New Occurrences from the Postmasburg Manganese Field, Northern Cape Province, South Africa

981

The Canadian Mineralogist Vol. 53, pp. 981-990 (2015) DOI: 10.3749/canmin.1500063

TOKYOITE, As-RICH TOKYOITE, AND NOÉLBENSONITE: NEW OCCURRENCES FROM THE POSTMASBURG MANGANESE FIELD, NORTHERN CAPE PROVINCE, SOUTH AFRICA

GELU COSTIN Department of Geology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa Department of Earth Science, MS-126 Rice University, 6100 Main Street, Houston, Texas 77005, USA

§ BRENTON FAIREY AND HARILAOS TSIKOS Department of Geology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa

ARNOLD GUCSIK Department of Geology, University of Johannesburg, Johannesburg, South Africa

ABSTRACT

Samples of manganese-rich rock containing two compositional varieties of tokyoite in association with noélbensonite were retrieved from a drill core obtained from the Postmasburg area in the Northern Cape Province of South Africa. The samples consist of fine-grained braunite, hematite, and hausmannite. Within this material abundant vugs are observed that are filled with witherite, baryte, and barytocalcite. In addition, As-rich tokyoite, tokyoite, and noélbensonite occur in the center of the vugs, in fine aggregates 0.1 to 1 mm in size. Individual As-rich tokyoite grains are typically 20–200 mm in size. The outer walls of the vugs are lined with microcrystalline K-feldspar, witherite, and/or sérandite. Textural evidence of the Ba-rich mineral phases in association with As-rich tokyoite suggest an epigenetic mode of formation of the observed assemblages, caused by V- and As-bearing alkali-rich fluids interacting with pre-existing Mn-rich minerals. Electron microprobe analysis (EMPA), electron backscatter diffraction analysis (EBSD), and Raman spectroscopy show that the As-rich tokyoite is a mineral belonging to the brackebuschite mineral group. Its measured mineral composition corresponds 3+ 3+ to the formula (Ba1.92Sr0.05Pb0.03)S2.00(Mn 0.98Fe 0.02)S1.00[(As1.050V0.950)S2.00O8)](OH). Electron backscatter diffraction analysis results point to a monoclinic, P21/m space group, with cell parameters a 9.121 Å, b 6.142 Å, c 7.838 Å, a = g = 90°, b = 112°, Z = 2. The structure is therefore similar to gamagarite. Raman spectra of As-rich tokyoite were compared to spectra of arsenbrackebuschite and arsentsumebite, and those of noélbensonite to spectra of hennomartinite and lawsonite. The indexing of Raman peaks in As-rich tokyoite, by similarity with arsentsumebite, suggests a possible ordering of the AsO4 and VO4 tetrahedra. This observation, correlated with mineral chemistry and specifically an As:V ratio of B1:1, suggest that the As-rich tokyoite may in fact represent a possible new mineral with ordering of As and V at tetrahedral positions, or at least an unknown As analogue of tokyoite.

Keywords: tokyoite, As-rich tokyoite, noélbensonite, Raman, EMPA, EBSD, South Africa.

INTRODUCTION many such minerals (Cairncross & Beukes 2013). The diversity of minerals found in the KMF is also The Northern Cape Province of South Africa is highlighted by Gutzmer & Beukes (1996a), who list endowed with a large and diverse set of rare minerals no less than 135 different species, including such rare associated with the commercial iron and manganese minerals as banalsite, kentrolite, kornite, sérandite, deposits that characterize the region. The Kalahari armbrusterite, norrishite, and sugilite. Further south, in manganese field (KMF; Figure 1), and particularly the the broad area between the towns of Kathu and northernmost area thereof containing high-grade man- Postmasburg, lies the Postmasburg manganese field ganese ores of the Hotazel Formation (Wessels and (PMF; Fig. 1), where a similarly impressive variety N’chwaning mines; Astrup & Tsikos 1998), is host to of minerals is found in association with karst-hosted y Corresponding author e-mail address: [email protected] 982 THE CANADIAN MINERALOGIST manganese deposits developed on dolomitic carbonate (Moore et al. 2011). The PMF occurs in two belts along a rocks of the Campbellrand Subgroup. These deposits 65 km long, broadly N–S transect across the Maremane have also been described by various authors (e.g.,Hall Dome (De Villiers 1960, Gutzmer & Beukes 1996b, 1926, Nel 1929, du Toit 1933, de Villiers 1944, 1960, Astrup & Tsikos 1998), referred to as the Western and Plehwe-Leisen & Klemm 1995, Gutzmer & Beukes Eastern Belts. The ferruginous-type ores of the Western 1996b), particularly with respect to their complex and Belt occur within paleo-sinkholes in the chert-free Reivilo diverse mineralogy. In one of the most recent such studies, Formation of the Campbellrand Subgroup (Plehwe-Leisen Moore et al. (2011) described a complex assemblage of & Klemm 1995, Gutzmer & Beukes 1996b). The fact that minerals near the Bruce iron-ore mine in the northern part the manganese ores rest directly on the Reivilo Formation of the PMF. Here sugilite, armbrusterite, albite, sérandite, and contain little to no chert is testament to the chert-free witherite, and norrishite occur in the footwall to con- nature of the substrate (Gutzmer & Beukes 1996b). glomeratic iron-ores that typify the locality. Mineralogically, the Eastern Belt ores range from De Villiers (1943) discovered in the PMF the rare fine-grained bixbyite- and braunite-rich to massive and 3+ barium–manganese vanadate gamagarite, Ba2(Fe , highly vuggy bixbyite-rich ores. The siliceous ores of 3+ Mn )(VO4)2(OH), which forms part of the brack- the Eastern Belt are intimately associated with the ebuschite structural group of minerals (Matsubara et al. Wolhaarkop chert breccia and the Fairfield Formation of 3+ 2004). Tokyoite, Ba2Mn (VO4)2(OH), was first the Campbellrand Subgroup (Plehwe-Leisen & Klemm describedbyMatsubaraet al. (2004) from the (now 1995, Gutzmer & Beukes 1996b). The matrix of the abandoned) Shiromaru manganese mine in Japan. Its breccia is composed of quartz, hematite, and braunite, structure is similar to gamagarite, which belongs to the becoming more siliceous and ferruginous toward the top brackebuschite group. These authors found tokyoite in and more manganiferous toward the base. association with braunite, hematite, albite, aegirine, Gutzmer & Beukes (1996b) describe the transition celsian, hyalophane, and rare minerals such as cymrite zones between chert breccia and manganese ore bodies and tamaite. They also described veinlets of banalsite, of the Eastern Belt as gradual and characterized by a baryte, strontianite, sérandite, piemontite-(Sr), marstur- rapid increase in the manganiferous matrix to chert ite, and ganophyllite-group minerals. The tokyoite at the fragment ratio. Cross-cutting veins of coarse baryte and type locality occurs as very fine aggregates, and is globular hematite, as well as pyrolusite, have been thought to have formed as a result of alkali-rich, Ba- and described, as well as replacement of chert fragments V-bearing circulating fluids reacting with pre-existing by baryte (Gutzmer & Beukes 1996b). The mineable assemblages containing braunite (Matsubara et al. 2004). manganese ores predominantly consist of massive This paper presents the first occurrence of tokyoite braunite and tend to be found at the contact between and noélbensonite on the African continent and the first the chert breccia and the underlying dolomite (Gutzmer occurrence ever of As-rich tokyoite. In order to define & Beukes 1996b). Whereas mixed ore-types to the the structural properties of tokyoite, its As-rich variety, aforementioned two endmembers are also reported, and noélbensonite, and their compositions, we used a they are generally considered akin to those of the combination of micro-Raman spectroscopy, electron Eastern Belt (Plehwe-Leisen & Klemm 1995). microprobe microanalyses (EMPA), and electron back- scatter diffraction (EBSD). The measured As content in SAMPLE SELECTION As-rich tokyoite indicates a possible new mineral, with V and As ordered at different sites. Samples with As-rich tokyoite were retrieved from a drill core provided by Kumba Iron Ore (a member of GEOLOGICAL SETTING the Anglo American Group) and obtained from the broader Kolomela area near the town of Postmasburg in Manganese ore deposits of the PMF were first the Northern Cape Province of South Africa. The discovered near the town of Postmasburg in the early part general drill core location is indicated in Figure 1. The of the 20th century and soon became South Africa’smost drill core intersects a lithological interval of laminated, important commercial resource of manganese (Gutzmer & massive, and vuggy manganese-rich lithologies at the Beukes 1996b). Some of the historically major manganese interface between minor, laminated hematite-rich mines in the area are those at Lohatlha, Glossam, and mineralization, and the Wolhaarkop Breccia stratigra- Bishop (Astrup & Tsikos 1998). The discovery and phically below (Fig. 1). The stratigraphy intersected subsequent exploitation of the giant KMF saw the gradual in the studied drill-core therefore bears similarities to decline of mining in the PMF, which today occurs only the one from the Bruce iron-ore mine studied for on a relatively small-scale (Gutzmer & Beukes 1996b). its intricate mineralogy by Moore et al. (2011). The A regional anticlinal structure known as the Maremane greatest modal abundance of As-rich tokyoite and Dome occurs in Northern Cape Province between the noélbensonite occurs towards the basal, vuggy ferro- towns of Postmasburg to the south and Kathu to the north manganiferous part of the examined intersection, (Fig. 1). The Maremane Dome consists, for most part, of between 95 and 97 m depth below the present surface dolomitic carbonate rocks of the Campbellrand Subgroup (Fig. 1). TOKYOITE, As-RICH TOKYOITE, AND NOÉLBENSONITE 983

o 20o 30 BOTSWANA Legend: 25o Laminated ferromanganese rock and brick-red shale

Transvaal SPG Silicious shale

30o Microbreccia/conglomerate R.S.A

Cape Town 0400 Massive ferromanganiferous rock Kilometres Vuggy manganese-rich rock

Vuggy ferromanganiferous rock G Wolhaarkop chert breccia V K KATHU 79 m V G 80 m

Approx. 81 m drill core location SISHEN 82 m MINE Fe K Dip 83 m

Thrust fault 84 m

V G 85 m Gamagara-Mapedi 86 m Unconformity 87 m Voëlwater LOHATHLA Kg 88 m V Ongeluk K f 89 m Makganyene Maremane 90 m Dome Kg Koegas 91 m Kg V G G Griquatown 92 m Manganore Fe Wolhaarkop V K Kuruman 93 m f 94 m Campbellrand V Fe POSTMASBURG V 95 m V 96 m Fe K G 97 m Kg 01020km V V 98 m

KOLOMELA MINE 99 m

FIG. 1. Regional geological map of the Postmasburg manganese field with approximate location of the studied drill-core site and log information for the intersected Fe- and Mn-rich rocks.

ANALYTICAL METHODS quantitative optimization on Co, dead time 40–45%, and acquisition rate 3.9–4.2 103 counts/s. Optical examination of petrographic polished thin Electron probe micro-analysis data were obtained at sections with transmitted, polarized light was conducted the Department of Geology, Rhodes University, using a at Rhodes University, Department of Geology, using an JEOL JXA 8230 Superprobe equipped with four WD Olympus BX60 microscope equipped with a 5 Mega- spectrometers and operated at 15 kV and 20 nA with a pixel PAXCam digital camera. Preliminary identification beam size of B1 mm. Wavelength-dispersive spectro- of minerals was done at the Electron Microscopy Unit (EMU) at Rhodes University, using a Scanning Electron scopy (WDS) qualitative scans were performed for Microscope (SEM; TESCAN Vega TS 5136LM) equip- several (As-rich) tokyoite grains in order to identify ped with an Si(Li) EDS INCAPentaFET-x3 detector. the elements present in measurable concentrations. The Energy-dispersive spectroscopy (EDS) data and images Virtual WDS software was used to simulate possible were processed using the Oxford Instruments INCA interferences between elements to be analyzed. The Energy350& EDS software. The analytical conditions counting time for most elements was set to 10 s per for the EDS analysis were: accelerating voltage 20 kV, peak and 5 s per background, except for Pb, S, P, and 984 THE CANADIAN MINERALOGIST

Sr, where a higher counting time was used in order to 30 mm, tilt 70°, emission current 90 mA. The error of the improve detection limits (20 s per peak and 10 s for best fit between the match unit and the experimental lower and upper backgrounds, respectively). Natural EBSD patterns is 5%. standards and ZAF correction matrices were employed for quantification. Raman spectra were obtained at Ecole Normale RESULTS AND INTERPRETATION Supérieure de Lyon using a multichannel Raman microp- robe (LabRam HR800 from DILOR) equipped with a Optical examination confocal microscope configuration (incident laser beam 3+ 514.53 nm line of Ar ion laser focused to less than 2 mmin Tokyoite, Ba2Mn [(V,As)O4]2(OH), As-rich tok- 3+ diameter). The scattered Raman light is focused through a yoite, Ba2Mn [(As,V)O4]2(OH), and noélbensonite, 3+ 100 mm slit into a spectrograph equipped with a 1800 lines/ BaMn 2(Si2O7)(OH)2 (H2O), occur associated with each mm grating and analyzed by a CCD detector, giving a other as minute, reddish, pink-reddish, and brown-reddish – resolution of approximately 2.5 cm 1. The accumulation grains, respectively, dispersed in fine (0.1 to 1 mm) vug- times for Raman spectra were typically 60–120 s. Precision filling aggregates. Individual grains are usually 20–200 mm – on the Raman peak positions is typically 0.2 cm 1 for in size. The minerals making up the vug fillings include – strong peaks, whereas accuracy is 1 cm 1. variable proportions of quartz, K-feldspar, sérandite– Electron backscatter diffraction analysis was performed pectolite, noélbensonite, aegirine, albite, natrolite, para- at Pascal University, Clermont-Ferrand, France, using a gonite, witherite, and baryte, and minor strontianite, JEOL 5910 LV scanning electron microscope, equipped piemontite, barytocalcite, and calcite. The matrix consists with a Bruker Quantax system, with Eflash EBSD camera of varying amounts of braunite, hematite, and hausmannite. coupled with an EDS-SDD Xflash detector. Analytical In thin sections (PPL mode), tokyoite and As-rich conditions were: low vacuum of 10 Pa, working distance tokyoite have a reddish-brown to pinkish dark red color

FIG. 2. A series of petrographic images (A–D) in plane-polarized light, illustrating typical mineralogical associations and textural relationships of tokyoite (Tk), As-rich tokyoite (As-Tk), and noélbensonite (Nbn), as observed in selected samples from this study (see text for detailed discussion). Associated phases: Kfs = K-feldspar with adularia habit; Srd = sérandite; Qz = quartz. TOKYOITE, As-RICH TOKYOITE, AND NOÉLBENSONITE 985

FIG. 3. Back scattered SEM images of characteristic textures. (A) Anhedral tokyoite partially replaced by noélbensonite, which appears to be also replaced by late sérandite. (B) Tokyoite replaced by an aggregate of noélbensonite and sérandite. Note baryte and witherite associated with the tokyoite and the fine, acicular terminations of noélbensonite aggregates. (C) Tokyoite and witherite replaced by K-feldspar. (D) Crystallographically controlled replacement of tokyoite by quartz. Sérandite and noélbensonite are present in the quartz mass. Abbreviations: As-rich tokyoite (astk), noélbensonite (nbn), sérandite (ser), K-feldspar (kfs), quartz (qz), baryte (bar), witherite (wth).

(more pink for the As-rich tokyoite), whereas noélbensonite some vugs, the tokyoite appears to be replaced by quartz has a lighter, honey-brown color (Fig. 2A–D). The As-rich along cleavage planes (Fig. 2D). tokyoite and tokyoite show distinct pleochroism with absorption from yellowish or brownish red to pinkish dark Mineral composition of tokyoite, As-rich tokyoite, red, respectively. With increasing As/V ratio, the absorp- and noélbensonite tion color of tokyoite becomes darker red. Petrographic evidence suggests that As-rich varieties of tokyoite (dark Energy-dispersive spectroscopy and electron red to pinkish dark red) have replaced As-poor ones microprobe analyses revealed two different data sets (reddish brown), whereas sérandite and noélbensonite seem for tokyoite—one of which is As-rich—which also to develop on the exterior of tokyoite grains (Fig. 2A–B correlate with two different phases having slightly and D). Kawachi et al. (1996) found noélbensonite different optical properties and representing two replacing sérandite in the Woods mine, New South Wales different textural contexts. The backscattered images (Australia). Our samples show a rather reverse relationship, (Figs. 3A–D) do not show any obvious difference in which noélbensonite grows later on the exterior of in contrast between As-poor tokyoite and As-rich the tokyoite grains (Fig. 2A). The tokyoite and As-rich tokyoite. Figure 3A shows anhedral tokyoite with tokyoite (and possibly noélbensonite as well) are replaced noélbensonite and sérandite grown at the exterior of or overgrown by sérandite (Fig. 2A–B). The outer walls of the tokyoite grains. Baryte and witherite are also the tokyoite-bearing vugs are lined with microcrystalline associated with tokyoite (Fig. 3B). In most cases, relics K-feldspar (adularia habit, Figs. 2A and C), sérandite, and of tokyoite along with lesser witherite are invaded by occasionally the barium carbonate mineral witherite. In K-feldspar (adularia; Fig. 3C), or can be embedded in 986 THE CANADIAN MINERALOGIST

V and As (Fig. 5), raising the possibility of a miscibility gap between the above-mentioned endmembers. Inter- estingly for this occurrence, both tokyoite and As-rich tokyoite can accommodate small amounts of Pb (repla- cing Ba), and minor Fe3+ replacing Mn3+, but only the As-rich tokyoite was found to accommodate measurable Sr replacing Ba. Moreover, even though PbO and Fe2O3 occur in minor amounts in tokyoite, a very weak positive correlation can be inferred between the two (not shown), suggesting a limited solution between tokyoite and 3+ arsenbrackebuschite, Pb2Fe (AsO4)2(OH). The crystal chemistry of tokyoite and As-rich tokyoite thus opens the way to the assessment of miscibility between Ba/Mn vanadates and Ba/Mn arsenates. Noélbensonite, a rare Ba–Mn silicate of the law- sonite structure type, has a composition presented in the EMPA data of Table 1 and Supplementary Table A3. Minor Sr (up to 0.13 apfu) and Ca (up to 0.04 apfu) can + substitute for Ba, and minor Fe3 (up to 0.01 apfu) can substitute for Mn3+. The formula of noélbensonite, FIG. 4. Detailed WDS As map of the largest grain in normalized to eight oxygen atoms per formula unit is: Figure 3C showing As-poor tokyoite (blue) and hetero- ÀÁ geneous patches of As-rich tokyoite (green to yellow). 3 þ 3 þ ðÞ: : : : : Ba0 85Sr0 13Ca0 04 Æ1:02 Mn 1 98Fe 0 01 Æ1:99 ðÞðÞ quartz, where partial and crystallographically con- Si1:95V0:04 Æ1:99O7 OH 2 H2O trolled replacement of tokyoite by quartz can be observed (Fig. 3D). The only way in which As-rich The composition of our noélbensonite is very close to tokyoite can be confidently detected within the (As- that described by Kawachi et al. (1996) from the Woods poor) tokyoite is by careful investigation of the WDS mine in New South Wales, Australia, with the only notable element maps (e.g., As map of Fig. 4 and Supp. difference being the high vanadium content (up to 1.3 wt.% Fig. A1), where small, heterogeneous spots of As-rich V2O5) in the noélbensonite described herein. areas can be observed in the tokyoite grains. fi A rst EMPA quantitative data set (Table 1 and Raman spectroscopy Supplementary Table A1) shows an As-poor tokyoite (throughout this text referred to simply as ‘‘tokyoite’’), The present work presents for the first time the Raman where V 4 As per formula unit. This phase appears as a spectra of As-rich tokyoite and noélbensonite (Figs. 6 relict in the vugs and seems to be heterogeneously replaced and 7). The spectrum of As-rich tokyoite is compared + (Figs. 2A–B, 4) by a second generation of As-rich tokyoite with the Raman spectra of arsenbrackebuschite, Pb Fe3 X 2 (As apfu V apfu; Table 1 and Supp. Table A2). The (AsO4)2(OH) (RRUFFt Project, RruffID 110041), and calculated formulae of tokyoite and As-rich tokyoite, arsentsumebite, Pb2Cu(AsO4)(SO4)(OH), from Frost normalized to 8.5 oxygens per formula unit are: et al. (2011). The comparison of multiple spectra was (As-poor) tokyoite: done with the CrystalSleuth software (Laetsch & Downs 2006). By comparison of Raman peaks for other minerals ÀÁ 3 þ 3 þ in the gamagarite group, the following indexation of the ðBa1:95Pb0:04Sr0:01ÞÆ : Mn 0:97Fe 0:03 ÂÃ2 00 Æ1:00 vibration bands can be made (sometimes with a certain ðÞðÞ degree of uncertainty, marked below with a question V1:750As0:250 Æ2:00O8 OH mark): the 170 and 241 cm–1 band corresponds to Ba–O As-rich tokyoite: – lattice vibrations, peaks at 308–340 cm 1 are attributed to ÀÁ– – ðÞ3 þ 3 þ (AsO )3 bending modes, the peak at 390 cm 1 probably Ba1:92Sr0:05Pb0:03 Æ : Mn 0:98Fe 0:02 Æ : 4 2 00 1 00 corresponds to SrO stretching vibrations, and the peaks at ½ðÞAs : V : O ðÞOH –1 3– 3– 1 050 0 950 Æ2:00 8 442–460 cm represent (AsO4) and (VO4) bending 2– modes and partially overlap doubly degenerate n2(SO4) The strong negative correlation between As and V in arsentsumebite (420 cm–1) and arsenbrackebuschite (Fig. 5) clearly suggests that the two elements occupy (464 cm–1). The peaks at 540–554 cm–1 are attributed to 3– –1 equivalent tetrahedral crystallographic sites, suggesting a (PO4) n4 bands and the peaks at 604–620 cm 2– continuous solid solution between tokyoite (V endmember) representing n4 bands bending modes of (SO4) are and a theoretical As endmember (not known). However, missing in As-rich tokyoite. The peak at 730 cm–1 is 3– asignificant gap can be observed in the distribution of attributed to the (AsO4) bending mode(?) while the peak TABLE 1. REPRESENTATIVE ELECTRON MICROPROBE ANALYSES OF TOKYOITE, As-RICH TOKYOITE, AND NOÉLBENSONITE (OXIDES IN wt.%)

Mineral tokyoite As-rich tokyoite noélbensonite

Analysis 23–32 23–34 23–35 Mean SD 1 6 9 Mean SD 15 16 21 Mean SD

V2O5 25.91 28.19 24.70 25.83 1.52 13.81 12.61 14.75 14.42 1.84 1.34 0.66 0.70 0.76 0.21 As2O5 5.95 3.99 7.73 6.42 1.68 20.71 21.92 19.84 19.92 2.37 0.04 0.00 0.00 0.02 0.03 SiO2 bd bd bd - - 0.14 bd bd --25.38 25.37 25.60 25.63 0.34 Mn2O3 13.17 13.24 12.89 13.13 0.17 12.06 12.18 11.96 12.29 0.31 34.31 34.30 33.50 34.27 0.41 NOÉLBENSONITE AND TOKYOITE, As-RICH TOKYOITE, Fe2O3 0.29 0.37 0.65 0.37 0.19 0.04 0.20 0.33 0.26 0.11 0.07 0.10 0.00 0.09 0.08 BaO 50.86 51.52 50.87 50.92 0.54 47.86 48.76 48.71 48.69 0.61 28.89 29.11 28.38 28.46 1.13 PbO 0.84 0.80 1.51 1.21 0.21 1.57 0.97 1.32 1.27 0.49 bd 0.09 0.49 0.16 0.16 SrO 0.21 0.29 0.18 0.20 0.05 1.55 0.91 1.17 1.03 0.24 3.53 3.14 2.84 2.86 0.69 CaO bd bd bd --0.11 0.04 0.06 0.03 0.03 0.20 0.26 0.40 0.51 0.29

H2O* 2.77 1.60 1.48 1.88 0.92 2.15 2.41 1.86 1.89 0.59 7.62 7.64 8.79 7.22 0.75 Total** 97.23 98.40 98.52 98.12 0.92 97.85 97.59 98.14 98.11 0.59 93.76 93.02 91.91 92.77 0.75 Cations normalized to 8.5 O apfu normalized to 8.5 O apfu normalized to 8 O apfu V5+ 1.688 1.801 1.598 1.668 0.089 0.919 0.845 0.978 0.957 0.117 0.067 0.033 0.036 0.038 0.011 As5+ 0.307 0.202 0.396 0.329 0.088 1.090 1.163 1.041 1.047 0.128 0.001 0.000 0.000 0.001 0.001 Si 0.000 0.000 0.000 0.003 0.004 0.014 0.006 0.005 0.013 0.020 1.918 1.940 1.970 1.950 0.014 Mn3+ 0.988 0.975 0.961 0.977 0.013 0.924 0.941 0.914 0.940 0.019 1.974 1.996 1.962 1.984 0.013 Fe3+ 0.021 0.027 0.048 0.027 0.014 0.003 0.016 0.025 0.019 0.008 0.004 0.006 0.000 0.005 0.004 Ba 1.965 1.953 1.952 1.951 0.010 1.889 1.939 1.916 1.917 0.026 0.855 0.872 0.856 0.848 0.037 Pb 0.022 0.021 0.040 0.032 0.006 0.042 0.027 0.036 0.034 0.013 0.000 0.002 0.010 0.003 0.003 Sr 0.012 0.016 0.010 0.011 0.003 0.090 0.053 0.068 0.060 0.014 0.155 0.139 0.127 0.126 0.031 Ca - - - --0.012 0.004 0.006 0.004 0.003 0.016 0.021 0.033 0.042 0.023 Total 5.003 4.995 5.005 4.998 0.005 4.983 4.993 4.989 4.991 0.015 4.990 5.009 4.994 4.997 0.006

*H2O estimated by difference; **Total oxides (wt.%) without H2O. SD = standard deviation 1s. bd = below detection limit. Means and SD: for 16 analyses of tokyoite, 15 analyses of As-rich tokyoite, and 10 analyses of noélbensonite, respectively (See Supp. Tables A1, A2, and A3). P2O5, SO3, Al2O3,andK2O concentrations are below detection limits and were eliminated from the table above. 987 988 THE CANADIAN MINERALOGIST

analyzed(?). The peak at 972 cm–1 is assigned to the 2– (SO4) n1 symmetric stretching mode. The four outstanding peaks between 830 cm–1 and – – 935 cm 1, a pair between 830–846 cm 1,andapair between 906 and 935 cm–1, could have two possible explanations. (1) The peaks at 830 and 846 cm–1 evidently originate from the As–O stretching vibrations within the AsO4 group (the strong one is from the symmetrical stretching vibrations and relatively weak one at 846 cm–1 from the asymmetrical stretching vibrations). When plotting the Raman data for arsenbrackebuschite against As-rich tokyoite data, one can see that these two FIG. 5. (A) Strong negative correlation between As and V peaks overlap quite well. The two peaks of As-rich (apfu) in tokyoite, suggesting a possible miscibility gap tokyoite at 870 and 930 cm–1, however, do not match between tokyoite and As-rich tokyoite. those for V–O stretching vibrations in brackebuschite (spectrum not shown in Fig. 6); they are at much higher –1 3– at 814–827 cm represents the (AsO4) n1 symmetric wave numbers. This shift could suggest that the bonding –1 stretching mode. The peak at 846 cm is attributed to the for V–O is stronger than expected for the VO4 group 3– (AsO4) asymmetric stretching mode(?), and the peak at in this mineral. The stronger V–O bonding could be –1 3– 3– 906 cm represents (VO4) and (AsO4) units of n1 achieved by weaker bonds of additional cations (Ca, Ba, –1 3+ symmetric stretching modes. The peak at 935 cm can Mn ) to the oxygen apices of VO4 tetrahedra or, less be attributed to the n1 symmetric stretching mode for likely, by minor replacement of the tetrahedrally 3– 2– 3– 5+ 6+ 6+ H–(VO4) or (HOVO3) and/or H–(AsO4) or coordinated cation (e.g.,P ,S ,orCr ), which were 2– (HOAsO3) units(?). Other possible interpretations not found in our samples. We can also assume that some include a shifting of O–V–O stretching modes owing to type of Jahn-Teller distortion characteristic of octahedral 3+ stronger bonding due to the small amount of SO4 Mn may be responsible for the observed shift in frequency. (2) VO4 and AsO4 groups in our sample may actually be ordered. In other words, they may occupy two

FIG. 6. Raman spectra of As-rich tokyoite (this study), arsenbrackebuschite (RRUFF sample 100184), and arsentsu- mebite (after Frost et al. 2011). Peak assignment: 170 and FIG. 7. Raman spectra of noélbensonite (this study), hennomarti- –1 –1 3– t 241 cm :Ba–O lattice vibrations; 308–340 cm :(AsO4) nite (RRUFF Project, RRUFF ID R130039), and lawsonite bending modes; 390 cm–1: SrO stretching vibrations (?); 442– (RRUFFt Project, RRUFF ID R050042). Peak assignment: –1 3– 3– –1 –1 460 cm :(AsO4) and (VO4) bending modes, partially 160 and 185 cm and a bigger peak at 245 cm : 2– – B –1 overlapping doubly degenerate n2(SO4) in arsentsumebite Ba O bending vibrations in noélbensonite; 170 cm and – (420 cm–1) and arsenbrackebuschite (464 cm–1); 540–554 B300 cm 1:Sr–O bending vibration in hennomartinite. –1 3– –1 B –1 – cm :(PO4) n4 bands and the peaks at 604–620 cm Single peak at 386 cm : stretching mode of Mn O, 2– representing n4 bands bending modes of (SO4) are missing commoninnoélbensoniteandhennomartiniteandmissingin –1 3– –1 – in As-rich tokyoite; 730 cm :(AsO4) bending mode(?); lawsonite. 436 cm :AlO stretching mode in lawsonite. –1 3– –1 – 814–827 cm :(AsO4) n1 symmetric stretching mode; Peaks between 500 and 600 cm :SiO bonding, with –1 3– 846 cm : (AsO4) asymmetric stretching mode (?); 906 shifting and splitting of these peaks in noélbensonite and –1 3– 3– 3+ cm :(VO4) and (AsO4) units of n1 symmetric hennomartinite probably due to the presence of Mn ; 650, – stretching modes; 935 cm–1: n1 symmetric stretching mode 660, and 696 cm 1: stretching vibrations of Ba–O, Sr–O, 3– 2– 3– – for H–(VO4) or (HOVO3) and/or H–(AsO4) or and Ca O molecules, in noélbensonite, hennomartinite, and 2– –1 – (HOAsO3) units (?). See text for further interpretations. lawsonite, respectively. 935 cm :Si O stretching vibration. TOKYOITE, As-RICH TOKYOITE, AND NOÉLBENSONITE 989 distinct crystallographic positions, as in arsentsumebite b 112°, which are similar to the cell parameters (AsO4 versus SO4) or in bushmakinite (PO4 versus VO4). of gamagarite (a 9.174, b 6.164, and c 7.882 Å, a = g The Raman spectrum of noélbensonite was compared = 90°, b 112.98°, RRUFF ID R130111) and tokyoite with the spectra of: (1) a Sr- analogue with the lawsonite (a 9.104, b 6.132, and c 7.895 Å, a = g =90°, structure called hennomartinite (RRUFFt Project, RRUFF b = 112.2°, see Matsubara et al. 2004).The results clearly ID R130039), and (2) lawsonite (RRUFFt Project, RRUFF show that As-rich tokyoite is a mineral of the gamagarite ID R050042) (Fig. 7). The main Raman peaks in group. The EBSD pattern for noélbensonite (not shown) noélbensonite are (in order of increasing Raman shift): was compared against the structure of lawsonite, –1 –1 160 and 185 cm ,245cm , a pair of asymmetric peaks CaAl2Si2O7(OH)2 H2O, with space group Cmcm and at 385 and 435 cm–1, a prominent peak at 523 cm–1 with cell parameters a 8.795 Å, b 5.847 Å, c 13.142 Å, a = – an acute shoulder at 572 cm 1, a smaller but acute peak b = g = 90°. The match between the pattern observed at 650 cm–1, followed by a significant peak at 935 cm–1 and that of lawsonite further confirmed that our mineral having a small shoulder at 904 cm–1. One minor peak is in fact noélbensonite. occurs at 1065 cm–1. We present here a possible assignment of the peaks to various types of molecule SUMMARY AND CONCLUSIONS and vibration (for all three minerals compared in Fig. 7). –1 The peaks situated at 160 and 185 cm , and a bigger A new occurrence of tokyoite and noélbensonite and a –1 peak at 245 cm , are attributed to Ba–Obending first occurrence of As-rich tokyoite from the Postmasburg vibrations in noélbensonite. In hennomartinite, the flat area of South Africa has been presented and described. The –1 –1 peak at ca. 170 cm and B300 cm is assigned to Sr– EBSD, Raman, and EMPA data illustrate that As-rich O bending vibrations. The single peak seen at B386 tokyoite is a mineral belonging to the brackebuschite –1 cm represents the stretching mode of Mn–O, common mineral group. The EBSD results suggest a monoclinic, in noélbensonite and hennomartinite and missing in P2 /m space group, and cell parameters a 9.121, b 6.142, –1 1 lawsonite. The peak at 436 cm is attributed to the and c 7.838 Å, a = g = 90°, b = 112°. The structure of Al–O stretching mode. The most interesting are the pro- As-tokyoite is similar to that of gamagarite and tokyoite. –1 minent peaks between 500 and 600 cm : here, the peaks The Raman spectrum of As-rich tokyoite was compared of noélbensonite and hennomartinite are split in two, but to spectra of arsenbrackebuschite and arsentsumebite. asymmetrically to each other. The prominent side of the The comparison with arsentsumebite suggests a possible –1 hennomartinite peak at 562 cm overlaps perfectly the ordering of the AsO4 and VO4 tetrahedra. This observation, peak of lawsonite. All these peaks represent stretching correlated with mineral compositions, indicates that the vibrations of Si–O bonding, with shifting and splitting of As-rich tokyoite may represent a new mineral with ordering these peaks in noélbensonite and hennomartinite prob- 3+ + of As and V and formula Ba2Mn (AsO4)(VO4)(OH), or 3 3 ably due to the presence of Mn . This could be a an unknown As-analogue of tokyoite with formula Ba2Mn possible effect of Jahn-Teller distortion characteristic of +[(As,V)O )] (OH). Further structural investigation of 3+ –1 4 2 octahedral Mn .Thepeaksat650,660,and696cm grains of suitable size and purity will be required to are attributed to the stretching vibrations of Ba–O, Sr–O, resolve that uncertainty. and Ca–O molecules in noélbensonite, hennomartinite, The occurrence of Sr-bearing noélbensonite in the and lawsonite, respectively. The prominent peak at PMF described herein can suggest that the Sr/Ba ratio of –1 935 cm represents the Si–O stretching vibration, the alkali-rich brines responsible for its formation was being common for all three investigated silicate phases. lower than that of the hydrothermal fluids forming its 3+ Sr-analogue, hennomartinite, SrMn 2Si2O7(OH)2 H2O, Electron backscatter diffraction described from the Wessels mine of the KMF (Armbruster et al. 1993). The discovery of tokyoite and As-rich The grain size of tokyoite, As-rich tokyoite, and tokyoite in vug-fills of manganese-rich rocks of the wider noélbensonite is very small, commonly below 100 mm. PMF, in conjunction with the occurrence of gamagarite in The extremely small size, coupled with the observed similar rocks as previously identified by De Villiers complex relationships of replacement and intergrowth (1943), provide an exciting new window into the natural rendered physically impossible the separation of a single diversity of the brackenbuschite mineral group. The crystal for X-ray diffraction or other structural techni- complexity of the mineral assemblage with which the ques. Instead, we opted for comparing the indexed tokyoite and As-rich tokyoite are associated is also similar EBSD patterns of our minerals with EBSD patterns of to that described from Mn-rich rocks near the Bruce iron known minerals that have similar symmetry and cell ore mine by Moore et al. (2011). The similarities between parameters. The EBSD pattern obtained for As-rich the two assemblages, combined with the new information tokyoite (not shown) was indexed and compared against presented in this paper, and particularly the multitude of 3+ 3+ its best fit, namely gamagarite, Ba2(Fe ,Mn ) Ba-bearing phases observed, such as noélbensonite and (VO4)2(OH). The results show that As-tokyoite has a witherite, further suggest that the alkali-rich brines thought monoclinic P21/m space group and cell parameters with to be responsible for the observed mineral paragenesis best fit a 9.121, b 6.142, and c 7.838Å, a = g = 90°, were also V- and As-bearing. This further reinforces the 990 THE CANADIAN MINERALOGIST notion that the formation of these assemblages was likely FAIREY,B.(2013)Genesis of karst-hosted manganese ores of the caused by a single hydrothermal event on a regional scale Postmasburg Manganese Field, South Africa, with emphasis on (Moore et al. 2011). evidence for hydrothermal processes. Unpublished M.Sc. thesis, Rhodes University, Grahamstown, South Africa, 122 pp. ACKNOWLEDGMENTS FROST, R.L., PALMER, S.J., & XI, Y. (2011) Raman spectro- scopy of the multi anion mineral arsentsumebite Pb Cu The findings presented in this manuscript are partly 2 (AsO4)(SO4)(OH) and its comparison with tsumebite derived from the M.Sc. thesis of one of the authors (Fairey Pb2Cu(PO4)(SO4)(OH). Spectrochimica Acta A: Molecular 2013), whose research was generously supported by the and Biomolecular Spectroscopy 83, 449–452. company Kumba Iron Ore. To this end, we wish to extend our deepest appreciation to Dr. V. Lickfold, Mr. D. Nel, GUTZMER,J.&BEUKES, N.J. (1996a) Mineral paragenesis of and Mr. S. MacGregor for making available to us the drill the Kalahari manganese field, South Africa. Ore Geology 11 – core in which the new minerals were discovered, for Reviews , 404 428. supporting our research, and for their permission to publish GUTZMER,J.&BEUKES, N.J. 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