Abstract:Hongheite (IMA 2017- 027), Ca 18 (□,Ca) 2Fe 2+ Al 4 (Fe 3+ ,Mg,Al) 8 (□,B) 4 BSi 18 O 69 (O,OH) 9 is a new menber of vesuvianite- group minerals found in the endoskarn of the Baishanchong granite (77. 43 Ma) that is adjacent to the Malage skarn Sn deposit, located at the northeast border of the world- class Gejiu tin polymetallic ore- field, Yunnan Province, southwest China. It occurs mostly as radiating acicular to prismatic aggregates 4 to 25 mm in diameter. When the mineral occurs in vugs, their well- formed, euhedral and prismatic crystals are 0. 5 to 4. 0 mm long and 0. 3 to 1. 0 mm thick, respectively. Associated minerals include danburite, fluorite, axinite- (Fe), datolite, cuspidine, nordenskildine, quartz and hydroxylapophyllite- (K) etc. Hongheite exhibits a dark green colour with a pale greyish green streak and a vitreous lustre. The mineral is translucent and brittle in tenacity, and has irregular fracture. Dominant crystal forms are {100}, {110}, {101} and {001}. The microhardness is 988. 3 N/mm 2, equivalent to Mohs hardness 6~7. The specific gravitiy is D meas =3. 446 g/cm 3 and D calc =3. 423 g/cm 3 respectively. The mineral is optically uniaxial (+) with ω =1. 720(2), ε =1. 725 (2) (inwhite light). Its pleochroism is weak. The chemical composition is: SiO 2 35. 85%; TiO 2 0. 01%; Al 2O 3 11. 00%; Fe 2O 3 7. 92%; FeO 2. 14%; CaO 33. 57%; MnO 0. 42%; MgO 3. 48%; B 2O 3 2. 82%; Cr 2O 3 0. 01%; Na 2O 0. 01%; F 0. 40%; Cl 0. 14%; (Cl, F≡O-0. 20); H 2O 0. 75% total 98. 32%. On the basis of crystal refinement and Si=18 apfu , the crystal- chemical formula of hongheite is Ca 18 (□,Ca) 2 Fe 2+ Al 4 (Fe 3+ , Mg,Al) 8 (□,B) 4 BSi 18 O 69 (O,OH) 9. The three strongest powder X- ray diffraction lines \[ d in ( I/I 0)( hkl )\] are: 2. 9289 (47) (004), 2. 7661 (100) (342) and 2.6079 (68) (243). The mineral is tetragonal, space group P 4/ nnc , unit- cell parameters a =15. 667 (3) , c = 11. 725 (1) , V =2878 (1) 3, Z =2. The crystal structure has been refined to R =0. 063. Hongheite is defined as a new member of the vesuvianite group with Fe 3+ and B as a dominant cation at the Y (3) and T (2) site separately. By the way, the paper discusses and proposes to revise the current crystal- chemical formula of the vesuvianite- group minerals calculation and writing.
