Introduction Fluorine-rich amphibole-group minerals may be more common than is generally recognized. Significant fluorine contents in amphiboles are reported since about a century (e.g. Doelter, 1914). An examination of the mineral-chem- istry of amphiboles based on literature data done by Petersen et al. (1982) revealed a number of amphiboles in which X F = F/(F+OH) is higher than 0.50. Since this publi- cation, nine fluoro-members of the amphibole group have been accepted by the IMA-CNMMN and published. Three of them are members of the calcic-amphibole group: fluor- cannilloite (Hawthorne et al., 1996), fluoro-edenite (Gianfagna & Oberti, 2001) and fluoropargasite (Lupulescu et al., 2005). High fluorine content in magnesiohastingsite is rarely reported. Forty-one magnesiohastingsite and hastingsite analyses are reported in Deer et al. (1997) but only five of them have significant fluorine-contents (X F > 0.25), and none of them have X F > 0.50. Gaeta & Freda (2001) reported strontian fluoro-magnesiohastingsite from Alban Hills (Central Italy) with X F up to 0.787. These submilli- metric fluoro-magnesiohastingsite crystals are poikilitic with numerous inclusions. Therefore they were not suitable for crystallographic investigations. The fluoro-magnesiohastingsite described in this paper has been approved by the Commission on New Mineral and Mineral Names of the International Mineralogical Association (#2005-002). The type material is deposited at the Department of Mineralogy of the Landesmuseum Eur. J. Mineral. 2006, 18, 503-508 Fluoro-magnesiohastingsite from Dealul Uroi (Hunedoara county, Romania): Mineral data and crystal structure of a new amphibole end-member HANS-PETER BOJAR 1, * and FRANZ WALTER 2 1 Department of Mineralogy, Landesmuseum Joanneum, Raubergasse 10, A-8010 Graz, Austria 2 Institute of Earth Sciences, Karl-Franzens-University Graz, Universitätsplatz 2, A-8010 Graz, Austria Abstract: The new mineral fluoro-magnesiohastingsite, ideally (Na,K,Ca)Ca 2 (Mg,Fe 3+ ,Al,Ti) 5 (Si,Al) 8 O 22 F 2 , 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) Å, b = 18.006(2) Å, c = 5.314(1) Å, β = 105.37(1)°, V = 910.7(2) Å 3 , Z = 2. The calcu- lated density is 3.18 g/cm 3 . 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 Fe 3+ 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 (Na 0.50 K 0.22 Ca 0.17 ) 0.89 B Ca 2.00 C (Mg 4.03 Fe 3+ 0.70 Al 0.13 Ti 0.13 ) 4.99 T (Si 5.89 Al 2.11 ) 8.00 O 22.00 F 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-magnesio- hastingsite) as well but they have a more variable Si/Al-ratio as the xenolith-hosted fluoro-magnesiohastingsites. Key-words: fluoro-magnesiohastingsite, new mineral, amphibole group, crystal structure, Dealul Uroi, Romania. 0935-1221/06/0018-0503 $ 2.70 © 2006 E. Schweizerbart’sche Verlagsbuchhandlung. D-70176 Stuttgart DOI: 10.1127/0935-1221/2006/0018-0503 *E-mail: hans-peter.bojar@stmk.gv.at