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

Abstract

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 = 26o). It is monoclinic, C2/m, with the unit-cell parameters a = 9.871(1) A, b = 18.006(2) A, c = 5.314(1) A, β = 105.37(1)°, V = 910.7(2) A3, Z = 2. The calcu- lated density is 3.18 g/cm3. The strongest lines in the X-ray powder diffraction pattern (dobs in A, (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 indi- rectly 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-magnesio- hastingsite) as well but they have a more variable Si/Al-ratio as the xenolith-hosted fluoro-magnesiohastingsites.