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Guidottiite, the Mn-Analogue of Cronstedtite: A New Serpentine-Group Mineral from South Africa

Published online by Cambridge University Press:  01 January 2024

Michael W. Wahle ,
Thomas J. Bujnowski ,
Stephen Guggenheim  and
Toshihiro Kogure
Michael W. Wahle
Affiliation:
Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607 USA
Thomas J. Bujnowski
Affiliation:
Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607 USA
Stephen Guggenheim*
Affiliation:
Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607 USA
Toshihiro Kogure
Affiliation:
Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033, Tokyo, Japan
*
* E-mail address of corresponding author: [email protected]
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Abstract

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The objective of this paper is to describe the new serpentine group mineral, guidottiite, which is analogous to cronstedtite. Guidottiite has an ideal chemical composition of (Mn2Fe3+)(SiFe3+)O5(OH)4. The sample is from the N’chwaning 2 mine, Kalahari manganese field, Republic of South Africa, and apparently forms from hydrothermal solutions. Grains are optically near opaque [average index of refraction 1.765, with variable extinction on the (001)], vitreous, and black, with perfect {001} platy cleavage. A non-separable fibrous substructure exists perpendicular to cleavage that results in a silky luster under optical examination. The average chemical analysis determined from electron microprobe based on four grains with ten analyses each resulted in a structural formula of (Mn1.86Fe0.613+Mg0.54)Σ=3.01(Si1.36Fe0.643+)Σ=2.00O5(OH)4, with calculated density of 3.236 g/cm3. Analysis from another area of the sample showed a slightly different chemical composition and resulted in a formula of (Mn1.70Fe0.963+Mg0.24Σ=2.89(Si1.26Fe0.743+)Σ=2.00O5(OH)4, with calculated density of 3.291 g/cm3. The measured density on a bulk sample (with impurities) was 3.33 g/cm3. Thermal analysis suggested a dehydroxylation temperature of 535°C, a decomposition/recrystallization temperature of 722°C, and weight loss (= H2O loss) of 9.4%. The derived Mohs hardness from nano-indentation is H = 4.25.

The sample is mostly the 2H1 polytype with minor amounts of the 2H2 polytype. Using a predominantly 2H2 crystal, which has better crystallinity, the strongest observed X-ray peaks are: 7.21 Å (Io/Io = 100%), 3.543 (50), 2.568 (39), 1.982 (26), and 2.381 (25). All Gandolfi simulations, even with three crystal remountings, showed preferred orientation effects. Transmission electron microscope (TEM) analysis showed stacking disorder within Group D serpentine polytypes. Thus, a regular alternation of the occupancy of octahedral sets within each layer along the stacking exists, but disorder of the layer displacements of 0 and ±b/3 (b defined here as the orthohexagonal cell) exists. Ordered 2H1 (no layer displacement) and 2H2 (alternating + and —b/3 displacement) domains were also frequently observed. X-ray diffraction analysis showed that even apparent single crystals contain impurity phases, presumably Mn-rich and Ca phases that were detected in the microprobe study. The single-crystal structure refinement used a well (stacking) ordered apparent 2H2 crystal with little to no streaking in the diffraction pattern. Results showed that the crystal has a random interstratification of 2H2 and 2H1. The 2H2 polytype is hexagonal, space group P63, with a = 5.5472(3), c = 14.293(2) Å, and Z = 2, and was refined to R1 = 0.072 and wR = 0.108 from 656 unique reflections. Because the two polytypes in the composite have only small differences in the lower 1:1 layer, a large displacement parameter for the basal oxygen atom results, which was constrained to B = 1.5 Å2 (Ueq = 0.0190) in the refinement. Half of the tetrahedral sites in the 2H1 upper layer superpose over half of the tetrahedral sites in the 2H2 upper layer (T1 sites only) per unit cell. This superposition produces an apparent excess of electron densities of the T1 site relative to the T2 site (T1 = 21.9 electrons, T2 = 15.8). Comparison with the microprobe data indicates that observed tetrahedral bond lengths are generally not affected by this intergrowth. Tetrahedral bond lengths indicated that the tetrahedral sites contain T1 = Si0.678 Fe0.3223+ and T2 = Si0.631Fe0.3693+. This excess of electron densities and other refinement problems associated with the guidottiite single-crystal refinement closely parallel all single-crystal cronstedtite-2H2 refinements to date, suggesting that these refinements also involve random interstratifications of 2H2 and 2H1 polytypes.

Keywords

CompositionCronstedtiteCrystal StructureDensityGuidottiitePhysical PropertiesSerpentineThermal AnalysisTransmission Electron MicroscopyX-ray Diffraction
Type
Article
Information
Clays and Clay Minerals , Volume 58 , Issue 3 , June 2010 , pp. 364 - 376
Copyright
Copyright © The Clay Minerals Society 2010

References

Bailey, S.W., 1969 Polytypism of trioctahedral 1:1 layer silicates Clays and Clay Minerals 17 355–371.CrossRefGoogle Scholar
Bailey, S.W., 1988 X-ray diffraction identification of the polytypes of mica, serpentine, and chlorite Clays and Clay Minerals 36 193–213.CrossRefGoogle Scholar
Bloss, F.D., 1971 Crystallography and Crystal Chemistry, An Introduction New York Holt, Rinehart and Winston, Inc..Google Scholar
Broz, M.E. Cook, R.F. and Whitney, D.L., 2006 Microhardness, toughness, and modulus of Mohs scale minerals American Mineralogist 91 135–142.CrossRefGoogle Scholar
Bruker AXS, Inc., 2001 TWINABS USA Madison, Wisconsin.Google Scholar
Dornberger-Schiff, K. and Ďurovič, S., 1975 OD-interpretation of kaolinite-type structures — II. The regular polytypes (MDO-polytypes) and their derivation Clays and Clay Minerals 23 231–246.CrossRefGoogle Scholar
Ďurovič, S. Hybler, J. and Kogure, T., 2004 Parallel intergrowths in cronstedtite-1T: Implications for structure refinement Clays and Clay Minerals 52 613–621.CrossRefGoogle Scholar
Frenz, B.A., 1997 SDP for Windows USA College Station, Texas.Google Scholar
Geiger, C.A. Henry, D.L. Bailey, S.W. and Maj, J.J., 1983 Crystal structure of cronstedtite-2H 2 Clays and Clay Minerals 31 97–108.CrossRefGoogle Scholar
Guggenheim, S., 2005 Simulations of Debye-Scherrer and Gandolfi patterns using a Bruker SMART/APEX Diffractometer system Bruker-AXS Application Note 373 USA Madison, Wisconsin.Google Scholar
Guggenheim, S. and van Koster Groos, A.F., 1992 High-pressure differential thermal analysis (HP-DTA): I. Dehydration reactions at elevated pressures in phyllosilicates Journal of Thermal Analysis 38 1701–1728.CrossRefGoogle Scholar
Hybler, J. Petříček, V. Ďurovič, S. and Smrcok, L., 2000 Refinement of the crystal structure of cronstedtite-1T Clays and Clay Minerals 48 331–338.CrossRefGoogle Scholar
Hybler, J. Petříček, V. Fabry, J. and Ďurovič, S., 2002 Refinement of the crystal structure of cronstedtite-2H 2 Clays and Clay Minerals 50 601–613.CrossRefGoogle Scholar
Kilaas, R., 1998 Optimal and near-optimal filters in highresolution electron microscopy Journal of Microscopy 190 45–51.CrossRefGoogle Scholar
Kogure, T. (2002) Investigation of micas using advanced TEM. Pp. 281–310 in: Micas: Crystal Chemistry & Metamorphic Petrology (Mottana, A., Sassi, F.P., Thompson, J.B. Jr. and Guggenheim, S., editors). Reviews in Mineralogy and Geochemistry vol. 46, Mineralogical Society of America, Washington, D.C.CrossRefGoogle Scholar
Kogure, T. Hybler, J. and Ďurovič, S., 2001 AHRTEM study of cronstedtite: Determination of polytypes and layer polarity in trioctahedral 1:1 phyllosilicates Clays and Clay Minerals 49 310–317.CrossRefGoogle Scholar
Kogure, T. Hybler, J. and Yoshida, H., 2002 Coexistence of two polytypic groups in cronstedtite from Lostwithiel, England Clays and Clay Minerals 50 504–513.CrossRefGoogle Scholar
Kogure, T. Eilers, P.H.C. and Ishizuka, K., 2008 Application of optimum HRTEM noise filters in mineralogy and related sciences Microscopy and Analysis 22 S11–S14.Google Scholar
van Koster Groos, A.F., 1979 Differential thermal analysis of the system NaF-Na2CO3 to 10 kbar Journal of Physical Chemistry 83 2976–2978.CrossRefGoogle Scholar
Marks, L.D., 1996 Wiener-filter enhancement of noisy HREM images Ultramicroscopy 62 43–52.CrossRefGoogle ScholarPubMed
Massa, W., 2004 Crystal Structure Determination Berlin Springer-Verlag Press.CrossRefGoogle Scholar
MDI, Materials Data, Inc., 2006 JADE USA Livermore, California.Google Scholar
Schissel, D. and Aro, P., 1992 The Major Early Proterozoic Sedimentary Iron and Manganese Deposits and their Tectonic Setting Economic Geology and the Bulletin of the Society of Economic Geologists 87 1367–1374.CrossRefGoogle Scholar
Shannon, R.D., 1976 Revised effective ionic-radii and systematic studies of interatomic distances in halides and chalcogenides Acta Crystallographica Section A 32 751–767.CrossRefGoogle Scholar
Sheldrick, G.M., 1997 SHELXTL97 version 5.1: Program for the Solution and Refinement of Crystal Structures Germany University of Gottingen.Google Scholar
Smrčok, L. Ďurovič, S. Petříček, V. and Weiss, Z., 1994 Refinement of the crystal structure of cronstedtite-3T Clays and Clay Minerals 42 544–551.CrossRefGoogle Scholar
Zvyagin, B.B., 1962 Polymorphism of double-layer minerals of the kaolinite type Soviet Physics, Crystallography 7 38–51.Google Scholar