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Olmiite and poldervaartite from the Kalahari manganese field northern cape province republic of South Africa.

Poldervaartite was described in 1993, based on attractive specimens collected at the Wessels mine in South Africa. Around 2001 a bonanza of fine specimens presumed also to be poldervaartite was discovered at the nearby N'Chwaning II mine. Subsequent investigations have shown the N'Chwaning II specimens to be a new species, olmiite, the Mn analog of poldervaartite. The two species cannot be distinguished visually.

INTRODUCTION

Poldervaartite, a calcium-manganese silicate, was described in 1993 by Dai, Harlow and McGhie as a new species from the Wessels mine, Kalahari Manganese Field, Republic of South Africa. It occurred sparingly in fine crystals and crystal aggregates, and, although several small finds are known, it can be considered a rather rare mineral, much in demand by collectors. Thus the mineral community was elated when, about eight years later, many fine specimens of a poldervaartite-like mineral in cream to pink to red crystals from another locality in the Kalahari Manganese Field, the N'Chwaning II mine, appeared on the market.

The external appearance of the N'Chwaning II mine mineral and that of the Wessels mine poldervaartite are similar but not identical; furthermore, the former mineral shows a wide variety of colors and habits of crystal aggregates. Surprisingly, no specific investigation seems to have been carried out until one of the authors (RP) acquired and examined a number of specimens from both localities and submitted his material to Paola Bonazzi and her colleagues at the Earth Science Department of the University of Florence, Italy. The work carried out there led to the approval of a new species, olmiite, the Mn-dominant analog of poldervaartite.

Some questions of interest regarding the specimens from the new find remained open: Are they all to be classified as olmiite? Do olmiite and poldervaartite occur together? Can olmiite be distinguished visually from poldervaartite? To answer these questions, an experimental investigation was carried out by the present authors.

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POLDERVAARTITE--WESSELS MINE

The Kalahari Manganese Field localities came to the attention of the curatorial and collector communities in the 1970s. Their fame was mainly due to the spectacular Hotazel rhodocrosite crystal groups, but many other mineral species were also reported (Wilson and Dunn, 1978; Cairncross et al., 1997, Von Bezing et al., 1991; Cairncross and Dixon, 1995). Some of these works include excellent photographs of the most important minerals from the area and also describe the geography and the geology of the Kalahari Manganese Field deposits, which, aside from their scientific and collecting appeal, are of great industrial importance.

Poldervaartite from the Wessels mine was described by Dai et al. (1993), who reported the general formula:

(C[a.sub.2-x]M[n.sub.x])Si[O.sub.4] [H.sub.2]O, with x = 0.28-0.52.

The formula adopted by the International Mineralogical Association's Committee on New Minerals and Mineral Names, was:

Ca(Ca,Mn)(Si[O.sub.3]OH)(OH).

The poldervaartite structure, discussed in detail by Dai et al. (1993), includes two sites, M1 and M2, hosting divalent ions: Ca resides in M1, while M2 hosts dominant Ca and subsidiary Mn. The type material came from a pocket in massive braunite and hausmannite and was associated with henritermierite, calcite, bultfonteinite and hematite. This association suggests that the minerals were deposited by hydrothermal fluids rich in Ca, [SiO.sub.2] and C[O.sub.2] circulating in the Mn-rich host rock. The original poldervaartite crystals were described by Dai et al. (1993) as having a wheat-sheaf to euhedral habit, with colorless, transparent cores and milky white rims. The electron probe microanalyses did not show any compositional variation related to this zoning.

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At least five (limited) finds of this mineral in fine pink to peach-colored crystal aggregates and sprays of small amber-colored prismatic crystals have been reported (Cairncross et al., 1997 and 2002). Some of these finds occurred before poldervaartite was studied and named, but all available specimens soon disappeared from the collector market. Thus the poldervaartite from the Wessels mine can be considered uncommon, if not rare.

"POLDERVAARTITE"--N'CHWANING II MINE

In the last months of 2001, very attractive new specimens from another location in the Kalahari Manganese Field, the N'Chwaning II mine, became available. Some of these specimens show whitish to cream to flesh-pink crystals. The broken sections are opaque and pink, with a sheaf-like structure. In other specimens the mineral forms cream to pink crystals on matrix, often as cauliform crystal aggregates up to 3 cm in diameter, or with cream to orange-colored crystals, commonly in globular or wheat-sheaf aggregates and as crystals on matrix. In some cases both the individual crystals and the crystal aggregates are glassy, pink to raspberry-red in color, shiny and sparkling, and form very showy and desirable matrix specimens. Frequently the prismatic crystals are nice and lustrous on the outside but, when broken, they show a very porous, corroded internal structure.

The various habits of this mineral are illustrated and described in detail by Cairncross et al. (2002). The N'Chwaning II material is similar to the Wessels mine poldervaartite in crystal shape, color and general external characteristics, and they both fluoresce a deep red color under shortwave ultraviolet light. Thousands of these specimens from the N'Chwaning II mine were collected and sold as poldervaartite.

OLMIITE--A NEW SPECIES

The purpose of the investigation carried out at the Earth Science Department of the University of Florence, Italy was to determine if the mineral from the N'Chwaning II mine is indeed poldervaartite. The results of this investigation are reported in detail by Bonazzi et al. (2007).

In summary, it was found that in the three N'Chwaning II mine samples examined, Mn largely prevails over Ca in the M2 site. The chemical analyses yielded the formula:

([Ca.sub.2-x][Mn.sub.x][Fe.sub.y]) [[SiO.sub.3](OH)] (OH)

with x = 0.84-0.86 and y = 0.01,

ideally:

CaMn[[SiO.sub.3](OH)] (OH).

This allowed the authors to define a new species, the Mn-dominant analog of poldervaartite, duly submitted to the International Mineralogical Association and approved with the name of olmiite. The name honors Filippo Olmi (1959-2005), in recognition of his mineralogical work at the CNR-Istituto di Geoscienze e Georisorse of Florence, Italy.

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Olmiite is orthorhombic, space group Pbca, and is isostructural with poldervaartite, with Mn prevailing over Ca in the M2 site, causing a significant difference in the unit cell parameters and cell volume. The optical properties are also a little different.

OLMIITE--AN EXPERIMENTAL REVIEW

The crystal habit and most other physical properties of the two minerals (Table 1) are very similar. Therefore the original description is not of much use in distinguishing olmiite from poldervaartite, nor does it indicate whether the two minerals coexist at the same locality or in the same specimen. To distinguish the two species from each other it is necessary to perform accurate analyses, either chemical (to determine the Mn: Ca ratio), or by X-ray diffraction (to measure the unit cell parameters)--not an easy task for the many collectors who own specimens of the N'Chwaning II material. Consequently, further work was carried out to address the questions still open on the poldervaartite vs. olmiite problem.

After a preliminary examination of the material available, consisting of about 25 specimens from the N'Chwaning II mine, the following samples, representing the various habits of crystals and crystal aggregates, colors etc. were selected, embedded in resin and polished:
no. 8280b     Red crystal, glassy rim, pink core

no. 8281b     Red crystal, glassy rim, pink core (cross-section)

no. 10278b    Pink-grayish crystal with oyelite (cross section)

no. 8787c     Pale brown crystal; porous, corroded-looking core

no. 10280b    Red crystal aggregate, glassy rim, pink radiating
              core structure

no. 10279     Pink cauliform crystal aggregate, radiating structure


A first set of data were obtained at the Natural History Museum of Milan (Italy) using their scanning electron microscope JEOL 5610LV equipped with a Gresham EDXS microprobe. Backscatter images were obtained and semi-quantitative analyses were conducted, showing significant contents of Mn which indicated that most or all the specimens examined were olmiite.

Quantitative analyses were then performed on the same samples at the Department of Geosciences, University of Padua (Italy), using a WDS electron microprobe CAMECA SX 50 with a detection limit of 0.02 wt%, 20 kV accelerating voltage, 20nA sample current and 1-2[micro]m beam diameter. The results of the analyses (average of three spots per area of the specimen) are reported in Table 2.

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Some of the material made available for the original work by Bonazzi et al. (2007) was also re-examined (L. Bindi, personal communication). The results of the microprobe and single crystal X-ray diffraction analyses performed with the same equipment and methods described by Bonazzi et al. (2007) in the original work indicated that all of the specimens examined are olmiite.

The first conclusion is that all of the analyses show a marked Mn dominance in the M2 site, even higher than that reported in Bonazzi et al. (2007): several of the specimens analyzed are very close to being end-member olmiite. Since the specimens are representative of the different colors and crystal aggregation habits of the material from the N'Chwaning II mine, it can safely be concluded that they should all be called olmiite.

The pink-reddish crystals similar to those of specimens 8280 and 8281 show a deeper pink inner core, and a glassy, almost colorless outer layer. Therefore both the central areas and areas near the external rim of some crystals were examined. Many spots of specimen 8280b were analyzed, traversing along a straight line from the rim to the core. The results show that the sample is all olmiite, with Mn content decreasing, and Ca increasing, from rim to core.

An X-ray powder diffraction analysis was carried out on specimen 8280b using a Phillips X-Pert Pro powder diffractometer. The results confirmed the presence of prevalent olmiite with minor hematite, most likely finely dispersed as inclusions in the olmiite.

The refined cell parameters of olmiite obtained by the Rietveld method were found to be:

a = 9.2672(2) [Angstrom]

b = 9.0640(2) [Angstrom]

c = 10.4042(2) [Angstrom]

V = 873.934(21) [[Angstrom].sup.3]

All are in good agreement with values reported by Bonazzi et al. (2007) for olmiite.

The present investigation addressed mainly the N'Chwaning II material, but two samples from the type locality, the Wessels mine, were also considered. A first specimen, similar to that shown by Cairncross (1997), page 121 (Dixon collection) has sharp, transparent pink crystals associated with yellow sturmanite and acicular celestine. A crystal taken from that specimen was analyzed at the University of Florence (L. Bindi, personal communication) by single crystal X-ray diffraction. The refined unit cell parameters were found to be:

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a = 9.396(2) [Angstrom]

b = 9.139(1) [Angstrom]

c = 10.378(3) [Angstrom]

V= 891.1(4) [[Angstrom].sup.3]

All are in good agreement with the parameters published by Dai et al. (1993) for poldervaartite.

A second specimen, very similar to that in the Desmond Sacco collection, shown in Cairncross et al. (2000), p. 338, and Cairncross (1997), p. 121, was analyzed by electron microprobe. The chemical analysis results (Table 3) show that the CaO content ranges between 40.54 and 41.26wt%, much higher than in any of the N'Chwaning II specimens. The Mn atoms per formula unit are slightly less than the Ca apfu at some points and slightly above at others. Thus this specimen contains both poldervaartite and olmiite.

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CONCLUSIONS

All the specimens of "poldervaartite" collected at the N'Chwaning II mine in 2001 and later can reasonably be attributed to olmiite, as no poldervaartite was detected in samples showing different habits, crystal aggregations and colors. The Wessels mine specimens most likely contain real poldervaartite and, in some cases, also olmiite.

Thus poldervaartite remains a rare mineral, while olmiite is a new species that, unlike many new minerals approved in recent years, is well crystallized, fairly abundant and forms very attractive specimens, from micromount size to cabinet specimens--a real blessing for all mineral collectors!
Table 1. Comparison of poldervaartite and olmiite.

                                                      Olmiite
                Poldervaartite from                 from Bonazzi
                 Dai et al. (1993)                  et al. (2007)

Author's       ([Ca.sub.2-x][Mn.sub.x])        ([Ca.sub.2-x][Mn.sub.x]
               Si[O.sub.4][dot]                [Fe.sub.y])[Si[O.sub.3]
               [H.sub.2], O, with              (OH)](OH) with
               x = 0.28-0.52                    x = 0.84-0.86, y = 0.01

Author's       Ca([Ca.sub.0.5][Mn.sub.0.5])    CaMn[Si[O.sub.3](OH)]
proposed       (Si[O.sub.3]OH)(OH),            (OH)
ideal
formula

IMA accepted   Ca(Ca, Mn)(Si[O.sub.3]OH)(OH)   CaMn[Si[O.sub.3](OH)]
formula                                        (OH)

Density,       2.91                            3.05
g/c[m.sup.3]

Hardness,      5                               5-5 1/2
Mohs

Cleavage       None                            None

Habit          Wheat-sheaf to euhedral         Wheat-sheaf to euhedral
               prismatic                       prismatic

Color          Colorless to milky white        Pale to reddish pink

Fluorescence   Deep red                        Deep red
(SW UV light)

Tenacity       Very brittle                    Very brittle

Symmetry       Orthorhombic                    Orthorhombic

Space group    Pbca                            Pbca

Cell           a 9.398 (1) [Angstrom]          9.249 (3) [Angstrom]
parameters

               b 9.139 (2) [Angstrom]          9.076 (9) [Angstrom]

               c 10.535 (2) [Angstrom]         10.342 (9) [Angstrom]

               V 904.8 (3) [[Angstrom].sup.3]  868(1)
                                               [[Angstrom].sup.3]

Table 2. Analyses of typical N'Chwaning II mine olmiite specimens.

                   8280b         8281b         10278b        8787c

                 R      C      R      C       R      C      R      C

[SiO.sub.2]    28.10  29.03  28.91  28.84   29.12  29.38  29.17  29.37
CaO            30.70  32.77  31.45  35.22   27.59  27.64  26.93  27.14
MnO            29.16  27.07  28.86  24.07   32.91  32.56  34.06  33.42
Total          87.97  88.87  89.22  88.13   89.62  89.58  90.16  89.93
Si              0.99   1.00   1.00   1.00    1.01   1.01   1.01   1.01
Ca              1.16   1.21   1.16   1.30    1.02   1.02   0.99   1.00
Mn              0.87   0.79   0.84   0.70    0.96   0.95   0.99   0.97

                    10280                    10279

               R      I      C      R      I      r      C

[SiO.sub.2]  29.62  29.01  29.46  29.25  29.10  29.17  29.31
CaO          28.65  29.04  27.96  27.78  27.61  27.31  28.12
MnO          31.67  31.25  32.16  33.55  32.62  34.16  32.76
Total        89.94  89.30  89.58  90.58  89.33  90.64  90.19
Si            1.01   1.00   1.01   1.00   1.01   1.00   1.01
Ca            1.05   1.08   1.03   1.02   1.03   1.00   1.03
Mn            0.92   0.92   0.94   0.97   0.96   0.99   0.95

Notes--Data are average of three spot analyses. R = crystal rim area.
C = crystal core area. FeO < 0.02%, instrument detection limit.

Table 3. Analyses of Wessels mine specimen no. 6810, poldervaartite-
olmiite.

               Point 1   Point 2

[SiO.sub.2]     29.50     29.03
CaO             41.26     40.54
MnO             17.02     18.05
Total           87.78     87.62
Si apfu          1.00      0.99
Ca apfu          1.51      1.49
Mn               0.49      0.52


ACKNOWLEDGEMENTS

The authors wish to thank Paola Bonazzi, Earth Science Department, and Luca Bindi, Natural History Museum, Mineralogy Section, University of Florence, for making available some unpublished data and for offering constructive suggestions. Thanks also to Federico Zorzi and Raoul Carampin, Geoscience Department. University of Padua, for their cooperation in performing X-ray diffraction and electron microprobe analyses.

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REFERENCES

BONAZZI, P., BINDI, L., MEDENBACH, O., PAGANO, R., LAMPRONTI, G. I, and MENCHETTI, S. (2007) Olmiite, CaMn[SiO3(OH)](OH), the Mn-dominant analogue of poldervaartite, a new mineral species from Kalahari manganese fields (Republic of South Africa). Mineralogical Magazine, 71(2), 229-238.

CAIRNCROSS, B., and DIXON, R. (1995) Minerals of South Africa. Geological Survey of South Africa, Linden 2104, Republic of South Africa. 296 p.

CAIRNCROSS, B., BEUKES, N., and GUTZMER, J. (1997) The Manganese Adventure: The South African Manganese fields. Associated Ore and Metal Cooperation Limited, Marshalltown, Johannesburg 2107, Republic of South Africa, 236 p.

CAIRNCROSS, B. (2000). The Desmond Sacco collection. Focus on Southern Africa. Published by Desmond Sacco, PO Box 61042, Marshalltown, Johannesburg 2107, Republic of South Africa. 408 p.

CAIRNCROSS, B., GUTZMER, J., and PRETORIUS, A. (2002) Spektakularer Neufund: Poldervaartit aus der N'Chwaning II--Mine, Kalahari, Sudafrika. Lapis, 27 (5), 30-34.

DAI, Y., HARLOW, G. E., and McGHIE, A. R. (1993) Poldervaartite, Ca(C[a.sub.0.5]M[n.sub.0.5])[(Si[O.sub.3]OH).sub.3], a new acid nesosilicate from the Kalahari manganese field, South Africa: Crystal structure and description. American Mineralogist, 78,1082-1087.

VON BEZING, K. L., DIXON, R. D., POHL, D., and CAVALLO, G. (1991) The Kalahari manganese field: an update. Mineralogical Record, 22, 279-297.

WILSON, W. E., and DUNN, P. J. (1978) Famous mineral localities: The Kalahari manganese field. Mineralogical Record, 9, 137-153.

Renato Pagano

P.O. Box 37, I-20092 Cinisello, Italy

email: renpagan@tin.it

Alessandro Guastoni

Dipartimento di Geoscienze, Universita degli Studi di Padova

Corso Garibaldi, 37, I-35137 Padova, Italy

Federico Pezzotta

Museo Civico di Storia Naturale, Corso Venezia, 55

I-20121 Milano, Italy
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