Wessels Mine Research Articles

Hydroxymcglassonite-(K), KSr4Si8O20(OH)·8H2O, the first Sr-bearing member of the apophyllite group, from the Wessels mine, Kalahari Manganese Field, South Africa

Abstract A new mineral species, hydroxymcglassonite-(K), ideally KSr4Si8O20(OH)·8H2O, has been found in the Wessels mine, Kalahari Manganese Field, Northern Cape Province, South Africa. It is granular (<0.05 mm), associated with meieranite, sugilite, aegirine, pectolite, and yuzuxiangite. The mineral is colorless, transparent with a white streak and a vitreous luster. It is brittle and has a Mohs hardness of 4.5–5.0; cleavage is perfect on {001} and no parting or twinning was observed. The measured and calculated densities are 2.60(3) and 2.614 g/cm3, respectively. Optically, hydroxymcglassonite-(K) is uniaxial (+), with ω = 1.555(5), ε = 1.567(5) (white light), and absorption O > E. Hydroxymcglassonite-(K) is insoluble in water or hydrochloric acid. An electron microprobe analysis yielded an empirical formula (based on 13 non-H cations pfu) K1.01(Sr2.99Ca1.03)Σ4.02Si7.99O20(OH)·8H2O, which can be simplified to K(Sr,Ca)4Si8O20(OH)·8H2O. Hydroxymcglassonite-(K) is tetragonal with space group P4/mnc and unit-cell parameters a = 9.0792(2), c = 16.1551(9) Å, V = 1331.70(9) Å3, and Z = 2. It is isostructural with hydroxyapophyllite-(K), KCa4Si8O20(OH)·8H2O, with Sr substituting for Ca. The crystal structure of hydroxy-mcglassonite-(K) is characterized by SiO4 tetrahedra sharing corners to form (Si8O20)8– sheets parallel to (001), which are connected by the K and B (= Sr + Ca) cations, as well as hydrogen bonding. The K cation is coordinated by eight H2O groups, and the average K–O distance of 2.941(3) Å is shorter than that of 2.950(3)–2.975(3) Å in hydroxyapophyllite-(K) or fluorapophyllite-(K). The B cation is sevenfold-coordinated (4O + 2H2O + OH), and the average B-O distance of 2.522(3) Å is noticeably longer than that of 2.422–2.435 Å in hydroxyapophyllite-(K) or fluorapophyllite-(K). The Raman spectra of hydroxymcglassonite-(K) and hydroxyapophyllite-(K) are very comparable, especially in the O-H stretching region. The discovery of hydroxymcglassonite-(K), the first Sr-bearing mineral of the apophyllite group, implies that more Sr-bearing members of the group may be found in nature or synthesized in laboratories, but the possibility for an incomplete solid solution between hydroxyapophyllite-(K) and hydroxymcglassonite-(K), due to the size difference between Sr2+ and Ca2+, cannot be ruled out.

The Where of Mineral Names: Wesselsite, Wessels Mine, Kalahari Manganese Field, South Africa

Wesselsite, SrCuSi4O10 (fig. 3), is a phyllosilicate member of the gillespite group of minerals found in, and named for, the Wessels mine, Kalahari manganese field, South Africa (Giester and Rieck ...

Meieranite, Na2Sr3MgSi6O17, a New Mineral from the Wessels Mine, Kalahari Manganese Fields, South Africa

Abstract A new mineral species, meieranite, ideally Na2Sr3MgSi6O17, has been found in the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. It occurs in isolated aggregates embedded in a matrix mainly of sugilite, along with minor aegirine and pectolite. Crystals of meieranite are up to 0.5 × 0.5 × 0.4 mm in size. No twinning is observed. The mineral is light blue to blue in transmitted and under incident lights, transparent with white streak, and has vitreous luster. It is brittle and has a Mohs hardness of 5.5; cleavage is good on {010} and no parting was observed. The measured and calculated densities are 3.41(3) and 3.410 g/cm3, respectively. Optically, meieranite is biaxial (–), with α = 1.610(1), β = 1.623(1), γ = 1.630(1) (white light), 2V (meas.) = 70(1)°, 2V (calc.) = 72°. The calculated compatibility index based on the empirical formula is –0.007 (superior). An electron microprobe analysis yields an empirical formula (based on 17 O apfu) of Na1.96(Sr2.91Ba0.03Ca0.03Pb0.02)Σ2.99(Mg0.62Mn0.28Co0.07Fe0.01)Σ0.98Si6.03O17, which can be simplified to Na2Sr3MgSi6O17. Meieranite is orthorhombic, with space group P21nb and unit-cell parameters a 7.9380(2), b 10.4923(3), c 18.2560(6) Å, and V 1520.50(8) Å3. Its crystal structure is characterized by two kinds of layers that alternate along [010]: layers of corner-sharing SiO4 and M2+O4 tetrahedra (M2+ = Mg, Mn, Co, Fe) and layers of NaO6 and SrO8 polyhedra. The tetrahedral layers consist of eight-, five-, and four-membered rings and are composed of [Si6O17] ribbons (parallel to [101]) linked together by MO4 tetrahedra. Most remarkably, the structure of meieranite is topologically identical to that of the nordite group of minerals, which has the general chemical formula Na3SrR3+M2+Si6O17, where R = Ce and La and M = Zn, Fe, and Mn. Accordingly, chemically, meieranite may be obtained through the coupled substitution of 2Sr2+ for (Na+ + R3+) in nordite.

Marshallsussmanite, NaCaMnSi3O8(OH), a new pectolite-group mineral providing insight into hydrogen bonding in pyroxenoids

AbstractMarshallsussmanite (IMA2013-067) is a new pyroxenoid mineral from the Wessels mine, Kalahari Manganese Field, Northern Cape Province, South Africa. Marshallsussmanite has ideal formula NaCaMnSi3O8(OH) and triclinic P$\bar{1}$symmetry. Marshallsussmanite forms vitreous pink bladed crystals to 2.1 cm. The mineral shows perfect cleavage on both {100} and {001}. The chemical composition from electron microprobe (average of 20 analyses) and inductively coupled plasma mass spectrometer analysis (average of three analyses) is Li2O 0.43, Na2O 8.06, MgO 0.08, CaO 15.33, MnO 21.79, SiO251.71; totalling 97.40 wt.%. The empirical formula, normalized to 3 Si and assuming 1 H apfu is Li0.100Na0.906Ca0.953Mg0.007Mn1.071Si3O8(OH). Unit-cell parameters from single crystal X-ray diffraction area= 7.7854(4),b= 6.9374(4),c= 6.8516(3) Å, α = 90.683(3)°, β = 94.330(3)°, γ = 102.856(3)°,V= 359.59(3) Å3;Z= 2. The crystal structure refinement converged withRobs= 0.0248 and site occupancy refinement gives crystal chemistry [Na0.948Li0.052][Ca0.793Mn0.207] [Mn0.937Ca0.063]Si3O8(OH). Marshallsussmanite is a single chain silicate with a repeat interval of three tetrahedra (i.e.dreierchain). Marshallsussmanite is a member of the pectolite group of pyroxenoids, which also includes barrydawsonite-(Y), cascandite, pectolite, serandite and tanohataite. Parallel silicate chains form layers, intercalated with well-ordered cation layers. Calcium and Mn both exhibit octahedral coordination, while Na has four bonded interactions in a coordination sphere (radius 3 Å) of seven separate O atoms. Procrystal electron density and bond valence modelling results are compared. The mineral has an unusually strong hydrogen bond with O4⋅⋅⋅O3 separation of 2.458(2) Å. Unlike pectolite and serandite, O4 in marshallsussmanite acts as an H-bond donor and O3 is an H-bond acceptor. Cation ordering in pyroxenoids has a substantial impact on the H position and corresponding H-bonding schemata.

Connoisseur's Choice: Sugilite, Wessels Mine, Kalahari Manganese Field, Northern Cape Province, South Africa

Purple is a relatively uncommon color in the mineral world, and one of the most recognizable purple species is sugilite. It is also one of the minerals that occurs in both massive form, used extens...

Crystal structure of potassic-mangani-leakeite from the Wessels Mine, Kalahari Manganese Field, South Africa

New crystallographic data for holotype potassic-mangani-leakeite, with the ideal composition KNa2(Mg2Mn3+2Li)[Si8O22](OH)2, are presented. This end-member of the amphibole supergroup was formerly reported as “kornite” by Armbruster et al. (1993a) with the empirical formula (K0.65Na0.31)(Na1.79Li0.21)(Mg2.13Mn3+1.43Fe3+0.52Li0.91)Si8O22(OH)2. X-ray crystal-structure refinements of four crystals taken from the holotype material (Wessels Mine, Kalahari Manganese Field, South Africa) showed that this amphibole composition has the standard space group C 2/ m , contrary to the original report. Differences in chemical composition (Mg vs . Li) were found based on scattering power determined by structure refinement. The crystals tested consistently show compositions significantly different from that reported in the original mineral description, especially as far as the Li content is concerned (0.45–0.85 vs . 1.11 apfu). The new data suggest that the Li content in the original paper is not characteristic of the studied single crystals and may have been overestimated. Three of the investigated crystals, represented by crystal no. 1, a = 9.9603(17), b = 17.830(3), c = 5.3056(9) A, β = 105.275(3)°, V = 909.0(3) A3, yielded a refined composition of (K0.62Na0.38)Na2(Mg1.94(Mn,Fe)2+0.36(Mn,Fe)3+1.85Li0.85)[Si8O22](OH)2 when using the simplified model without vacancies at the (K, Na)-bearing A site, M (1) and M (2) filled with Mg and (Mn,Fe), and M (3) occupied by Li and Mn. The range of Li at M (3) was bracketed by two models either with (0.64 Li + 0.36 Mg) or with (0.85 Li and 0.15 Mn) having the same scattering power. The choice of the above formula was made considering the agreement between observed and calculated 〈 M (3)–O〉 bond lengths. On the same grounds, the one crystal with the lowest Li content of 0.46 apfu belongs to the compositional space of eckermannite. Small concentrations of A group vacancies cannot be excluded.

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

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.

Redetermination of ruizite, Ca2Mn(3+) 2[Si4O11(OH)2](OH)2·2H2O.

The crystal structure of ruizite, ideally Ca2Mn(3+) 2[Si4O11(OH)2](OH)2·2H2O [dicalcium dimanganese(III) tetra-silicate tetra-hydroxide dihydrate] was first determined in space group A2 with an isotropic displacement parameter model (R = 5.6%) [Hawthorne (1984 ▸). Tschermaks Mineral. Petrogr. Mitt. 33, 135-146]. A subsequent refinement in space group C2/m with anisotropic displacement parameters for non-H atoms converged with R = 8.4% [Moore et al. (1985 ▸). Am. Mineral. 70, 171-181]. The current study reports a redetermination of the ruizite structure by means of single-crystal X-ray diffraction data of a natural sample from the Wessels mine, Kalahari Manganese Field, Northern Cape Province, South Africa. Our data (R 1 = 3.0%) confirm that the space group of ruizite is that of the first study rather than C2/m. This work improves upon the structure reported by Hawthorne (1984 ▸) in that all non-H atoms were refined with anisotropic displacement parameters and all hydrogen atoms were located. The crystal structure consists of [010] chains of edge-sharing MnO6 octa-hedra flanked by finite [Si4O11(OH)2] chains. The Ca(2+) cations are situated in the cavities of this arrangement and exhibit a coordination number of seven.

Cairncrossite, a new Ca-Sr (-Na) phyllosilicate from the Wessels Mine, Kalahari Manganese Field, South Africa

Cairncrossite is a new phyllosilicate species found in manganese ore on dumps of the Wessels Mine, Kalahari Manganese Field, South Africa. Associated minerals are richterite, sugilite, lizardite and fibrous pectolite. It occurs as radiating platy micaceous aggregates of up to 1 cm in size. Cairncrossite is colourless, appearing white, and the crystals are translucent to transparent with a white streak and vitreous to pearly lustre. The crystals are sectile before brittle fracture, with a Mohs hardness of 3. A perfect cleavage parallel (001) is observed. The calculated density is 2.486 g cm −3 . The mineral is biaxial positive with n α = 1.518(2), n β = 1.522(2), n γ = 1.546(2), 2 V obs = 33.9(6)° (2 V calc = 44.97°) at 589.3 nm and 24°C. The orientation of the indicatrix is Z ^ c * = 10°. The dispersion is weak ( r v ) and no pleochroism is observed. An intense light-blue fluorescence is emitted under shortwave UV radiation. Cairncrossite is triclinic, space group P 1 ¯ , a = 9.6265(5), b = 9.6391(5), c = 15.6534(10) A, α = 100.89(1), β = 91.27(1)°, γ = 119.73(1)°, V = 1227.08(13) A 3 , Z = 1. The strongest lines in the Gandolfi X-ray powder-diffraction pattern [ d in A( I )( hkl )] are 15.230 (100)(001), 8.290 (15)(1–10), 5.080(25)(003), 3.807(30)(004), and 3.045(20)(005). The chemical composition obtained by electron-microprobe analysis is Na 2 O 3.06, K 2 O 0.11, CaO 18.61, SiO 2 54.91, SrO 11.75, total 88.44 wt%. The relevant empirical formula, based on 16 Si atoms per formula unit ( apfu ) and TGA data is: Sr 1.99 K 0.02 Ca 5.81 Na 1.73 Si 16 O 55.84 H 30.33 . Taking variable sodium contents into account, the idealized structural formula is Sr 2 Ca 7–x Na 2x (Si 4 O 10 ) 4 (OH) 2 (H 2 O) 15–x with 0 ≤ x ≤ 1, and the simplified formula for sodium-rich crystals is SrCa 3 Na(Si 4 O 10 ) 2 (OH)(H 2 O) 7 with Z = 2. The structure of cairncrossite was refined on single-crystal X-ray data (Mo K a radiation) to R 1 = 0.047. Cairncrossite belongs to the gyrolite and reyerite mineral groups, it is characterized by sheets consisting of edge-sharing CaO 6 octahedra, which are corner-linked on both sides to silicate layers. These units are intercalated by layers formed by SrO 8 polyhedra, which are arranged in pairs via a common edge, and further bound to disordered NaO 6 polyhedra. A complex system of hydrogen bonds strengthens the linkage to adjacent silicate layers. Cairncrossite exhibits a two-phase endothermic weight loss of the H 2 O molecules in the range 25–400°C; however, the mineral shows a nearly complete rehydration capability up to 400°C. The new mineral is named in honour of Bruce Cairncross, Professor and Head of the Department of Geology, University of Johannesburg.

Colinowensite, BaCuSi2O6, a new mineral from the Kalahari Manganese Field, South Africa and new data on wesselsite, SrCuSi4O10

AbstractA new silicate, colinowensite, BaCuSi2O6, has been found in the Wessels mine, Kalahari Manganese Field, South Africa. It is associated with effenbergerite, wesselsite, lavinskyite, scottyite, diegogattaite, as well as with pectolite, quartz, aegirine, richterite, minerals of the garnet group and a number of different manganese and iron oxides, especially hausmannite and hematite. The mineral was named for the mineral collector and finder of the new species, Colin R. Owens, of Somerset West, South Africa. Colinowensite is brittle, with uneven fracture, and the estimated Mohs hardness is ∼4. It occurs as subhedral crystals <100 μm in size. The forms {100} and {110} are observed while {001} is always present in cleavage plates. The calculated density is 4.236 g cm–3. It is the natural analogue of the synthetic pigmentreferred to as Chinese or Han purple, which is found on artifacts from ancient and imperial China. The mineral is of dark blue to purple colour, with a purple streak, and is uniaxial (–), with ω = 1.740 (20), ε = 1.735 (20) (420 nm) and ω = 1.745 (20), ε = 1.730(20) (650 nm). The lustre is vitreous and no fluorescence is observed under either shortwave or longwave ultraviolet radiation. Avery strong pleochroism occurs from purple along the c axis to blue in a perpendicular direction. Colinowensite is not soluble in acids except HF. Electron microprobe analyses gave an average composition (wt.%) of CuO 22.53, BaO 43.43 and SiO2 34.04 yielding the empirical formula (based on 6 O a.p.f.u.) BaCuSi2O6. The new mineral is tetragonal, space group I41/acd with Z = 16, anda = 9.967(1), c = 22.290 (2) Å. Colinowensite is a cyclosilicate with [Si4O12]8– 4-membered single rings, arranged in sheets parallel to (001). The structure is further characterized by CuO4 squares sharing corners with four neighbouring silicate rings within a sheet. Ba2+ cations are bonded to ten O atoms in irregular coordination. Average Si–O, Cu–O and Ba–O bond lengths are 1.619, 1.934 and 2.943 Å, respectively. Colinowensite belongs to subdivision 9. CE of the Strunz Mineralogical Tables. In addition, based on single-crystal X-ray work, new structural data for wesselsite of chemical composition Sr0.9Ba0.1CuSi4O10 are provided.

Connoisseur's Choice: Marshallsussmanite, Wessels mine, Kalahari Manganese Field, Northern Cape Province, South Africa

Connoisseur's Choice: Marshallsussmanite, Wessels mine, Kalahari Manganese Field, Northern Cape Province, South Africa

Lavinskyite, K(LiCu)Cu6(Si4O11)2(OH)4, isotypic with plancheite, a new mineral from the Wessels mine, Kalahari Manganese Fields, South Africa

A new mineral species, lavinskyite, ideally K(LiCu 2+ )Cu6 2+ (Si4O11)2(OH)4 (IMA 2012-028), has been found in the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. Associated minerals include wesselsite, pectolite, richterite, sugilite, and scottyite. Lavinskyite crystals are tabular [parallel to (010)]. The mineral is light blue, transparent with very pale blue streak and vitreous luster. It is brittle and has a Mohs hardness of ~5; cleavage is perfect on {010} and no parting was observed. The measured and calculated densities are 3.61(3) and 3.62 g/cm 3 , respectively. Optically, lavinskyite is biaxial (+), with a = 1.675(1), b = 1.686(1), g = 1.715(1), 2Vmeas = 64(2)°. An electron microprobe analysis produced an average composition (wt%) of SiO2 42.85(10), CuO 46.13(23), K2O 4.16(2), MgO 1.53(17), Na2O 0.27(4), BaO 0.18(6), and MnO 0.08(1), plus Li2O 1.38 from the LA-ICP-MS measurement and H2O 3.22 (added to bring the analytical total close to 100%), yielding a total of 99.79% and an empirical chemical formula (K0.99Ba0.01)S=1.00(Li1.04Cu0.93Na0.10)S=2.07 (Cu5.57Mg0.43Mn0.01)S=6.01(Si4.00O11)2(OH)4. Lavinskyite is isotypic with plancheite, Cu8(Si4O11)2(OH)4·H2O, an amphibole derivative. It is orthorhombic, with space group Pcnb and unit-cell parameters a = 19.046(2), b = 20.377(2), c = 5.2497(6) A, and V = 2037.4(4) A 3 . The key difference between lavinskyite and plancheite lies in the coupled substitution of K + and Li + in the former for H2O and Cu 2+ in the latter, respectively. The structure of lavinskyite is characterized by the undulating, brucite-like layers consisting of three distinct octahedral sites occupied mainly by Cu. These layers are sandwiched by the amphibole-type double silicate chains extending along the c axis, forming a sheet structure of compact silicate-Cu-silicate triple layers. Adjacent sheets are linked together by K and M4 (= Cu + Li) cations, as well as hydrogen bonding. The M4 site is split, with Cu and Li occupying two different sites. Lavinskyite exhibits more amphibole-like structural features than plancheite, as a consequence of K in the large cavity between the two back-to-back double silicate chains.

Diegogattaite, Na2CaCu2Si8O20·H2O: a new nanoporous copper sheet silicate from Wessels Mine, Kalahari Manganese Fields, Republic of South Africa

AbstractDiegogattaite (IMA2012-096), Na2CaCu2Si8O20·H2O, is a new mineral from the Wessels mine in the Kalahari manganese fields of South Africa. It occurs as a minor phase with other copper-bearing silicates, Cu-rich pectolite, sugilite, quartz, aegirine and undifferentiated Fe-Mn oxides. Diegogattaite is pale turquoise through teal blue. It is found as sub-mm sized grains in a main crystalline patch 3–4 mm in size, and is currently known from only one sample. The mineral is transparent with a vitreous lustre and may have a good cleavage on {001}. It is brittle, with an uneven fracture and a very pale-blue streak. It is non-fluorescent in short- and long-wave UV light and has an estimated Mohs hardness of ∼5–6. Diegogattaite is biaxial (–), α = 1.598(2), β = 1.627(2), γ = 1.632(2); 2Vmeas = 44.0(6)°, 2Vcalc = 44.5°; dispersion: strong r < v, orientation: X = b, Y ≈ ⊥(001), Z ≈ a; pleochroism X colourless << Y ≈ Z blue green. The calculated density is 3.10 g/cm3. Electron-microprobe analysis gave: Na2O 8.07, CaO 7.3, CuO 20.5, FeO 0.36, SiO262.4, H2O(calc) 2.34, total 100.97 wt.%. A charge-balanced formula on the basis of 21 oxygen a.p.f.u. is: Na2.00Ca1.00Cu1.98Fe0.04Si7.99H2O21. Diegogattaite is monoclinic, space group C2/m, a = 12.2439(6) Å, b = 15.7514(4) Å, c = 10.6008(3) Å, β = 125.623(2)°, V = 1661.87(10) Å3 and Z = 4. The five strongest lines in the X-ray powder pattern are [dobs in Å (Iobs)(hkl)]: 4.25(75)(002,22,220), 3.951(77)(040), 3.261(100)(31,13), 2.898(89)(042,03,003), 2.332(66)(331,43,62,260,043). The crystal structure of diegogattaite was determined by single-crystal X-ray diffraction to final agreement indices of R1 = 0.027, wR2 = 0.071 and GoF = 1.090. It represents a completely new silicate topology based upon a double-sheet of SiO4 tetrahedra composed of connected 6482 cages. The structure of diegogattaite is related to those of synthetic nanoporous Na-Cu-Si-O-(OH)-H2O (CuSH) compounds, which are of interest to the solid-state chemistry community as potential ion-exchangers, catalysts and molecular sieves. The structure of diegogattaite forms a bridge between these structures and those of the gillespite-group minerals, including wesselsite. The close spatial association of wesselsite and diegogattaite suggests a possible reaction between them that may point to a synthetic route for the production of novel alkaline-earth-based nanoporous copper silicates.

Scottyite, the natural analog of synthetic BaCu2Si2O7, a new mineral from the Wessels mine, Kalahari Manganese Fields, South Africa

A new mineral species, scottyite, ideally BaCu 2 Si 2 O 7 , has been found in the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. The mineral appears to have formed as a result of a hydrothermal event and is associated with wesselsite, pectolite, richterite, sugilite, and lavinskyite. Scottyite forms blocky grains with striations parallel to the c axis. Crystals are found up to 0.4 x 0.3 x 0.3 mm. No twinning is observed. The mineral is dark-blue in transmitted and under incident lights, transparent with pale blue streak and vitreous luster. It is brittle and has a Mohs hardness of 4~5; cleavage is perfect on {100} and {010} and no parting was observed. The calculated density is 4.654 g/cm 3 . Optically, scottyite is biaxial (-), with α = 1.750(1), s = 1.761(1), and γ = 1.765(1), 2 V meas = 66(2)°. It is insoluble in water, acetone, or hydrochloric acid. An electron microprobe analysis produced an average composition (wt%) (8 points) of CuO 36.98(31), BaO 35.12(16), SiO 2 27.01(61), SrO 0.28(5), and Na 2 O 0.06(2), and total = 99.45(65), yielding an empirical formula (based on 7 O apfu) Ba 1.00 Sr 0.01 Na 0.01 Cu 2.04 Si 1.97 O7. Scottyite is the natural analog of synthetic BaCu 2 (Si,Ge) 2 O 7 , which exhibits novel one-dimensional quantum spin-1/2 antiferromagnetic properties with tunable super-exchange interactions. It is ortho-rhombic, with space group Pnma and unit-cell parameters a = 6.8556(2 ), b = 13.1725(2), c = 6.8901(1) A, and V = 622.21(6) A 3 . The structure of scottyite is characterized by flattened CuO 4 tetrahedra sharing corners with one another to form chains parallel to the c axis. These chains are interlinked by Si 2 O 7 tetrahedral dimers and Ba 2+ . The Ba 2+ cations are bonded to seven O atoms in an irregular coordination. The average Si-O, Cu-O, and Ba-O bond lengths are 1.630, 1.941, and 2.825 A, respectively. Scottyite is topologically related to a group of compounds with the general formula BaM 2+ 2 Si 2 O 7 , where M = Be (barylite and clinobarylite), Fe (andremeyerite), Mg, Mn, Co, and Zn.

A sugilite-bearing assemblage from the Wolhaarkop breccia, Bruce iron-ore mine, South Africa: Evidence for alkali metasomatism and 40Ar-39Ar dating

An occurrence of the alkali-rich, Fe-Mn silicate sugilite [KNa2(Fe3+, Mn3+, Al)2Li3Si12O30] is reported from the Wolhaarkop manganese-rich chert breccia at the Bruce iron-ore mine, Northern Cape Province, South Africa. The upper portion of the breccia is dominated by braunite, albite, K-feldspar and aegirine, and locally hosts irregular-shaped void-fills (generally <5 cm) consisting of both zoned and unzoned assemblages of alkali-rich mn-silicates. the zoned void-fills are characterised by marginal needles of a mineral with norrishite-like composition [kmn3+2LiSi4O10(O)2] penetrating into a persistent wall zone that comprises tabular serandite [NaMn2Si3O8(OH)], albite, K-feldspar and lesser granular sugilite. The cores of larger vugs are occupied by masses of interlocking armbrusterite [K5Na6Mn3+Mn2+14(Si9O22)4(OH)10.4H2O] and/or fibrous sugilite. Unzoned assemblages contain a similar mineralogy, comprising albite, K-feldspar, quartz, sugilite, norrishite, serandite, armbrusterite and minor witherite, strontia- nite and kentrolite [PbMn2O2(Si2O7)]. Similarities to sugilite-bearing assemblages at the Wessels mine in the Kalahari Manganese Field suggest that a single regional hydrothermal alteration event in the vicinity of the unconformity between the Olifantshoek and Transvaal Supergroups was the probable genetic cause for both occurrences. 40Ar-39Ar dating of fibrous sugilite, however, yields an age of 620.2 ± 3.3 Ma (2σ) that is distinctly younger than the ages reported for the assemblages from the Kalahari Manganese Field.

Connoisseur's Choice: Inesite, Wessels Mine, Northwest of Kuruman, Kalahari Manganese District, Cape Province, South Africa

Connoisseur's Choice: Inesite, Wessels Mine, Northwest of Kuruman, Kalahari Manganese District, Cape Province, South Africa

A 57Fe MOSSBAUER SPECTROSCOPIC STUDY OF SUGILITE, KNa2(Fe3+,Mn3+,Al)2Li3Si12O30

Sugilite, ideally KNa(2)(Fe(3+),Mn(3+),Al)(2)Li(3)Si(12)O(30), from the Wessels mine, Kalahari, South Africa, was studied using (57)Fe Mossbauer spectroscopy. The spectra confirm published electron-microprobe data and X-ray single-crystal results, which indicate that Fe(3+) is ordered at the A octahedral site. The magnitude of the quadrupole splitting, DEQ for Fe(3+) at both 84 K and room temperature is 0.00 mm/s. This value is very low for a silicate; reported DEQ values for octahedrally coordinated Fe(3+) are generally greater than 0.20 mm/s. Octahedron distortion, calculated from published X-ray-diffraction data, gives an octahedron-angle variance of 7 and a mean elongation of the octahedron of 1.002. This indicates nearly regular octahedral geometry, and hence a symmetric electric-field gradient around Fe(3+). A crystal-chemical interpretation for the regular octahedral geometry and the resulting low DEQ value for Fe(3+) in the Mossbauer spectrum of sugilite is that structural strain is largely taken up by weak Li-O bonds. They permit highly distorted LiO(4) tetrahedra, whose variance in the O-Li-O angle, 502, and mean elongation of the tetrahedron, 1.148, are among the largest observed values for inorganic crystal structures. Weak Li-O bonds allow the edge-shared, more strongly bonded Fe(3+)O(6) octahedra to remain regular in geometry.

Norrishite, K(Mn23+Li)Si4O10(O)2, an oxymica associated with sugilite from the Wessels Mine, South Africa: Crystal chemistry and40Ar-39Ar dating

Norrishite, K(Mn 2 3+ Li)Si 4 O 10 (O) 2 , an OH-free mica, is found in a silicate-rich sample from the Wessels mine, Kalahari manganese field, South Africa. The mineral forms centimeter-sized black aggregates in dark-purple sugilite, and is associated with kornite and Mn-rich pectolite. Single-crystal X-ray data, collected on norrishite of end-member composition, yielded a monoclinic C2/m lattice with a = 5.291(2), b = 8.904(2), c = 10.049(4) Å, β = 98.15(5)°, V = 920.6 Å 3 , and Z = 2, characteristic of the 1M polytype. 40 Ar- 39 Ar analyses on a gem-quality sugilite yielded an age of 1048.1 ± 5.9 Ma interpreted as a mineral crystallization age that dates the main hydrothermal event that led to enrichment of the Mn ore. The norrishite age data are more complex and suggest a crystallization age ≈ 1010 Ma and a subsequent alteration at ca. 850 Ma. The age results obtained suggest that the hydrothermal activity is related to the collision and tectonism affecting the edges of the Kalahari craton, and show that sugilite, and probably also unaltered norrishite, are suitable for dating metamorphism.

The real structure of tobermorite 11A: normal and anomalous forms, OD character and polytypic modifications

Specimens of tobermorite 11A from Wessels mine, South Africa, an 'anomalous' tobermorite, and from BaIscenov, Urals, Russia, a 'normal' tobermorite, have been studied through X-ray diffraction. Their structuraldisorder, evidencedby the diffusenessof the reflectionswithk odd, has beendealtwiththrough an OD approach, which allowed us to derive a reliable model of the structure, to define the two main polytypes (MDO structures),which were subsequently refined, revealingthe distinctive structural details of the two specimens. The MDO1polytype oftobermorite11A fromWesselsmine (orthorhombic, spacegroup F2dd,a=11.265(2), b = 7.386(1), c = 44.970(9) A) was refinedto R = 0.128, whereas the refinement carriedon the MDO2 polytype (monoclinic, space group B11m, a = 6.735(2), b = 7.385(1), c = 22.487(4) A, = 123.25(1)°) converged to R = 0.051. Two refinements were carried out for the MDO2 polytype of the specimen from the Urals (space group B11m, a = 6.732(2), b = 7.368(1), c = 22.680(4) A, = 123.18(1)°) with data collectedwith conventional source (R = 0.089) and with synchrotron radiation(R = 0.110). Common structural features are the infinite layers of calcium polyhedra, parallel to (001), with tetrahedral chainsof wollastonite-type running along b and attachedon both sides of these layers.The composite layers are stacked along c and connected through formation of double tetrahedral chains, with 'ideal' composition (Si6O15(OH)2)and(Si6O16(OH))inthespecimensfromWesselsmineandUralsrespectively.Themostdistinctive featureof the two specimensisthe content of the structuralcavities:whereasonly watermoleculeswerefound in the specimen from Wessels mine, 'zeolitic'calcium cations and water molecules were found in that from Urals. Thesestructuralaspectsarecomparedandtheirrelevanceinexplaining thedifferentthermalbehaviour ofthe two types of tobermorite 11A is indicated.

Transmission electron microscopy study of gaudefroyite,

A transmission electron microscopy (TEM) study of gaudefroyite, \(Ca_{8}Mn_{6}^{3+}[(BO_{3})_{6}(CO_{3})_{2}O_{6}]\) , from the Wessels mine, Kalahari manganese field, South Africa, shows strong, continuous streaking in selected-area electron diffraction (SAED) patterns. The pseudo-hexagonal parameters for gaudefroyite are a = 10.606(1) and c = 5.896(1) A. The symmetry of gaudefroyite is lower than hexagonal because of the streaked reflections. The CO 3 groups in the large 6 3 channels are ordered in two different ways and this ordering gives rise to two different types of channels, A and B. The A and B channels occur in a 1:1 ratio and these channels are partially ordered and give rise to the continuous streak reflections in gaudefroyite.