Lithium tourmalines from the Meldon aplite, Devonshire, England

Autor: R. A. Howie, M. N. Chaudhry
Rok vydání: 1976
Předmět:
Zdroj: Mineralogical Magazine. 40:747-751
ISSN: 1471-8022
0026-461X
DOI: 10.1180/minmag.1976.040.315.08
Popis: SUMMARY. Seven chemical analyses of pink or green tourmalines belonging to the elbalte-schorl series, along with their physical, optical, and X-ray data are presented. Linear variation diagrams showing the relationship between composition and refractive indices, specific gravities, and cell parameters are constructed. Relationship between colour and chemical composition is also discussed. THE Meldon aplite, Devonshire, is a soda-lithia aplite dyke about 6o to 8o ft in thickness, occurring three-quarters of a mile north-west of the main Dartmoor granite (Worth, 192o). The aplite has been cut and metasomatized by a multitude of successive generations of thin (2 to 7 cm) K- and Li-rich veins containing tourmaline crystals. The magmatic veins of earlier generations and surrounding contemporaneously metasomatized aplites contain green tourmalines whereas the later pneumatolytichydrothermal veins and associated or contemporaneously metasomatized aplite contains pink tourmaline (rubellite). Four samples (Table I), M.P.T., MI8, M2oo, and MI7 belong to the rubellite variety of elbaite (MI7 being paler pink than the others) and three samples, M5, MK653, and R. A. H. are verdelites (green-coloured elbaites). The samples MI8 and M.P.T. are from veins composed of orthoclase, lepidolite, quartz, and rubellite; the sample M2oo comes from a very late greisen-like vein composed chiefly of rubellite, lepidolite, quartz, and a subordinate amount of feldspar; the samples MI7, MK653, M5 and R. A. H. have been obtain from reconstituted aplite, composed mainly of albite, quartz, and lepidolite. Chemistry. The green tourmalines are richer in iron and poorer in aluminium than the pink tourmalines. In the green tourmalines ferrous iron and manganese are inversely related. There is no clear difference in the lithium or manganese content of the two groups. At first sight there is poor correspondence between the theoretical and the calculated formulae (Table I) as there is so much A1 included in the R3 group that, despite o'74 to o.96 Li, the total valency of the R3 group is around 7. In the formula NaR3AlnBaSi602v(OH)4, R3 should have only a total of 6 vaIencies; however, the deficiency of Si, B, and OH restores the over-all balance. From an examination of published analyses of tourmalines, Foit and Rosenberg (1974) have identified several analyses that show a variation of the trivalent ions (R 3+) with (R++OH-+F-). This �9 Copyright the Mineralogical Society.
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