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Subsolidus rubidium-dominant feldspar from the Morrua pegmatite, Mozambique: paragenesis and composition

Published online by Cambridge University Press:  05 July 2018

D. K. Teertstra
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
P. Černý
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
F. C. Hawthorne
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

Abstract

At the Morrua pegmatite, Mozambique, alkali feldspar has replaced pollucite under low-temperature (250–150°C) hydrothermal conditions. Fluids invading a fracture system in pollucite formed round granular aggregates of (K-Rb)-feldspar in three stages: (1) a compositionally heterogeneous core of the feldspar cluster (+cookeite ± apatite) with 7–20 mol.% RbAlSi3O8, grading outward into a Rb-dominant feldspar with 66 mol.% RbAlSi3O8 (20 wt.% Rb2O); (2) an intermediate layer of non-porous, inclusion-free, end-member K-feldspar; (3) an outer layer of porous end-member K-feldspar. Feldspars of all three stages seem to be monoclinic and disordered, with metastable sanidine structure. Zoning in K/Rb, preserved on a fine scale, was formed during growth at a temperature too low for subsequent alkali-cation diffusion or (Al,Si)-ordering.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1999

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References

Černý, P. (1982) Mineralogy of rubidium and cesium. In Granitic Pegmatites in Science and Industry (Černý, P., ed.), Mineral. Assoc. Canada Short-Course Handbook, 8, 149–61.Google Scholar
Černý, P. (1994) Evolution of feldspars in granitic pegmatites. In Feldspars and Their Reactions (I. Parsons, ed.), NATO ASI Series C, 421, 501–40.CrossRefGoogle Scholar
Černý, P. and Chapman, R. (1984) Paragenesis, chemistry and structural state of adularia from granitic pegmatites. Bull. Minéral., 107, 369–84.CrossRefGoogle Scholar
Černý, P. and Chapman, R. (1986) Adularia from hydrothermal vein deposits: extremes in structural state. Canad. Mineral., 24, 717–28.Google Scholar
Černý, P., Pentinghaus, H. and Macek, J.J. (1985) Rubidian microcline from Red Cross Lake, north-eastern Manitoba. Bull. Geol. Soc. Finland, 57, 217–30.CrossRefGoogle Scholar
Černý, P., Ercit, T.S. and Vanstone, P.J. (1996) Petrology and mineralization of the Tanco rare- element pegmatite, southeastern Manitoba. Field Trip Guidebook A4, Geol. Assoc. Canada — Mineral. Assoc. Canada Annual Meeting Winnipeg, Manitoba, 63 pp.Google Scholar
Chakoumakos, B.C. and Lumpkin, G.R. (1990) Pressure-temperature constraints on the crystallization of the Harding pegmatite, Taos County, New Mexico. Canad. Mineral., 28, 287–97.Google Scholar
Correia Neves, J.M. (1981) Pegmatitos Graníticos. Morfologia, Mineralogia, Geoquímica, Gênese, Metalogênese. Habil. thesis, Universidade Federal de Minas Gerais, A1-R23.Google Scholar
Dong, G. and Morrison, G.W. (1995) Adularia in epithermal veins, Queensland: morphology, structural state and origin. Mineral. Deposita, 30, 11–9.CrossRefGoogle Scholar
Giletti, B.J. (1994) Isotopic equilibrium/disequilibrium and diffusion kinetics in feldspars. In Feldspars and Their Reactions (I. Parsons, ed.), NATO ASI Series C, 421, 351–82.CrossRefGoogle Scholar
Henderson, C.M.B. and Manning, D.A.C. (1984) The effect of Cs on phase relations in the granite system: stability of pollucite. Nat. Env. Res. Council, Progr. Exper. Petrol., 25, 41–2.Google Scholar
Khalili, H. and von Knorring, O. (1976) Pollucite from some African and Scandinavian pegmatites. Inst. African Geol. Leeds Univ., Ann. Rept., 1976, 5960.Google Scholar
London, D. (1984) Experimental phase equilibria in the system LiAlSiO4-SiO2-H2O: a petrogenetic grid for lithium-rich pegmatites. Amer. Mineral., 69, 995–1004.Google Scholar
London, D. (1986) Magmatic-hydrothermal transition in the Tanco rare-element pegmatite: evidence from fluid inclusions and phase-equilibria experiments. Amer. Mineral., 71, 376–95.Google Scholar
Pauwels, H., Zuddas, P. and Michard, G. (1989) Behaviour of trace elements during feldspar dissolution in near-equilibrium conditions: Preliminary investigation. Chem. Geol., 78, 255–67.CrossRefGoogle Scholar
Pouchou, J.L. and Pichoir, F. (1985) ‘PAP’ (phi-rho-Z) procedure for improved quantitative microanalysis. In Microbeam Analysis (Armstrong, J.T., ed.) San Francisco Press, 104–6.Google Scholar
Smeds, S.-A. and Černý, P. (1989) Pollucite from the Proterozoic petalite-bearing pegmatites of Utö , Stockholm archipelago, Sweden. Geol. Foren. Foren. Forh., 111, 361–71.CrossRefGoogle Scholar
Teertstra, D.K. (1991) Compositional heterogeneity and alteration of pollucite. Unpubl. M.Sc thesis, Univ. Manitoba, Winnipeg.Google Scholar
Teertstra, D.K. (1997) Reactions of (K-Rb)-feldspars from rare-element granitic pegmatites. Unpubl. Ph.D. thesis, Univ. Manitoba, Winnipeg.Google Scholar
Teertstra, D.K. and Černý, P. (1995) First natural occurrences of end-member pollucite: A product of low-temperature reequilibration. Eur. J. Mineral., 7, 1137–48.CrossRefGoogle Scholar
Teertstra, D.K. and Černý, P. (1997) The compositional evolution of pollucite from African granitic pegmatites. J. Afr. Earth Sci., 25, 317–31.CrossRefGoogle Scholar
Teertstra, D.K., Černý, P. and Hawthorne, F.C. (1997) Rubidium-rich feldspars in a granitic pegmatite from the Kola Peninsula, Russia. Canad. Mineral., 35, 1277–81.Google Scholar
Teertstra, D.K., Černý, P., Hawthorne, F.C., Wang, Lu-Min, Pier, J. and Ewing, R.C. (1998 a) Rubicline, a new feldspar from San Piero in Campo, Elba, Italy. Amer. Mineral., 83, 335–9.CrossRefGoogle Scholar
Teertstra, D.K., Hawthorne, F.C. and Černý, P. (1998 b) Identification of normal and anomalous compositions of minerals by electron-microprobe analysis: K-rich feldspar as a case study. Canad. Mineral., 36, 8795.Google Scholar
Teertstra, D.K., Lahti, S.I., Alviola, R. and Černý, P. (1993) Pollucite and its alteration in Finnish pegmatites. Bull. Geol. Surv. Finland, 368, 39 pp.Google Scholar
Vidal, O. and Goffé, B. (1991) Cookeite LiAl4(Si3Al)O10(OH)8: Experimental study and thermodynamic analysis of its compatability relations in the Li2O-Al2O3-SiO2-H2O system. Contrib. Mineral. Petrol., 108, 7281.CrossRefGoogle Scholar
von Knorring, O. and Condliffe, E. (1987) Mineralized pegmatites in Africa. Geol. J., 22, 253–70.CrossRefGoogle Scholar
Worden, R.H. and Rushton, J.C. (1992) Diagenetic K-feldspar textures: a TEM study and model for diagenetic feldspar growth. J. Sed. Petrol., 62, 779–89.Google Scholar