Cyprine, Ca19 Cu2+ (Al, Mg, Mn)12 Si18 O69 (OH)9, a new vesuvianite-group mineral from the Wessels mine, South Africa

Abstract

Cyprine, ideally Ca 19 Cu 2+ (Al,Mg,Mn) 12 Si 18 O 69 (OH) 9 , was found at the Wessels mine near Hotazel, Kalahari Manganese Field, North Cape province, South Africa. It occurs as chaotic aggregates (up to 5 cm across) in open cavities or embedded in coarse-grained colourless calcite. Associated minerals are calcite, apatite, andradite, henritermierite and rhodochrosite. Single crystals of cyprine up to 1 cm are dark red with a lilac hue and vitreous lustre. Dominant crystal forms are {1 0 0}, {1 1 0} and {3 3 1}. The Mohs hardness is 6.5. D meas and D calc are 3.40(3) and 3.41 g/cm 3 , respectively. Cyprine is optically uniaxial, negative, ω = 1.744(2), e= 1.732(2) (in white light). Pleochroism is strong; ranging from O = dark purple, E = pale red to O = dark reddish brown, E = pale yellowish brown; O ≫ E . Chemical composition (electron microprobe, H 2 O by thermogravimetric analysis (TGA)) of two chemically different zones (zone-1/zone-2, wt.%) is: SiO 2 36.98/36.51; Al 2 O 3 14.98/13.70; CaO 36.70/36.18; MgO 3.24/1.10; Mn 2 O 3 2.27/6.04; CuO 2.39/1.86; Fe 2 O 3 0.62/2.50; Cr 2 O 3 0.23/0.04; H 2 O 3.30/3.30, total 100.71/101.23. The crystal-chemical formulae (based on the FTIR, TGA/differential scanning calorimetry, SREF, WDS and optical spectra) are Ca 8.00 Ca 1.00 (Cu 0.95 Mg 0.05 ) Ʃ1.00 Al 4.00 (Al 5.50 Mg 1.00 Mn 3+ 1.19 Fe 3+ 0.22 Cr 0.09 ) Ʃ8.00 ([SiO 4 ] 9.91 [H 4 O 4 ] 0.09 ) Ʃ10.00 [Si 2 O 7 ] 4 ((OH) 9 O) 9.91 and Ca 18.00 Ca 1.00 (Cu 0.90 Mg 0.10 ) Ʃ1.00 (Al 3.22 Mn 3+ 0.60 Fe 30.18 ) Ʃ4.00 (A l4.72 Mn 3+ 1.20 Fe 3+ 1.06 Mn 2+ 0.60 Mg 0.40 Cr 0.02 ) Ʃ8.00 ([SiO 4 ] 9.91 [H 4 O 4 ] 0.09 ) Ʃ10.00 [Si 2 O 7 ] 4 ((OH) 9 O) Ʃ10.00 for zone-1 and zone-2, respectively. Absorption bands in the IR spectrum are 443, 490, 574, 604, 671, 814, 905, 972, 1015, 3354, 3640 cm −1 . The polarized optical absorption spectra have two strong absorption bands at 415 and 555 nm and a less intense band at 645 nm. The eight strongest lines of the powder X-ray diffraction pattern are ( I–d (A)– h k l ): 12–5.89–002, 12–3.007–431, 47–2.950–004, 100–2.75–432, 76–2.594–522, 35–2.459–620, 10–2.324–144, 28–1.6224–672. Cyprine is tetragonal, space group P 4/ n , unit-cell parameters refined from the powder data are a = 15.5652(5), c = 11.7921(4), V = 2863.2 (2) A 3 , Z = 2. The crystal structure has been refined to R 1 = 0.034 and R 1 = 0.035 for 2848 and 2071 unique observed reflections with | F o | ≥ 4σ F for the zones 1 and 2, respectively. The structure refinements provide scattering factors of the Y 1A,B sites close to 29 e − , which supports occupancy of these sites by Cu 2+ ions. The copper coordination polyhedra possess strong Jahn–Teller distortion: Y 1A–O6(4×) = 2.043(3) A and Y 1A–O10 = 2.258(6) A for zone-1 and Y 1B–O6(4×) = 2.010(4) A and Y 1B–O10 = 2.274(7) A for zone-2. Cyprine is defined as a member of the vesuvianite group with Cu 2+ as a dominant cation in the Y 1 site. The historical name cyprine (derived from Latin cuprum , copper) given by J.J. Berzelius in 1821 for Cu-bearing vesuvianite is transferred to the new mineral.