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Blue Euclase from Zimbabwe — a Review Susan Stocklmayer (Née Anderson), FGS, FGA Philip Antrobus Ltd., London

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Blue Euclase from Zimbabwe — a Review Susan Stocklmayer (Née Anderson), FGS, FGA Philip Antrobus Ltd., London The Journal of Volume Gemmolog26 No. 4 October 1998 y The Gemmological Association and Gem Testing Laboratory of Great Britain Gemmological Association and Gem Testing Laboratory of Great Britain 27 Greville Street, London EC1N 8TN Tel: 0171 404 3334 Fax: 0171 4048843 e-mail: [email protected] Website: www.gagtl.ac.uk/gagtl President: Professor RA. Howie Vice-Presidents: E.M. Bruton, A.E. Farn, D.G Kent, RK. Mitchell Honorary Fellows: RA Howie, RT. Liddicoat Jnr, K. Nassau Honorary Life Members: D.J. Callaghan, E.A Jobbins, H. Tillander Council of Management: CR. Cavey, T.J. Davidson, N.W. Deeks, RR Harding, M.J. O'Donoghue, 1. Thomson, VP Watson Members' Council: AJ. Allnutt, P. Dwyer-Hickey, S.A. Everitt, R. Fuller, AG. Good, J. Greatwood, B. Jackson, J. Kessler, J. Monnickendam, 1. Music, J.B. Nelson, P.G Read, R Shepherd, P.J. Wates, CH. Winter Branch Chairmen: Midlands - G.M. Green, North West - 1. Knight, Scottish - B. Jackson Examiners: AJ. Allnutt, M.Sc., Ph.D., FCA, 1. Bartlett, BSc., M.Phil., FCA, DCA, E.M. Bruton, FCA, DCA, C.R Cavey, FCA, S. Coelho, BSc., FCA, DCA, Prof. AT. Collins, BSc., PhD, AG. Good, FCA, DCA, G.M. Howe, FCA, DCA, GH. Jones, BSc., Ph.D., FCA, M. Newton, BSc., D.Phil., CJ.E. Oldershaw, BSc. (Hons), FCA, H.L. Plumb, BSc., FCA, DCA, RD. Ross, BSc., FCA, DCA, PA Sadler, BSc., FCA, DCA, E. Stern, FCA, DCA, S.M. Stocklmayer, BSe. (Hons), FCA, Prof. I. Sunagawa, D.Sc., M. Tilley, cc, FCA, CM. Woodward, BSc., FCA, DCA The Journal of Gemmology Editor: Dr RR Harding Assistant Editors: M.J. O'Donoghue, P.G. Read Associate Editors: Dr CE.S. Arps (Leiden), G Bosshart (Zurich), Prof. AT. Collins (London), Dr J.W. Harris (Glasgow), Prof. R.A Howie (Derbyshire), Dr J.M. Ogden (Cambridge), Prof. AH. Rankin (Kingston upon Thames), Dr J.E. Shigley (Carlsbad), Prof. D.C Smith (Paris), E. Stern (London), S.M. Stocklmayer (Perth), Prof. I. Sunagawa (Tokyo), Dr M. Superchi (Milan), CM. Woodward (London) Production Editor: M.A Burland Vol 26, No.4, October 1998 ISSN: 1355-4565 Blue euclase from Zimbabwe — a review Susan Stocklmayer (née Anderson), FGS, FGA Philip Antrobus Ltd., London ABSTRACT: Blue euclase from Mwame, Hurungwe District, Zimbabwe, which was first collected in the late 1970s has recently been available for study. This paper presents information on mineral inclusions for the first time and the complex surface morphology of the crystals is shown in a series of photomicrographs. Keywords: euclase, inclusions, pegmatite, Zimbabwe Introduction Euclase in Africa uclase, a rare beryllium mineral Tanzania 174 (BeAlSi04 (OH)), occurs worldwide as Spencer (1934) first described euclase a late-stage hydrothermal mineral often from Africa. Spectacular crystals were found associated with beryl and other beryllium with quartz in pegmatite at Lukangasi Mica minerals in a variety of host rocks, usually Claims, 8 km south of Mikese Station, quartz veins, granitic pegmatites or greisens. Morogoro District. The largest of these 209 Although euclase can occur as a primary crystals measuring 72 x 38 x 35 mm (b axis mineral, more usually it is secondary in direction) is in the collection of the Natural origin formed by replacement of other History Museum, London. It is described as beryllium minerals. being white and cloudy but bluish green and First recorded from Brazil in 1785 (Dana, transparent on the face parallel to b. Other 1892; Bastos, 1969), euclase has been found crystals were sold in the 1930s to collections more recently from sources in Colombia, in the USA (Pough, 1991; see also Dirlam et al., Russia, Tanzania and Zimbabwe; spectacular 1992; McKie, 1955). crystals have been described from all these localities. Of less importance gemmologically, Mozambique occurrences with small crystals or non- A reference to the occurrence of euclase transparent types of euclase have been from Muiane pegmatite, Alto Ligonha recorded from New Zealand, USA (Sharp, District, was made by von Knorring et al. 1961), Europe, Guyana, Sri Lanka (Deen, (1964). This pegmatite was mined for micro- 1984), Kashmir, Mozambique, Uganda and lite and tantalite and was an important Namibia. source of gem beryl, producing both Euclase occurs in a range of colours aquamarine and pink beryl; it was also noted including pale green, pale to medium blue, for giant crystals of microcline. Two crystals, medium greenish blue, pale yellow and apparently similar in morphology to the colourless; the gemmological interest in the Zimbabwean euclase were described, one crystals from Zimbabwe is for the deep azure was used for analysis, and the other colour. reportedly weighed about 20 gm. No © Gemmological Association and Gem Testing Laboratory of Great Britain ISSN: 1355-4565 minerals, herderite and bertrandite. Beyer inferred that the euclase formed after a first generation of beryl but was older than the tabular crystals of bertrandite. The associated euclase and herderite showed no signs of etching or dissolution and euclase was not considered to be the source of beryllium for a younger generation of beryl (aquamarine) which also occurred in the pegmatite. Uganda In a report by von Knorring (1962) on the mineralogy of pegmatites in Uganda, euclase was recorded in the Namapherere tin mine, Ankole. Beryl is a common minor constituent in all the pegmatites mined in south-west Uganda and at Kihandu mine, an unusual form of beryl with acute bipyramidal form Figure 1: Partial euclase sheath on beryl crystal. has been pseudomorphed by muscovite and Photo: E.A. Jobbins albite together with crystals of clear flaky bertrandite. No further information was information was given about the found on this locality. 209210 accompanying minerals or the paragenesis of the euclase. Zimbabwe Euclase has been found in several of the Namibia pegmatites mined for mica since the 1920s and A report by Beyer (1980) described colour­ for beryl since the 1950s in the Mwame area less to slightly yellow crystals up to 5 mm long 19 km NNE of Karoi, in northern Zimbabwe from Spitzkopje. The euclase grew freely on (Figures 1, 2, 3 and 4). These occurrences are microcline together with quartz, green described in more detail in this paper. feldspar, fluorite, and the other beryllium Euc1ase in Zimbabwe Figure 2: Group of euclase crystals, each about 10 mm across. Photo: E.A. Jobbins History Euclase was first recorded in Zimbabwe in 1952, from a location 16 km east of Mwame (Geological Survey, 1952) and again (Wiles, 1961) where it is mentioned in a review of the economic minerals of the Mwame Mica Fields, Hurungwe District. Mwame was formerly known as Miami, Urungwe District, Rhodesia or Southern Rhodesia; most references cited give the earlier names, which are used in this paper where. relevant. Euclase is one of a group of rare beryllium minerals that had been found in pegmatites in this area; hurlbutite and herderite occur in the Grand Slam Beryl mine, phenakite in an unnamed beryl working and chrysoberyl in the Renda mine. No bertrandite J. Gemm., 1998, 26, 4, 209-218 has been recorded. Only chrysoberyl and euclase have been exploited commercially. The crystal aggregates and faceted gems of blue euclase described in 1978 (Gübelin) 1980 (Anderson) and in this paper, came from the Last Hope claims located on Haslemere and Momba Estates 13 km NNE of the town of Karoi. Recent information unsupported by geological reports refers to the production of some euclase specimens from the MWM claims, Haslemere and Momba Estates and from Trim, Mukwichi District. Both localities are within the general Mwame area. Samples of euclase at the Geological Museum, Harare, are recorded from the Eurythmie and Lion Hill claims, situated N and NNW of Mwame; the euclase from Last Hope claims, however, remains the Figure 3: Faceted euclase, 4 mm across. Photo: SM. Stocklmayer most significant source for the blue variety. Euclase specimens displayed in mineral collections at the Natural History Museum, granite plugs termed the Miami Granites. London, and the National Museum of Radiometric data gave K-Ar ages of 520 and Scotland in Edinburgh, originate from the 530 Ma on muscovite from the Catkin and Last Hope claims. Esquire pegmatites (Wiles, 1961) and K-Ar and Rb-Sr ages of 480 ± 20 Ma and 500 ± 20 Ma on biotite from the Miami 209211 Geology Granites (Clifford et al, 1967). The country rocks are sillimanite gneisses and sillimanite garnet schists; formed by Figure 4: Faceted euclase of Figure 3 immersed high-grade metamorphism of former to show the colour distribution. Photo: SM. argillaceous sediments of the mid- Stocklmayer Precambrian Piriwiri supergroup. There have been several metamorphic episodes, the most significant phase being the Palaeozoic to Proterozoic (400-650 Ma) event referred to as Miami metamorphism (Stagman, 1978). Pegmatites are of two main groups representing distinct episodes in the metamorphic history of the area. An 'older' group, secretionary in type and unzoned, are composed principally of plagioclase, quartz and some tourmaline and belong to a phase of sodic metasomatism; these are of no economic importance. The 'younger' group include the economic pegmatites and belong to a later potassic phase of metasomatism considered to be part of the Miami metamorphic event. These pegmatites contain abundant microcline and may also have mica, beryl and other gem species. They are clearly associated with late kinematic intrusive Blue euclase from Zimbabwe - a review The nature of the mineral content of these recrystallization and replacement of pegmatites is complex with source material pre-existing beryl by secondary euclase. The not only derived from the granites but, in the mineralogical sequence of replacement was case of the mica-bearing type, from described in a preliminary report (Anderson, assimilated country rock. The close 1980).
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