concentrations than previously thought but have not clarified the ultimate origin of these melts. Because of the hybrid nature of kimberlites and their common hydrothermal alteration by fluids of controversial origin (magmatic and/or crustal), the composition of primary kimberlite melts remains unknown.

Research paper thumbnail of Mineralogy and genesis of pyrochlore-apatitite from The Good Hope Carbonatite, Ontario: A potential Nb deposit

Mineralogical Magazine

The Good Hope carbonatite is located adjacent to the Prairie Lake alkaline rock and carbonatite c... more The Good Hope carbonatite is located adjacent to the Prairie Lake alkaline rock and carbonatite complex in northwestern Ontario. The occurrence is a heterolithic breccia consisting of diverse calcite, dolomite and ferrodolomite carbonatites containing clasts of magnesio-arfvedsonite + potassium feldspar, phlogopite + potassium feldspar together with pyrochlore-bearing apatitite clasts. The apatitite occurs as angular, boudinaged and schlieren clasts up to 5 cm in maximum dimensions. In these pyrochlore occurs principally as euhedral single crystals (0.1–1.5 cm) and can comprise up to 25 vol.% of the clasts. Individual clasts contain compositionally- and texturally-distinct suites of pyrochlore. The pyrochlores are hosted by small prismatic crystals of apatite (~100–500 μm × 10–25 μm) that are commonly flow-aligned and in some instances occur as folds. Allotriogranular cumulate textures are not evident in the apatitites. The fluorapatite does not exhibit compositional zonation under ...

Research paper thumbnail of Mineralogy of the baotite-bearing Gundrapalli lamproite, Nalgonda district, Telangana, India

Mineralogical Magazine

We describe the mineralogy of a lamproite dyke from Gundrapalli village (Nalgonda district), Tela... more We describe the mineralogy of a lamproite dyke from Gundrapalli village (Nalgonda district), Telangana, India. The dyke consists of a mineral assemblage characteristic of lamproites in terms of the presence of amphiboles (mainly potassic-richterite together with potassic-arfvedsonite, magnesio-riebeckite, Ti-rich potassic-magnesio-arfvedsonite, potassic-magnesio-arfvedsonite, katophorite and potassic-ferri-katophorite), Al-poor pyroxene, phlogopite (Ti-rich, Al-poor), pseudomorphed leucite, spinel (chromite-magnesiochromite), fluorapatite, baryte, titanite, rutile, barytocalcite, calcite, ilmenite, hydro-zircon, baotite, strontianite, allanite, quartz and pyrite. The absence of wadeite and priderite have been compensated for by the presence of baotite, rutile, titanite, baryte and hydro-zircons. The presence of the secondary phases: allanite, hydro-zircon, chlorite, quartz and cryptocrystalline silica, implies that the dyke has undergone deuteric alteration. On the basis of its typo...

Research paper thumbnail of Theoretical aspects of gaseous and isotopic equilibria in the system C-H-O-S with application to kimberlites

International Kimberlite Conference: Extended Abstracts

Research paper thumbnail of Mineralogy of the Tunraq Kimberlite Somerset Island, N.W.T., Canada

Kimberlites, Diatremes, and Diamonds: Their Geology, Petrology, and Geochemistry, 1979

Research paper thumbnail of A structural study of the perovskite series Na0.75Ln0.25Ti0.5Nb0.5O3

J Solid State Chem, 2005

The ternary stoichiometric perovskite compounds, Na 0.

Research paper thumbnail of Rare Earth Element Geochemistry of Potassie Lavas from the Birunga and Toro-Ankole Regions of Uganda, Africa

Contrib Mineral Petrol, 1976

Research paper thumbnail of A structural study of the perovskite series Ca1-xNaxTi1-xTaxO3

J Solid State Chem, 2004

Compounds in the solid solution series Ca 1Àx Na x Ti 1Àx Ta x O 3 were synthesized at 1300 1C, f... more Compounds in the solid solution series Ca 1Àx Na x Ti 1Àx Ta x O 3 were synthesized at 1300 1C, followed by annealing at 850 1C or 800 1C with quenching and/or slow cooling to room temperature. Rietveld refinement of their powder X-ray diffraction patterns show that all compounds are single-phase ternary perovskites which adopt the space group Pbnm (aEbEO2a p ; cE2a p ; Z=4) at ambient conditions. The unit cell parameters and cell volumes of the compounds increase regularly with increasing values of x. The coordination of the A-site cations changes throughout the series from eight for CaTiO 3 to nine for NaTaO 3. Compounds with 0px p0.4 have A-site cations in eight fold coordination, whereas the coordination of those with 0.4oxo0.9 is ambiguous. Analysis of the crystal chemistry of the compounds shows that the change in coordination at x=0.4 is related to the departure of the B-site cations from the second coordination sphere of the A-site cations, as in compounds with x40.4 the A-II O distances become less than the A-B intercation distances. Contemporaneous with these coordination changes, the tilt angles of the BO 6 polyhedra decrease with increasing values of x. This solid solution series is unusual in that these structural and coordination changes occur regardless that Goldschmidt tolerance factors remain essentially constant at approximately 0.89, and observed tolerance factors, assuming eight fold coordination of the A-site cations, range only from 0.91 to 0.93 (0pxp0.8).

Research paper thumbnail of Petrology of hypabyssal kimberlites: Relevance to primary magma compositions

Journal of Volcanology and Geothermal Research, Jun 1, 2008

Hypabyssal kimberlites are similar in their mineralogy and geochemistry across the world, and thu... more Hypabyssal kimberlites are similar in their mineralogy and geochemistry across the world, and thus form a distinct magma type that is produced repeatedly in time and space. This small volume magma is derived from the asthenosphere (200–?650 km), and as it nears the surface consists of olivine macrocrysts, i.e. xenocrysts (~25 vol.%), olivine phenocrysts (~25 vol.%), entrained in a Ba–Ti–Mg-rich, Al-poor carbonate-silicate liquid

Research paper thumbnail of Shcherbakovite in Leucite Phlogopite Lamproites from the Leucite Hills, Wyoming

Mineralogical Magazine, Dec 1, 1990

Shcherbakovite in leucite phlogopite lamproites from the Leucite Hills, Wyoming POTASSIUM barium ... more Shcherbakovite in leucite phlogopite lamproites from the Leucite Hills, Wyoming POTASSIUM barium titanosilicates belonging to the shcherbakovite-batisite series are relatively rare Flo. 1. Back-scattered electron image of shcherbakovite crystal(s) occurring in a vesicle associated with apatite, amphibole and sanidine. Emmons Mesa,

Research paper thumbnail of The Diamond-Bearing Kokchetav Massif, Kazakhstan: Petrochemistry and Tectonic Evolution of a Unique Ultrahigh-Pressure Metamorphic Terrain.: C.D. Parkinson, I. Katayama, J. Liou & S. Maruyama, eds. Frontiers Science Series No. 38, Universal Academy Press Inc., Tokyo, Japan. US$70. ISBN 4-946443-78-9

Research paper thumbnail of The Mineralogy of the diamond : by . Wiley-Interscience. 235 pp., 84 figs. $32.00, 1978

Geochim Cosmochim Acta, 1978

Research paper thumbnail of Crystal structures of CsFe2S3 and RbFe2S3: synthetic analogs of rasvumite KFe2S3

Journal of Solid State Chemistry, 2004

The isostructural alkali thioferrate compounds CsFe2S3, RbFe2S3 and KFe2S3 have been synthesized ... more The isostructural alkali thioferrate compounds CsFe2S3, RbFe2S3 and KFe2S3 have been synthesized by reacting Fe and S with their corresponding AFeS2 (A=K, Rb, Cs) precursors. The crystal structures of these and binary compounds of intermediate composition were determined by Rietveld analysis of laboratory powder X-ray diffraction patterns. All of the synthesized compounds adopt the space group Cmcm (#63), Z=4 with:

Research paper thumbnail of Diamonds and their Source rocks in India

Gsi Publications, Sep 4, 2012

Research paper thumbnail of A structural study of the perovskite series Ca{sub 1-x}Na�Ti{sub 1-x}Ta�O�

Research paper thumbnail of Second International Kimberlite Conference

Geoscience Canada, Mar 3, 1978

Research paper thumbnail of Rb-Sr geochronology of the Coldwell Complex, northwestern Ontario, Canada: Reply

Can J Earth Sci, 1984

The Coldwell Complex of Northwestern Ontario is North America's largest structurally and petrolog... more The Coldwell Complex of Northwestern Ontario is North America's largest structurally and petrologically complex alkaline intrusion. Situated on the north shore of Lake Superior, it consists of at least three intrusive centres and is cross-cut by a diverse suite of coeval-cogenetic dikes. The main intrusive rocks range from gabbros to ferroaugite syenites, nepheline syenites, and quartz syenites. The dikes are predominantly lamprophyric. A seventeen point whole rock Rb-Sr isochron (MSWD 2.22) gives an age of 1044.5 * 6.2 Ma (2 4 and an initial ratio of 0.70354 + 0.00016 (20). The age is late Neohelikian and is younger than the bulk of igneous activity (Keweenawan activity) prevalent in the Lake Superior Basin during the Neohelikian. The low initial ratio indicates an upper mantle origin for the parental magma of the complex. Le complexe de Coldwell du nord-ouest de I'Ontario constitue le plus grand complexe intrusif alcalin en Amtrique du Nord, tant s w le plan structural que pCtrologique. SituC s w la rive nord du Lac Sugrieur, il est form6 d'au moins trois centres intrusifs et il est entrecoup6 par diffkrentes stries de dykes contemporains et cogCnCtiques. Les principales roches formant I'intrusif varient de gabbros B des syCnites & ferroaugite, des syCnites & nCphCline et des syCnites quartzifkres. Les dykes sont en gCnCral lamprophyriques. Une courbe isochrone tracCe avec 17 points pour Rb-Sr sur roche totale fournit un Lge de 1044,5 + 6,2 Ma (2u) et un rapport initial de 0,70354 + 0,00016 (2u). L'Lge correspond au NCohClikien su@rieur et est plus jeune que celui de I'ensemble de I'activitC ign6e (activit.6 keweenavienne) qui a prCvalu dans le bassin du Lac SupCrieur durant le NCohClikien. Le faible rapport initial indique que la source du magma de ce complexe est localisCe dans le manteau supCrieur.

Research paper thumbnail of Latrappite: A re-investigation

The Canadian Mineralogist, 1998

Research paper thumbnail of Crystal structure of a synthetic aluminoan tantalian titanite: a reconnaissance study

Mineralogical Magazine, Feb 1, 2006

A synthetic analogue, Ca(Ti0.6Al0.2Ta0.16Nb0.04)OSiO4, of an aluminoan tantalian titanite previou... more A synthetic analogue, Ca(Ti0.6Al0.2Ta0.16Nb0.04)OSiO4, of an aluminoan tantalian titanite previously described from Craveggia pegmatite (Piemonte, Italy) has been prepared by a ceramic synthesis technique and its crystal structure determined by Rietveld analysis of the powder X-ray diffraction pattern. The synthetic Al-Ta-Nb-rich titanite adopts space group A2/a implying that substitutions at the single octahedral site destroy the coherence of the off-centering of octahedral chains in the titanite structure resulting in a P21/a → A2/a phase transition. Unit-cell dimensions obtained for the Al-Ta-Nb-rich titanite are: a = 7.0649(1) Å; b = 8.7187(1) Å; c = 6.5701(1) Å; β = 113.755(1)°, V = 370.41(1) Å3. The extensive replacement of Ti by Al, Ta and Nb results in a considerable decrease in the distortion of all coordination polyhedra in the structure of this titanite. These structural data suggest that a solid solution CaTi1-x(Alx/2 [Ta,Nb]x/2)OSiO4 (0 ≤ x ≤ 0.4) adopting the titanite structure might exist.

Research paper thumbnail of Instability of Perovskite in a CO2-RICH Environment : Examples from Carbonatite and Kimberlite

Canadian Mineralogist, 1998

Intricate multiphase pseudomorphs after perovskite (Nb-, LREE-poor) from calcite carbonatite (Seb... more Intricate multiphase pseudomorphs after perovskite (Nb-, LREE-poor) from calcite carbonatite (Sebljavr complex, Kola Peninsula, Russia) and serpentine calcite kimberlite (Iron Mountain, Wyoming) are described. In the kimberlite, the major products of perovskite replacement are (in order of crystallization): kassite, anatase and titanite plus calcite, ilmenite, LREE-Ti oxide [? lucasite-(Ce)]. In the carbonatite, perovskite is initially replaced by anatase plus calcite and, subsequently, ilmenite and ancylite-(Ce). In both instances, the development of calcite and Ti-bearing phases after perovskite involved initial progressive leaching of Ca 2+ from the structure followed by crystallization of ilmenite and LREE minerals in the final stages, after the precipitation of groundmass calcite. The formation of kassite and titanite in the pseudomorphs in kimberlite was controlled by a lower Ca leach-rate and higher activity of SiO 2 in this system, compared with the carbonatite. The similarity between the two types of pseudomorphs results from the instability of Nb-LREE-poor perovskite in a CO 2-rich fluid at low temperatures. Perovskite is considered an unsuitable host for radioactive 90 Sr and REE isotopes due to the low resistance of CaTiO 3 to leaching and incompatibility of Sr and the rare earths with the products of perovskite replacement.