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Fluoro-magnesiohastingsite from Dealul Uroi (Hunedoara county, Romania): Mineral data and crystal structure of a new amphibole end-member

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The new mineral fluoro-magnesiohastingsite, ideally (Na,K,Ca)Ca2(Mg,Fe3+,Al,Ti)5(Si,Al)8O22F2, a member of the calcic-amphibole-group, occurs in small cavities of an altered hematite-rich xenolith in the quaternary trachyandesite at Dealul Uroi, Hunedoara county, Romania. Associated minerals are: titaniferous hematite, augite, phlogopite, enstatite, feldspar, tridymite, titanite, fluorapatite, ilmenite and pseudobrookite. The mineral is reddish-brown to yellowish, with a light reddish-brown streak, and has a perfect {110} cleavage. The tenacity is brittle and the Mohs hardness is 6. Optically the mineral is biaxial (+) and weakly pleochroic, α = 1.642 (yellow-brown), β = 1.647 (light brown), γ = 1.662 (light brown) at 589 nm 2V (meas. and calc.) is 61°, dispersion was not observed. The orientation of β is parallel to b. The axial plane is (010) (γ^c = 26°). It is monoclinic, C2/m, with the unit-cell parameters a = 9.871(1) Å, b = 18.006(2) Å, c = 5.314(1) Å, β = 105.37(1)°, V = 910.7(2) Å 3, Z = 2. The calculated density is 3.18 g/cm3. The strongest lines in the X-ray powder diffraction pattern (d obs in Å, (hkl), I) are: 9.008, (020), 27; 8.421, (110), 61; 3.377, (131), 44; 3.271, (240), 61; 3.124, (310), 100; 2.932, (221), 35; 2.805, (330), 28; 2.700, (151), 54.

The crystal structure of fluoro-magnesiohastingsite was refined to R(F) = 0.054 using reflection intensities collected with MoKα X-radiation. Refined site-scattering values and considerations of mean bond lengths demonstrated an ordering of Fe3+ at M2. Electron-microprobe data and the A site-scattering values show significant Ca (0.19 apfu) together with K and Na at the Am and A2 sites. The site-scattering value and bond-length of O3 to cations indicate that this site is occupied by F, which was indirectly confirmed by infrared spectroscopy. The emp-analyses correspond to the empirical formula: A(Na0.50K0.22Ca0.17)0.89 BCa2.00 C(Mg4.03Fe3+ 0.70Al0.13Ti0.13)4.99 T(Si5.89Al2.11)8.00O22.00F2.00.

Most companion minerals of fluoro-magnesiohastingsite containing fluorine, like phlogopite, fluorapatite and chondrodite, have a XF = F/(F+OH) > 0.79. Amphiboles of the hosting trachyandesite are fluoro-members (fluoro-edenite to fluoro-magnesiohastingsite) as well but they have a more variable Si/Al-ratio as the xenolith-hosted fluoro-magnesiohastingsites.
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Keywords: AMPHIBOLE GROUP; CRYSTAL STRUCTURE; DEALUL UROI; FLUORO-MAGNESIOHASTINGSITE; NEW MINERAL; ROMANIA

Document Type: Research Article

Publication date: 2006-08-01

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  • The European Journal of Mineralogy publishes original papers, review articles and letters dealing with the mineralogical sciences s.l. These include primarily mineralogy, petrology, geochemistry, crystallography and ore deposits, as well as environmental, applied and technical mineralogy. Nevertheless, papers in any related field, including cultural heritage, will be considered.
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