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Rubidium-Rich Feldspars and Associated Minerals from the Luolamäki Pegmatite, Somero, Finland

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Rubidium-Rich Feldspars and Associated Minerals from the Luolamäki Pegmatite, Somero, Finland RUBIDIUM-RICH FELDSPARS AND ASSOCIATED MINERALS FROM THE LUOLAMÄKI PEGMATITE, SOMERO, FINLAND DAVID K. TEERTSTRA, PETR ÖERNY and FRANK C. HAWTHORNE TEERTSTRA, D.K., CERNY, P. and HAWTHORNE, F.C. 1998. Rubidium-rich feldspars and associated minerals from the Luolamäki pegmatite, Somero, Fin- land. Bulletin of the Geological Society of Finland 70, Parts 1-2, 43^f9. Rubidium feldspar occurs near the core zone of the highly fractionated pet- alite-subtype Luolamäki granitic pegmatite in intimate intergrowth with other feldspars which are part of a characteristic sequence of alteration of pollucite. Pods of pollucite are cut by 5-20 cm-wide veins of albite, petalite, non-perthit- ic microcline, lepidolite and quartz, by thinner veins of fine-grained micas and spodumene, and are replaced by metasomatic adularia. Grains of rubidium feld- spar occur as a potentially ordered phase in the vein microcline in association with earlier-exsolved albite, and also as late thin (< 5 |J.m) veinlets. Rubidium feldspar also occurs as a potentially disordered phase which crystallized along with metasomatic adularia. Both generations of (Rb,K)-feldspar have a similar compositional range, close to the join KAlSi308-RbAlSi308, typically with up to -21 wt.% Rb,0 (-70 mol.% Rbf) and with minor Cs, but neglible Na, Ca, Fe or P. Extreme compositions have 26.0 wt.% Rb,0 (89.0 mol.% Rbf) and 1.26 wt.% Cs20 (2.8 mol.% Csf). The diffuse compositional gradients from micro- cline to rubicline are consistent with a solid-state exsolution origin, followed by fluid-assisted textural coarsening which generates distinct phase boundaries. In contrast, metasomatic adularian (Rb,K)-feldspar was precipitated at low tem- perature (250-150°C) and fine-scale zoning with variable K/Rb is preserved as a growth feature. Key words: pegmatite, feldspar group, rubidium, K-feldspar, rubicline, micro- cline, adularia, Proterozoic, Luolamäki, Finland David K. Teertstra, Petr Cerny and Frank C. Hawthorne: Department of Geo- logical Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2. E-mail: [email protected] INTRODUCTION 1998a), and a natural (Al,Si)-disordered species (analogous to synthetic rubidian high sanidine) The rubidium-dominant feldspars are a recent ad- also probably exists. Description of rubicline from dition to the feldspar group and are the first kno- the type locality, a highly fractionated granitic wn mineral(s) with rubidium as an essential consti- pegmatite at San Piero in Campo, Elba, Italy, was tuent. Rubicline has been characterized as the largely restricted to its characterization as a new Rb>K analogue of microcline (Teertstra et al. species. A fine-grained Rb>K feldspar of unkno- 44 David K. Teertstra, Petr Cerny and Frank C. Hawthorne wn structural state was briefly described from a Volkesfeld, Eifel, Germany (K Ka, Al Ka, Si Ka). pegmatite in the Kola Peninsula (Teertstra et al. The procedure of Teertstra et al. (1997) was used 1997), but genetic interpretation was limited due to accurately measure the composition of the sa- to the small quantity of material available. A de- nidine and to provide conformity with the ideal tailed petrologic description of rubidium feldspar feldspar stoichiometry. Minor elements were me- and the associated minerals is required, to charac- asured using albite (Na Ka), pollucite (Cs La), terize their mode of formation. The Luolamäki fayalite (Fe Ka), barite (Ba Lß), SrTiOj (Sr La) pegmatite affords sufficient material, in which the and VP205 (P Ka). The elements Mg, Ti, F, Mn, rubidium feldspars are relatively common, for Ga, Ca and Pb were sought but not detected in any such a description and interpretation. generation of feldspar; limits of detection and ad- The Luolamäki pegmatite is a member of the ditional experimental details are provided in Somero-Tammela pegmatite field, located in SW Teertstra et al. (1997). Finland about 100 km NW of Helsinki. The pegm- Feldspar formulae were calculated on the ba- atite belongs to the petalite subtype of complex sis of eight atoms of oxygen per formula unit pegmatites, with negligible quantities of lithian (apfu). Monovalent and divalent cations were as- micas and no primary spodumene. The upper half signed to the A/-site and higher-valency cations to of the pegmatite is albite-rich, and the lower half the T-site of the general formula MT408. The com- is K-feldspar-rich. Petalite, quartz and pollucite positions of phosphorus-bearing and phosphorus- occur in the core zone, cut by veins of cleavelan- free feldspars were compared via the berlinite dite, quartz, schorl and muscovite (Vesasalo 1959, (AlP)Si.2 substitution (e.g., Kontak et al. 1996), Neuvonen & Vesasalo 1960, Teertstra et al. 1993). using T02~ = (Al-P) for the negative charge of the Bulk compositions of microcline-perthite have framework and Si+2P, where P equals P plus Al- 1.51 wt.% Rb20 (Neuvonen & Vesasalo 1960); equivalent of P. Compositional vectors thereby however, the K-feldspar most enriched in rubid- directly correspond to the plagioclase-like ium occurs near the core of the pegmatite in close (M^AIXM'+Si).! substitution, and to the "excess"- association with pollucite. Here, we describe silicon DSi^Og substitution (i.e., •Si(KAl).,). (Rb,K)-feldspars found as part of a characteristic Comparison of charge of the framework with M- sequence of alteration of the Luolamäki pollucite, cation charge, M* = (monovalent + 2 x divalent) examined in detail by Teertstra et al. (1993). Study cations, gives a trend for incorporation of light- of the alteration sequences of pollucite has great- element M-cations which cannot be directly meas- ly constrained the possible physical and chemical ured by the electron microprobe. conditions under which the associated rubidium- In the following text, the term (K,Rb)-feldspar rich feldspars may form (Teertstra & Cerny 1995). is used for K-dominant compositions, and (Rb,K)- feldspar for Rb-dominant compositions; (K-Rb)- feldspars designates members of the substitution- EXPERIMENTAL al series. Rbf and Csf signify the end-member components of ideal rubidium feldspar and cesi- Mineral compositions were measured using wave- um feldspar, respectively. length-dispersion (WDS) analysis on a CAMECA SX-50 electron microprobe (EMP) operating at 15 kV and 20 nA with a beam diameter of 5 p.m. THE RUBIDIUM-RICH FELDSPARS Using these conditions, gain or loss of alkali ele- ments from the EMP-analysed volume is negli- The rubidium-rich feldspars occur in intimate in- gible. Data were reduced using the PAP procedu- tergrowth with two generations of K-rich feldspars re of Pouchou and Pichoir (1985). Major elements which form part of the sequence of alteration of were measured using a glass of Rb2ZnSi5012 pollucite. Pods of pollucite near the core of the stoichiometry (Rb La) and gem sanidine from pegmatite are largely monomineralic, but contain Rubidium-rich feldspars and associated minerals from the Luolamäki pegmatite, Somero, Finland 45 scattered grains of quartz, apatite and K-feldspar. (Neuvonen & Vesasalo 1960). However, it is ac- Pollucite is cut by 5-20 cm-wide veins of albite, tually zoned with variable K/Rb, and contains petalite, non-perthitic microcline (4- lepidolite), abundant inclusions of cookeite, with minor apa- lepidolite (+ quartz) and quartz, and by later 1-2 tite and small (<1 |0.m diameter) grains of pollu- mm-wide veins of muscovite + quartz + apatite, cite (Fig. IE). Adularia locally overgrows micro- followed by thinner (< 0.5 mm) veinlets of fine- cline in parallel orientation, indicating that the grained muscovite + spodumene. Aggregates of microcline substrate provided a surface for easy fine-grained adularian (K-Rb)-feldspar overgrow epitactic nucleation and subsequent growth. Late the microcline and mica veins, and metasomati- overgrowths of adularia tend to be extremely mi- cally replace microcline veins and pollucite. Mi- croporous and are associated with leaching and nor cation exchange and leaching of pollucite is analcimization of pollucite. Most of the adularia followed by local replacement by clay minerals. has end-member composition of KAlSi308, but The vein microcline is tartan-twinned and forms local gradations of (K,Rb)- to (Rb,K)-feldspar are a vein network with uniform optical orientation relatively common (Fig. IF). The adularian (Rb- throughout the pollucite. The microcline has a K)-feldspar has a similar compositional range and patchy compositional pattern, particularly in mi- upper compositional limit as the (Rb-K)-feldspar croporous areas, with wt.% ranges of 1.32-5.64 in microcline, with up to 20.95 wt.% Rb20 (70 Rb20, 0.09-0.29 Na20, 0.12-0.32 Cs20, and 0.13- mol.% Rbf). Representative compositions are giv- 0.62 P205. The variation in Rb20 corresponds to en in Table 1. 4-17 mol.% Rbf. Areas of minimal porosity tend Mean values for (Al-P) of 0.995(15) apfu and to have higher concentrations of Rb, Na and P, and M-cation charge of 0.987(16) pfu suggest on av- a more homogeneous composition than areas erage 1 at.% light-element substitution, with in- which have high pore density (Fig. IA, 1C). dividual values up to 4 at.% (Fig. 2A). Composi- (Rb,K)-feldspar is associated with irregular veins tional trends indicate widespread substitution of of patch albite which are oriented approximately •Si408, with individual values attaining 4 mol.% parallel to planes of microcline cleavage, and thin •Si408 substitution (Fig. 2B). Mean values for veinlets of (Rb.K)-feldspar crosscut this assem- (Si+2P) of 3.007(14) and mean values of TT of blage (Fig. 1 A). (Rb,K)-feldspar adjacent to grains 4.002(4) indicate good overall agreement with the of lepidolite commonly shows a gradation of K/ feldspar formula, taking into account the DSi^g Rb from the host microcline (Fig. IB). Small and berlinite substitutions. Sums of (K+Rb) are round blebs of (Rb,K)-feldspar also occur fully slightly lower than the ideal 1.0 apfu line because embedded in the microcline (Fig.
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