----- American Mineralogist, Volume 78, pages 1082-1087, 1993 Poldervaartite, Ca(C80.sMIIo.s)(Si030H)(OH), a new acid nesosilicate from the Kalahari manganese field, South Africa: Crystal structure and description Y ONGSHAN DAI, * GEORGE E. HARLOW Department of Mineral Sciences, The American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024-5192, U.S.A. ANDREW R. MCGHIE The Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6202, U.S.A. ABSTRACT Poldervaartite, Ca(Cao.5Mno.s)(Si030H)(OH), space group Pbca, a = 9.398(1), b = 9.139(2), c = 10.535(2) A, V = 904.8(3) A3, is a new mineral from the Wessels mine in the Kalahari manganese field of northwest Kuruman Hill, Republic of South Africa. It occurs as wheat-sheaf polycrystals to euhedral crystals associated with bultfonteinite, braunite, hausmannite, henritermierite, calcite, and hematite. Poldervaartite is colorless to milky white, has a density of 2.91(2) glcm3 and a hardness (Mohs) of 5. Optically it is biaxial(+) with a = 1.634(2),(3= 1.640(4),and 'Y= 1.656(2), and with pleochroic color: T/-y= bluish gray, 17{3= light gray, and 17a= colorless. The five strongest powder diffraction lines are [dI(hkl)] 3.231x(122), 4.185(210), 2.8464(311), 2.3914(223), and 2.7894(131) A. The structure was solved by direct methods and refined to R = 0.024 using X-ray diffractometer data (1028 independent reflections). The structure is a hydroxyl nesosilicate with a single isolated Si tetrahedron and two M2+sites. Ca is accommodated in Ml, and both Ca and Mn share M2. OH groups are strongly bonded in the structure, preventing H20 loss at temperatures below 460°C. One OH group is bound to Si, making polder- vaartite an acid silicate. The Si030H tetrahedron is externally regular but internally dis- torted. Poldervaartite is probably isostructural with dicalcium silicate a hydrate, a com- mon phase in steam-cured Portland cement. INTRODUCTION ruman Hill, northwest Cape Province, Republic of South Poldervaartite, ideally Ca(Cao.sMno.s)(Si030H)(OH), Africa (Yon Bezing et aI., 1991). The pocket occurred in occurs in the Wessels mine of the famous Kalahari man- massive braunite: and hausmannite, in which poldervaar- ganese field in northwest Cape Province, South Africa tite is intimately associated with henritermierite and cal- (Wilson and Dunn, 1978; Yon Bezing et aI., 1991). We cite and is coated by bultfonteinite and hematite. The take great pleasure in naming this new mineral in honor observed mineral assemblage suggests that the minerals of Arie Poldervaart (1918-1964), professor of petrology were formed by rnodification of low-grade Mn-rich meta- at Columbia University, who made great contributions sediments by hydrothermal fluids rich in Ca, Si02, to understanding the South African Karoo dolerites and and C02. Archean rocks in the Kakamas area of northwest Cape Mineral data Province. The new mineral and name have been ap- proved by the Commission on New Minerals and Min- Poldervaartite crystals have colorless and transparent eral Names, IMA. Type material is preserved in the cores and milky white and translucent rims and have a American Museum of Natural History under catalogue distinct deep red fluorescent color under short-wave- number TI00728. length ultraviolet radiation. However, electron micro- probe analyses do not show any compositional variation DESCRIPTION correlated with the aforementioned zonation. Luster is Occurrence and paragenesis vitreous on fractured surfaces and subvitreous on crystal faces. Other physical and optical data for poldervaartite Poldervaartite was found in a pocket at the Wessels are presented in 'fable 1. mine in the Kalahari manganese field of northwest Ku- Chemical composition * Present address: Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washing- Chemical analysis was performed on an ARL SEMQ ton, DC 20560, U.S.A. nine-spectromete'r electron microprobe, and data were 0003-004X/93/091 0-1 082$02.00 1082 DAI ET AL.: POLDERV AARTITE, Ca(Cao.5Mno.5)(Si030H)(OH) 1083 TABLE 1. Physical and optical properties of poldervaartite The Gladstone-Dale relationship for optical and com- positional data yields a superior compatibility index. Habit: wheat-sheaf to euhedral prismatic Forms: {11 0},{1 00},{01 0},{001} (Mandarino, 1981). Hardness: 5 (Mohs scale) Tenacity: very brittle X-RAY CRYSTALLOGRAPHY AND CRYSTAL Density: STRUCTURE DETERMINATION Measured: 2.91 (2) g/cm3 (Berman balance, toluene at 25 °C) Calculated: 2.90 g/cm3 The space group ofpoldervaartite was established from Optics: biaxial positive 2V: 65 (:t 5)0 (meas), 630 (calc) both overexposed precession photographs and three-di- a = 1.634(2) X = b mensional structure data from a crystal measuring 0.07 (3 = 1.640(4) Y = a x 0.07 x 0.10 mm. X-ray powder diffraction data from 1.656(2) Z 'Y = = c Dispersion: weak; r < II an automated Philips PW1710 diffractometer (graphite- Pleochroism: weak monochromatized CuKa radiation) are given in Table 2. 1/-y = bluish gray light gray Indexing of the powder diffractions is conformable with 11;J = 11"= colorless the single-crystal diffraction intensity data. The least- squares refined unit-cell parameters are a = 9.398(1), b = 9.139(2), c = 10.535(2) A, and V= 904.8(3) A3 (Z = corrected for ZAF effects using Magic-IV (Colby, 1968). 8), with axes defined to conform with space group Pbca. H20 content was determined using a DuPont 951 X-ray intensity data from a quadrant of reciprocal space ThermoGravimetric Analyzer (TGA), and, up to 1000 were measured on a Krisel-automated Picker FACS-I °C, H20 was the only component recorded by the at- four-circle diffractometer utilizing Zr-filtered MoKa ra- tached VG Micromass 300D mass spe'ctrometer. TGA diation. Details of the data measurement and reduction showed no observable H20 loss at temperatures lower are given in Table 3. Absorption corrections employed than 460 °C, indicating strong bonding of the H20 com- an empirical t/; scan technique, utilizing intensity data ob- ponents and high thermal stability of the hydrated phase. tained from ::t1800 t/;scans at 100intervals for one reflec- The analyses yield a general formula of (Ca2-xMnx)Si04 . tion (006). Symmetry equivalent reflections were aver- H20, with x values ranging from 0.28 to 0.52. Results in aged, and only the reflections with Fa > 3eTFwere used in weight percent (range in parentheses) for a relatively ho- the structure determination and refinement. mogeneous grain are Si02 29.8 (28.1-28.9); CaO 41.5 Crystallographic calculations were undertaken using an (40.7-41.9); MnO 18.7 (17.9-19.8); F(~O 0.1 (0.0-0.1); XTAL3.0 package [both UNIX and PC versions (Hall MgO 0.0 (0.0-0.1); and H20 9.4 (average of TGA), with and Stewart, 1990; Grossie, 1991 )]. Interpolated form the average formula being Ca1.47Mno.53Sio.9903.96.1.04H20. factors of neutral atoms, including terms for anomalous The composition of the crystal used in the structure de- dispersion (Davenport and Hall, 1990); were used in all termination was estimated by site refine:ment (see below); calculations, and all reflections were weighted equally in mounting the crystal for probe analysis 'was not practical. the least-squares refinements. Direct methods were used TABLE2. X-ray powder diffraction data for poldervaartite 1110 dmeas(A) deale(A) hkl 1110 dmeas(A) deale(A) hkl 11 5.24 5.27 002 10 2.229 2.233 041 45 4.18 4.18 210 13 2.215 2.217 124 11 4.09 4.11 112 11 2.137 2.139 331 18 3.87 3.88 211 13 2.091 2.096 042 11 3.82 3.83 121 13 2.085 2.089 420 24 3.50 3.51 202 15 2.063 2.067 233 11 3.44 3.45 022 28 2.042 2.046 142 26 3.27 3.28, 3.27 220,212 9 2.019 2.017, 2.017 332,241 100 3.231 3.240 122 12 1.939 1.942 422 14 3.091 3.095 113 20 1.911 1.914, 1.910 242,413 42 2.846 2.853 311 12 1.871 1.875 125 35 2.789 2.794, 2.784 131,023 12 1.829 1.834,1.832 234,431 16 2.685 2.688, 2.692 213,302 20 1.770 1.773, 1.772 243,225 17 2.665 2.67 123 11 1.748 1.753 404 26 2.624 2.634 004 12 1.723 1.722 414 26 2.576 2.583 312 14 1.713 1.717 315 23 2.554 2.556 230 20 1.696 1.697 144 12 2.531 2.536 104 14 1.647 1.644 433 25 2.504 2.509 321 21 1.634 1.633 325 11 2.478 2.484 231 12 1.613 1.620 244 11 2.436 2.444 114 20 1.559 1.564 442 42 2.391 2.395 223 13 1.522 1.520 434 12 2.296 2.297 204 13 1.481 1.484, 1.484 425,443 13 2.276 2.282, 2.276 024,410 13 1.467 1.471 245 0.010, and Si as internal standard. Note: scan rate = 0.6 °/min, step size = ------------ ---~---- 1084 DAI ET AL.: POLDERVAARTITE, Ca(Cao.sMno.s)(Si030H)(OH) TABLE3. Experimental details for poldervaartite TABLE5. Selected bond lengths (A) and O-Si-O angles (0)of poldervaartite 28 limit: 1-60 Scan type: 8/28 M1a*-01a 2.506(2) M2a-01c 2.807(2) 2.00 + 0.70 tan 8 Scan width (200): -02a 2.374(2) -01a 2.881(2) Scan duration: :5180 s -03a 2.333(2) -02c 2.340(2) Intensity standards: 3 per 5 h -04a 2.336(2) -03f 2.204(2) Total reflections: 3052 -05a 2.330(2) -04d 2.265(2) Unique reflections: 1330 -02b 2.447(2) -04a 2.275(2) Rmerge 0.02 -05e 2.259(2) 3UF) No.