Caesian Bazzite in Granite Pegmatite in Tordal, Telemark, Norway

Transcription

1 Mineralogy and Petrology (1990) 43: Mineralogy RtlCI Petrology by Springer-Verlag 1990 Caesian Bazzite in Granite Pegmatite in Tordal, Telemark, Norway G. Juve I and S. Bergstol z 1Norges geologiske undersokelse, Trondheim, Norway 2 Inst. for Geologi og Bergteknikk, NTH, Universitetet i Trondheim, Norway With 4 Figures Received February 10, 1990; accepted June 11, 1990 Summary Bazzite, Be3Sc2Si6018, the scandium analogue of beryl, is a rare accessory mineral together with ixiolite and pyrochlore of the cleavetandite-amazonite pegmatites at Heftetjern, Tordal, Telemark, Norway. Chemical investigation shows that it contains ca. 3 weight % CszO. It is, therefore, a caesian bazzite. Zusammenfassung Caesium-haltiger Bazzit in Granitpegmatiten yon Tordal, Telemark, Norwegen Bazzit, Be3SczSi6Oa8,das Scandium-Analogon zu Beryll ist zusammen mit Scandiumhaltigem Ixiolit und Pyrochlor ein seltenes akzessorisches Mineral der Cleavelandit- Amazonit-Pegmatite von Heftetjern, Tordal, Telemark, Norwegen. Seine Chemische Analyse zeigt ca. 3 Gew.-% CsaO; es handelt sich folglich um einen Caesium-haltigen Bazzit. The Heftetjern cleavelandite-amazonite pegmatite lies between Hoydalen and Skarsfjell, where two earlier described lepidolite-amazonite pegmatites, rich in rare minerals (Oftedal, 1942 and 1943) are located. The Heftetjern pegmatite is one of a larger number (Fig. 1) occurring in a volcano-sedimentary outlier which may be correlated with the Rjukan Group of the Telemark Supracrustal Suite ( My.) (Dons, 1960), see map in Bergstol and Jure (1988). Both the outlier (< 1300 My.) and the basement (< 1500 My.) are intruded by the Tordal granite ( My.), which is regarded as the source of the swarms of pegmatite in the area (Fig. 1). The dominant pegmatite minerals are amazonite, rnicrocline, albite-oligoclase, cleavelandite, quartz (partly as a smoky variety) and various dark-and lightcoloured micas, which are currently under study. Among these zinnwaldite with

2 132 G. Juve and S. Bergstol, ' fjell,, ~,.,~I ~zite localities.,., ",,\ q Hgydalen ",,,~ x +q%+. ~Heftetjern ~..4 ";/d-,_,. q4':, /. " "'4'.~+,,.+ "'~ ""~'~ PEGMATITE-OUTCROP "ITE-IlUTCR( MAP.-,!,,,, ~.. ~. T~RDAL, TELEMARK 0 5~Om COMPILATION GUNNAR JUVE AND SVEINUNG BERGST~L ' -; ~% AIJ, NTH AL~I, NGU 1989 Fig. 1. Pegmatite-outcrop map showing the large number of pegmatites in a small part of the Nissedal supracrustal outlier. Bazzite localities encircled cassiterite inclusions up to 0.5 millimetre in size has recently been identified. Both the zinnwaldite and other light-coloured micas are extremely rich in caesium, scandium, rubidium and zinc. Zinnwaldite also occurs as an alteration product of garnet. In addition to bazzite accessory minerals at Heftetjern are: ixiolite, pyrochlore-group minerals, yellowish and colourless beryl, bertrandite, bavenite (pers. comm. A.O. Larsen), garnet, gadolinite, cassiterite, allanite, zircon, monazite, magnetite, and fluorite. The ixiolite is a scandian variety with up to 1.4 Sc-atoms in the unit cell based on 8 oxygen atoms (Bergstol and Jure 1988). The pyrochloregroup minerals occur as alteration products of ixiolite and are mostly of microlite type. /Light yell ow-~~ I Quartz ~\, 4ram, \ ) \ b = bazzite,/ \ 7 Fig. 2. Sketch showing small bazzite crystals intergrown with a normal beryl, section perpendicular to their longest axes

3 Caesian Bazzite in Granite Pegmatite in Tordal, Telemark, Norway 133 Fig. 3. Well developed bazzite crystal in quartz-filled vug on a larger crystal of beryl The caesian bazzite occurs as clear, sky-blue crystals, both as imperfect plates in feldspars and quartz as well as with hexagonal habitus, often as needles along the faces and edges of larger beryl crystals as shown in Fig. 2, where at least five small crystals, millimetre in diameter, have grown together with an ordinary light yellow beryl. The longest axes of all the individuals, included the beryl, are parallel. Another peculiar configuration is shown in Fig. 3, where a 5 millimetre well developed crystal of bazzite has grown in a quartz-filled vug on a larger yellow beryl crystal. A.O. Larsen (pers. comm. 1989) has also found bazzite at Heftetjern as very small crystals in a fibrous mass of bavenite, which again is an alteration product of beryl. X-ray diffraction data. Unit-cell dimensions: Bazzite a = A c = Yellow beryl a = ~ c = 9.191/~ Colourless beryl a = 9.215/~ c = c/a = c/a = c/a = V = /~3 V = ~3 V = /~3 The calculations were made using the program: Indexing and Least-Squares Refinement of PDF Data by D.E. Appleman, H.T. Evans Jr. and P. Benoit of the USGS. The figures for bazzite correspond well with the ones given by Frondel (1968) for synthetic bazzite and for the natural mineral described by Chistyakova et al. (1966). Electron-microprobe analyses were carried out on three different grains of bazzite by T. Boassen at the Institute of Continental Shelf Research (IKU), Trondheim, using a Jeol Super Probe. A back-scatter electron picture showed that some of the bazzite grains contain inclusions of K-feldspar with 16.2~ K20 (Fig. 4). The beryllium and the KiO-content was determined on a bulk sample by atomicabsorption spectrometry at the Geological Survey of Norway by G. Faye and A. Flgtronnin9 and Li20 by I. Romme at The Norwegian Institute of Technology

4 Fig. 4. Back-scatter electron picture showing inclusions of K-feldspar in bazzite Table 1. Electron-microprobe analyses of three grains and atomic-absorption analyses of a bulk sample of caesian bazzite from H eftetjern, Tordal, Norway, and data from one published analysis Oxide Grain 1 Grain 2 Grain 3 Average/bulk Kazakhstan* BeO** SczO Fe203(tot. ) A MgO MnO Li20** 0.24 n.d. Na K Rb CsaO SiO H20*** SUM One XRF spectrogram shows traces of gallium. n.d. not detected. * Chistyakova et al. (1966). ** Determined by AAS on a bulk sample. *** Determined on a bulk sample. For determination of Cs a pollucite standard was used.

5 G. Juve and S. Bergstol: Caesian Bazzite 135 (NTH). The analyses are presented in Table 1, together with data for the Kasakhstan bazzite. The BeO-content in the bulk sample of bazzite was determined to 14.18%, and the K20 to 0.48%. Since the electron-microprobe result was 0.13% K20 on an average, this means that 0.35% K20 in the bulk sample for determination of beryllium is due to K-feldspar inclusions in the bazzite. The electron-microprobe analyses show that it is a nearly pure K-feldspar, and its amount could be calculated to 2.2%. Based on these figures the corrected BeO-content in the bazzite is: x % BeO Determination of H20 was also done on a bulk sample. The formula, based on an average of the three analyzed grains and the bulk sample calculated to 18 oxygen atoms and grouping the elements as suggested by V.V. Bakakin and N.V. Belov (1962), is as follows: Be3.1 (Sc 1.24Feo.42Mno. 12Alo. 1 omgo.o 2)1.9 (Nao. 30Cso. 12Li0.05Rbo.o2Ko.01 )o. 5 (Si5.7Be0.3)6 O18" 0.36H20. Compared to beryl most of the aluminium is replaced by different di- and trivalent elements of which scandium is the most important. Since Cs exceeds 0.1 in the formula unit, the mineral is preferably called a caesian bazzite. The small deviation from stoechiometry can not be readily explained. Because of the many individual determinations, it is not likely that it derives from analytical errors. The oxide sum is also within the expected limits for the Jeol microprobe and the AAS. The specific gravity was determined by the pycnometer method (P.-R. Graft, NGU) to be This value is slightly higher than earlier published data: (Hutttenlocher et al. 1954) and 2.77 (Chistyakova et al. 1966), and is certainly due to the higher caesium content of the Tordal bazzite. The index of refraction were determined to be ne = and nco = According to Oftedal (1956) there are only small quantities of caesium in the beryls of the amazonite pegmatites in Tordal, although the element is enriched in the micas of the cleavelandite-rich parts. Regarding scandium Oftedal (1953) found only up to 300 ppm in beryls and up to 2000 ppm in zinnwaldite from Hoydalen and Skarsfjell, Tordal. The only scandium-rich beryl reported from Norway, of which the authors are aware, is a green variety with about 1% Sc described by Oftedal (1953) from Ljoslandsknipan in Iveland. Neumann (1961) has found 1000 ppm Sc in another beryl from the same locality. As stated in the paper on ixiolite etc. (Bergstol and Jure, 1988) the scandium and some of the tin in the Tordal amazonite-cleavelandite pegmatites may have their source in the volcanogenic rocks of the Nissedal supracrustal outlier. The caesium which is enriched in the bazzite and in the micas is, like Li, Nb and Ta, probably of primary granitic origin. Acknowledgements Constructive criticism of the manuscript was given by Frank M. Vokes and Svein Olerud, Trondheim as well as by two anonymous referees.

6 136 G. Juve and S. Bergstol: Caesian Bazzite References Bakakin VVBelov NV (1962) Crystal chemistry of beryl. Geochemistry 5: Bergstol S, Jure G (1988) Scandian ixiolite, pyrochlore and bazzite in granite pegmatite in Tordal, Telemark, Norway. A contribution to the mineralogy and geochemistry of scandium and tin. Mineral Petrol 38: Chistyakova NB, MoIeva VA, Razmanova SP (1966) The first find of bazzite in the U.S.S.R. Dokl Akad Nauk SSSR 169: Dons JA (1960) The stratigraphy of supracrustal rocks, granitization and tectonics in the Precambrian Telemark area, southern Norway. Guide to Excursion A 10, 21st Int Geol Congr Norden, Nor geol unders 212h, 30 pp Frondel C, lto J (1968) Synthesis of the scandium analogue of beryl. Amer Mineral 53: Huttenlocher H, Hftgi Th, Nowacki W (1954) R6ntgenographische und Spektrographische Untersuchungen an Bazzit. Schw Min Pet Mitt 34: Jure G, Bergstol S (1989) The occurrence of bazzite, the scandium analogue of beryl, and its relation to a Precambrian province of rare metals in South Norway. TERRA cognita 2 (abstract) Neumann H (1961) The scandium content of some Norwegian minerals and the formation of thortveitite, a reconnaissance survey. Nor Geol Tidsskr 41: Oftedal I (1942) Lepidolit- og tinnstensforende pegmatit: Tordal, Telemark. Nor Geol Tidsskr 22: (1943) Scandium in biotite as a geologic thermometer. Nor Geol Tidsskr 23: (1956) Contribution to the geochemistry of granite pegmatite. Nor Geol Tidsskr 36: Authors' addresses: G. Jure, Norges geologiske undersokelse, N-7002 Trondheim, Norway; S. BergstoI, Inst. for Geologi og Bergteknikk, NTH, Universitetet i Trondheim, Norway.