Wessels Mine Research Articles

The real structures of clinotobermorite and tobermorite 9 Å OD character, polytypes, and structural relationships

Clinotobermorite, Ca 5 Si 6 O 17 ·5H 2 O, is a rare mineral, structurally related to tobermorite 11 A. It is characterized by the presence of structural disorder (evidenced by the diffuseness of the reflections with k odd) which, until now, prevented the determination of its real structure. In this paper, a model for the real structure of clinotobermorite is proposed on the basis of OD theory, through examination of the X-ray diffraction pattern of a sample coming from the Wessels mine (South Africa). The proposed model, which assumes the presence of silicate double chains of wollastonite-type, is confirmed by structural refinements carried out for the two polytypes with maximum degree of order (MDO). The MDO 1 polytype of clinotobermorite (monoclinic, space group Cc, a = 11.276(2), b = 7.3427(8), c = 22.642(4) A, β = 97.28(1)°) was refined up to R = 0.15, whereas the two refinements performed on the MDO 2 polytype (triclinic, space group C1, a = 11.274(2), b = 7.3439(7), c = 11.468(2) A, α = 99.18(1), β = 97.19(1), γ = 90.09(1)°) converged to R = 0.12 and R = 0.10, respectively. In clinotobermorite infinite calcium polyhedral layers parallel to (001) are connected through double silicate chains [Si 6 O 17 ] 10- running along b; additional calcium cations and H 2 O molecules are placed in the channels of the resulting framework. By dehydration at 225°C, clinotobermorite transforms topotactically into a new phase, which also displays an OD character. The results of the structural refinement carried out for its triclinic MDO 2 polytype (space group Cc 1, a = 11.156(5), b = 7.303(3), c = 9.566(5) A, α = 101.08(4), β = 92.83(5), γ = 89.98(4)°) indicate that this phase, with crystal chemical formula Ca 5 Si 6 O 16 (OH) 2 , exhibits single chains of wollastonite-type, resulting from decondensation of the double chains. On the basis of the new detailed structural information a possible explanation for the enigmatic thermal behaviour of tobermorite 11 A has been proposed.

Clinotobermorite, Ca5[Si3O8(OH)]2 · 4 H2O \p=n-\ Ca5[Si6O17] · 5 H2O, a natural C\p=n-\S\p=n-\H(I) type cement mineral: determination of the substructure

Abstract The substructure of natural clinotobermorite Ca5[Si3O8(OH)]2 · 4 H2O \p=n-\ Ca5[Si6Ol7] · 5 H2O from the Wessels mine, Kalahari manganese field, South Africa, was determined from single-crystal X-ray data. The diffraction pattern (monoclinic: a = 11.19 Å, b = 7.29 Å, c = 22.46 Å, β = 96.97°) is characterised by rather diffuse reflections of the type hkl with h, k = 2n + 1. The substructure (space group I2/m, Z = 1, a/2 = 5.593(6) Å, b/2 = 3.645(4) Å, c = 22.46(3) Å, β = 96.97(2)°) was solved from the sharp Bragg reflections (hkl, with h, k = 2n) and refined on the basis of 1877 unique reflections to R(F) = 6.7%. The structure of clinotobermorite is closely related to the one of so called 11 Å tobermorite Ca5Si6O16(OH)2 · 2 H2O (pseudo-orthorhombic, a = 11.17 Å, b = 7.38 Å, c = 22.6 Å). In both minerals layers parallel (001), formed by seven-fold co-ordinated Ca, are flanked on either side by wollastonite-like [Si3O9] single chains (ca. 7.3 Å periodicity along b) creating a sandwich-like assemblage. The short b/2 axis (3.645 Å) and the short a/2 axis (5.593 Å) of the subcell are caused by an overlay of two half-occupied single chains of tetrahedra with one shifted by b/2 against the other. In clinotobermorite the sandwich-like layers are stacked along c yielding an interlayer distance of 4.5 Å with a free aperture of 3.2 Å where additional Ca ions and H2O molecules connect the sandwich units. Clinotobermorite with 4–5 H2O represents a much denser structure than 11 Å tobermorite with only 2 H2O or 14 Å tobermorite with ca. 6 H2O (pseudo-orthorhombic, a = 11.17 Å, b = 7.38 Å, c = 27.94 Å). The substructure of clinotobermorite can be interpreted as either composed of ‘Dreiereinfachketten’ leading to the formula Ca5[Si3O8(OH)]2 · 4 H2O, or of ‘Dreierdoppelketten’ leading to the formula Ca5[Si6O17] · 5 H2O, or of a mixture of both variants. The surface structure of clinotobermorite was studied by atomic force microscopy (AFM) where low magnifications reveal a complex layering or twinning parallel (001). High resolutions scans parallel (100) establish enhanced contrast fringes with a periodicity of ca. 10 Å interpreted as the repeat distance of the Ca polyhedral layers parallel to c.

Wesselsite, SrCu[Si4O10], a further new gillespite-group mineral from the Kalahari Manganese Field, South Africa

AbstractWesselsite, SrCu[Si4O10], is a new mineral species from the Wessels mine, Kalahari Manganese Field, South Africa, and it belongs to the gillespite group. Wesselsite is tetragonal, space group P4/ncc; the unit cell parameters, refined from Gandolfi film data, are a = 7.366(1), c = 15.574(3) Å V = 845.01 Å3. The strongest lines are (dobs/lobs/hkl) (7.79/35/002), (4.33/20/112), (3.89/20/004), (3.44/40/104), (3.33/100/202), (3.12/55/114), (3.03/50/212), (2.68/25/204), (2.61/30/220) and (2.32/30/116). Wesselsite is associated with hennomartinite, embedded in a matrix of sugilite, xonotlite, quartz and pectolite. Microprobe analyses of 111 samples show that it is the end-member of a solid solution series with effenbergerite, BaCu[Si4O10], with substitutions of Sr by Ba up to 50 mol.%. Wesselsite forms tiny subhedral plates in sizes not exceeding 50 × 50 × 5 µm, arranged in clusters of up to 200 µm. It shows a perfect cleavage parallel to {001}, has blue colour, white to light blue streak, and is uniaxial negative with ω = 1.630(2), ε = 1.590(5), strongly pleochroic from blue (ω) to pale blue (ε). The calculated density is 3.32 g cm−3, the measured density is 3.2(1) g cm−3.

Effenbergerite, BaCu[Si4O10], a new mineral from the Kalahari Manganese Field, South Africa: description and crystal structure

AbstractEffenbergerite, ideally BaCu[Si4O10], structure determined by single crystal X-ray methods in space group P4/ncc, a = 7.442(2)Å, c = 16.133(5)Å, V = 893.50 Å3 Z = 4, is a new mineral from the Wessels mine, Kalahari Manganese Field, South Africa. It is associated with native copper, calcite, quartz and clinozoisite within pectolite veinlets, embedded in a matrix of braunite, sugilite and hausmannite. Effenbergerite occurs as transparent blue platelets with perfect cleavage parallel to {001} in sizes up to 8.0 × 8.0 × 0.1mm. It has a pale blue streak, subconchoidal fracture, a calculated density of 3.52gcm−3 and an estimated Mohs' hardness of 4–5. Effenbergerite is uniaxial negative with ω = 1.633(2), ε = 1.593(2), strongly pleochroic from intense blue (ω) to nearly colourless (ε). The strongest lines in the X-ray powder diffraction pattern (with refined lattice parameters a = 7.440(1)Å, c = 16.133(2)Å) are: (dobs/Iobs/hkl) (8.0624/100/002), (4.0325/39/004), (3.5443/29/104), (3.1998/44/114), (2.6892/21/006), (2.3943/41/116), (2.0169/34/008), (1.9466/22/108) and (1.4802/21/2.0.70).Effenbergerite is the natural analogue to synthetic BaCu[Si4O10], isotypic with SrCu[Si4O10] and CaCr[Si4O10] as well as with the minerals cuprorivaite, CaCu[Si4O10] and gillespite, BaFe[Si4O10]. The structure consists of silicate sheets [Si8O20]8− parallel (001) formed by corner-linkage of silicate 4-membered rings. The copper(II) atom is nearly planar 4-coordinated; the barium atom has a distorted cubelike environment of oxygen atoms. The mineral is named for Dr. Herta S. Effenberger of the University of Vienna, Austria.

Chemical composition and structural formula of manganoan sugilite from the Wessels Mine, Republic of South Africa

Chemical composition and structural formula of manganoan sugilite from the Wessels Mine, Republic of South Africa

Crystal structure of SrMn2[Si2O7](OH)2·H2O, a new mineral of the lawsonite type

Crystal with the composition SrMn 2 [Si 2 O 7 ](OH) 2 .H 2 O were found in a sugilite, serandite-pectolite rich sample from the Wessels Mine, Kalahari, South Africa. The crystal structure of the nex compound with space group Cmcm, a = 6.255(1), b = 9.034(2), c = 13.397(2) A, Z = 4 was determined from X-ray single-crystal data (R = 0.048). The structure is of the lawsonite type where Al is completely replaced by Mn 3+ and Ca by Sr. [Mn 3+ O 6 ] octahedra forming edge-sharing chains parallel to a exhibit a Jahn-Teller distortion with four short and two long Mn 3+ -O distances. Sr is in eight-fold coordination and H 2 O is disordered on a split position. Together with orientite, macfallite, ruizite, and okhotskite the new compound belongs to a family of structures which have the fundamental building block [Mn 3+ (OH) 2 (SiO 4 ) 2 ] and 6 x 9 A cell dimensions in common

The Occurrence and Gemological Properties of Wessels Mine Sugilite

Examination of jewelry-quality siigilite shows that i t consists o f manganese-bearing sugilite and other minerals i n a polycrystalline aggregate. The material occurs in a large stratiform manganese orebody at the Wessels m ine near Kuruman, South Africa. Some 12 to 1.5 tons o f sugilite of varying quality lire estimated to occur at the mine. The attractive purple color is due to the presence of about 1-3 wt.% n~anganese oxide. The research reported here revealed that there are actually two types of gem materials that have heretofore been called sugilite: one that is predominantly manganoan sugilite with minor impurity minerals, and the other that is chalcedony mixed wi th (and colored by ) sugjlite.