• journal_title：European Journal of Mineralogy
• Contributor：Hans-Peter BOJAR ; Franz WALTER
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2006-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2006/0018-0503
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：18
• issue：4
• firstpage：503
• section：Articles

The new mineral fluoro-magnesiohastingsite, ideally (Na,K,Ca)Ca₂(Mg,Fe³⁺,Al,Ti)₅(Si,Al)₈0₂₂F₂ 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) ų, 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), l) 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 03 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(Na_0.50K_0.22Ca_0.17)_0.89BCa_2.00^c(Mg_4.03Fe³⁺_0.70Al_0.13Ti_0.13)_4.99T(Si_5.89Al_2.11)_8.00O_22.00F_2.00.

Most companion minerals of fluoro-magnesiohastingsite containing fluorine, like phlogopite, fluorapatite and chondrodite, have a X_F = 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.