<p>Baddeleyite (monoclinic; <em>m</em>-ZrO<sub>2</sub>) is an important U-Pb chronometer within mafic lithologies from many planetary bodies. Recent <em>in-situ</em> U-Pb dating of micro-baddeleyite within shergottites has been key in confirming recent magmatic activity on Mars. However, despite a high U-Pb closure temperature (&#8805;900 &#176;C) and the retention of robust U-Pb isotope systematics to ~57 GPa within experimental studies, up to 80% Pb loss within baddeleyite has been reported from the highly-shocked shergottite Northwest Africa (NWA) 5298. Significantly, U-Pb isotopic disturbance has been shown to be strongly linked with baddeleyite internal microstructure, generated by partial to complete reversion from meta-stable, high P-T zirconia polymorphs during shock metamorphism. NWA 5298 has experienced elevated shock metamorphism, and particularly post-shock temperatures, in comparison to many other shergottites; in the absence of microstructural analyses, the magnitude of baddeleyite U-Pb isotopic disturbance within more moderately shocked shergottites remains unknown.</p><p>To address this, we combine electron backscatter diffraction (EBSD) microstructural analysis and in-situ U-Pb chronology of baddeleyite within three enriched shergottites: NWA 7257, NWA 8679 and Zagami. Studied samples have undergone shock conditions typical of shergottites, with complete transformation of plagioclase to maskelynite and pervasive fracturing of pyroxene, phosphates and oxides. Small veinlets of shock melt cross-cut NWA 8679 and Zagami, and shock melt pockets are present in all samples. Baddeleyite is abundant and ubiquitously associated with late-stage igneous assemblages, rather than shock melt.</p><p>We document a wide range of baddeleyite microstructures. These include crystal-plastically deformed magmatic twins, domains with a marked decrease in crystallinity, and complex, nanostructured domains with orthogonal orientation relationships that are interpreted to have resulted from complete reversion from high P-T polymorphs. Magmatic twins are only locally preserved due to shock heterogeneity. Despite this, and in contrast to NWA 5298, we find no link between baddeleyite microstructure and U-Pb isotope systematics. Analyses fall along well-defined discordia within Tera-Wasserburg plots for each sample, with the U-Pb isotopic composition of analyses controlled by overlap with surrounding phases and fractures rather than baddeleyite microstructure. We therefore determine two new, microstructurally constrained ages from discordia lower intercepts: 195 &#177; 15 Ma (95% confidence; MSWD 5.6) for NWA 7257 and 220 &#177; 23 Ma (95% confidence; MSWD 2.2) for NWA 8679. For Zagami, our findings support the previously reported magmatic crystallisation age of ~180 Ma. These results provide further confirmation that high post-shock temperatures are required to induce resolvable U-Pb isotopic disturbance baddeleyite, even within highly shocked samples, and that reversion from high P-T zirconia polymorphs alone does not necessitate U-Pb isotopic disturbance. While we caution the continued requirement for detailed microstructural analyses of baddeleyite prior to isotopic analyses, this study underlines the utility of baddeleyite chronology within martian meteorites and other shocked planetary materials.</p>
Announcement| May 01, 2013 New Mineral Names* Fernando Camara; Fernando Camara 1Dipartimento di Scienze della Terra, Università di degli Studi di Torino, Via Valperga Caluso, 35-10125 Torino, Italy Search for other works by this author on: GSW Google Scholar G. Diego Gatta; G. Diego Gatta 2Dipartimento Scienze della Terra, Università degli Studi di Milano, Via Botticelli, 23-20133 Milano, Italy Search for other works by this author on: GSW Google Scholar Dmitriy Belakovskiy; Dmitriy Belakovskiy 3Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow 119071, Russia Search for other works by this author on: GSW Google Scholar Dorian G.W. Smith; Dorian G.W. Smith 4Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Kimberly T. Tait Kimberly T. Tait 5Department of Natual History, Royal Ontario Museum, 100 Queens Park, Toronto, Ontario M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar American Mineralogist (2013) 98 (5-6): 1078–1083. https://doi.org/10.2138/am.2013.616 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Fernando Camara, G. Diego Gatta, Dmitriy Belakovskiy, Dorian G.W. Smith, Kimberly T. Tait; New Mineral Names. American Mineralogist 2013;; 98 (5-6): 1078–1083. doi: https://doi.org/10.2138/am.2013.616 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyAmerican Mineralogist Search Advanced Search This New Mineral Names has entries from several different journals about anorpiment, cossaite, fassinaite, lammerite-β, lileyite, megawite, perrierite-(La) and various unnamed minerals. A.R. Kampf, R.T. Downs, R.M. Housley, R.A. Jenkins, and J. Hyršl (2011) Anorpiment, As2S3, the triclinic dimorph of orpiment. Mineralogical Magazine, 75(6), 2857–2867. Anorpiment, ideally As2S3, is the triclinic dimorph of orpiment. It occurs at the Palomo mine, Castrovirreyna Province, Huancavelica Department, Peru. The Palomo mine belongs metallogenetically to the Huachocolpa ore district. The rocks in the area of the mine are exclusively volcanic, comprising andesitic lavas, pyroclastic rocks, tuffs,... You do not currently have access to this article.
A new mineral species, bobdownsite, the F-dominant analogue of whitlockite, ideally Ca 9 Mg(PO 4 ) 6 (PO 3 F), has been found in Lower Cretaceous bedded ironstones and shales exposed on a high ridge on the west side of Big Fish River, Yukon, Canada. The associated minerals include siderite, lazulite, an arrojadite-group mineral, kulanite, gormanite, quartz, and collinsite. Bobdownsite from the Yukon is tabular, colorless, and transparent, with a white streak and vitreous luster. It is brittle, with a Mohs hardness of ~5; no cleavage, parting, or macroscopic twinning is observed. The fracture is uneven and subconchoidal. The measured and calculated densities are 3.14 and 3.16 g/cm 3 , respectively. Bobdownsite is insoluble in water, acetone, or hydrochloric acid. Optically, it is uniaxial (−), ω = 1.625(2), ɛ = 1.622(2). The electron-microprobe analysis yielded (Ca 8.76 Na 0.24 ) ∑9.00 (Mg 0.72 Fe 3+ 0.13 Al 0.11 Fe 2+ 0.04 ) ∑1.00 (P 1.00 O 4 ) 6 (P 1.00 O 3 F 1.07 ) as the empirical formula. Bobdownsite was examined with single-crystal X-ray diffraction; it is trigonal with space group R 3 c and unit-cell parameters a 10.3224(3), c 37.070(2) A, V 3420.7(6) A 3 . The structure was refined to an R 1 factor of 0.031. Bobdownsite is isotypic with whitlockite, whose structure and relationships with other phosphate compounds have been studied extensively. Its structure is characterized by the [Mg(PO 4 ) 6 ] 16− ligand, or the so-called “Mg pinwheel”. The isolated pinwheels are held together by intralayer Ca cations to form layers parallel to (001), which are linked together by interlayer Ca cations along [001]. The Raman spectra of bobdownsite strongly resemble those of whitlockite and merrillite. Bobdownsite represents the first naturally formed phosphate known to contain a P–F bond. It has subsequently been found in the Tip Top mine, Custer County, South Dakota, USA. On the basis of our study, we conclude that the “fluor whitlockite” found in the Martian meteorite SaU 094B also is bobdownsite.
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