Occurrence of Cymrite in Nevada

Winter 1991

The rare mineral cymrite, a barium-aluminum silicate hydrate, was discovered during the course of the NBMG Geochemical Sampling and Characterization Program. The mineral occurs exclusively in thin-bedded chert containing anhedral pyrite crystals up to 1 cm in size. The coarse pyrite crystals are rimmed with dark brown goethite, while the fine ones are completely replaced by goethite and limonitic matter which impart the rock its brownish color. Cymrite forms rectangular grains about 40 microns in size distributed throughout the chalcedonic quartz matrix (fig. 1). Other associated minerals include barite and sericite. The sample containing cymrite was collected from the Little Britches Barite North Pit, a few miles northeast to Golconda, Nevada. It was interbedded with dark- to light-gray bedded barite with layers 1 mm up to 1 cm thick. The barite overlies gray chert and carbonaceous shale with sulfides and underlies black bedded limestone with each bed up to 6 m thick. The whole sequence belongs to the Cambrian Preble Formation.

The rectangular shape of the Nevada cymrite rather than the hexagonal shape reported for other cymrite prompted us to take a closer examination. The scanning electron microscope image (fig. 2) of the enlarged portion of a rectangle shows that it is filled by tiny crystal aggregates instead of a single crystal. The x-ray energy spectrum obtained from this portion reveals the presence of only Ba, Si, and Al (fig. 3) confirming the chemistry for heavier elements present in cymrite. Thus it appears that cymrite might have replaced a preexisting mineral with rectangular crystal habit, most likely barite.

The x-ray powder diffraction data for Nevada cymrite after heavy liquid separation, using the associated quartz as an internal standard, are given in table 1, along with Runnells' (1964) data on cymrite from Alaska.

Cymrite was first discovered in 1949 as colorless crystals in veinlets that cut across the hydrothermally derived manganese-silicate orebody at the Benalt mine in Carnarvonshire, Wales (Runnells, 1964). Many of the occurrences reported later are associated with low and medium grade metamorphic rocks. Cymrite was found in high-pressure metamorphosed Mn-rich rocks from the islands of Andros, Greece where it replaces celsian. Cymrite was formed during penetrative deformation in the Foss celsian-barite-sulfide deposit at Aberfeldy, Scotland and has since largely been replaced by celsian. Cymrite was found in a regionally metamorphosed strata-bound Ba-Zn deposit in the Scottish Dalradian and was considered as the precursor to the platy celsian. Cymrite was discovered in a jadeite metagraywacke of the Franciscan Group on the Pacheco Pass, California. However, cymrite is also known from little-metamorphosed black shale and chert on South Island, New Zealand and from stratabound base-metal mineralization in the Brooks Range, Alaska. Nevada cymrite occurs in Cambrian bedded barite sequence that exhibits low-grade metamorphism.

Various chemical formulae have been proposed for cymrite: Ba2Al5Si5O19.5-3.65H2O, BaAlSi3O8(OH), and BaAl2Si,O8-H2O. In the latest study Fortey and Beddoe-Stephens (1982) concluded that the last formula was the correct one. The crystal structure of the mineral also went through a series of revisions. In the latest structural determination, Drits et al. (1975) proposed that cymrite has a monoclinic symmetry (space group P2,) with pseudo-hexagonal nature and that those crystals studied previously that yielded hexagonal symmetry were triplets and those that yielded orthorhombic symmetry were twins.

An earlier study (Seki and Kennedy, 1964) of the bulk composition of BaAlSi3O8-OH indicated that cymrite is stable at pressures above 18 kbar at 300C and above 20 kbar at 500C; the pressure requirement is inconceivably high in view of its natural occurrence. More recent experimental work on the bulk composition of BaAl2Si2O8-H2O (Nitsch, 1980) has shown that cymrite equilibrates with celsian at pressures above 4.3 kbar at 300C and above 6.7 kbar at 500C. Such discrepancies may have resulted from the use of differing chemical compositions and apparatus. Clearly, more careful experimental work is needed, particularly in terms of compositional effects on cymrite stability.

References

Drits, V. A., Kashaev, A. A., and Sokolova, G. V., 1975, Crystal structure of cymrite: Soviet Physics and Crystallography, v. 20, p. 171-175.

Fortey, N. J. and Beddoe-Stephens, B., 1982, Barium silicates in strata-bound Ba-Zn mineralization in the Scottish Dalradian: Mineralogical Magazine, v. 46, p. 63-72.

Nitsch, K.-H., 1980, Reaktion von Bariumfeldspat [celsian] mit H2O zu Cymrrit unter metamorphen Bedingungen: Fortschritt der Mineralogie, v. 58, p. 98-100.

Runnells, D. D., 1964, Cymrite in a copper deposit, Brooks Range, Alaska: American Mineralogist, v. 49, p. 158-165.

Seki, Y. and Kennedy, G. C., 1964, Phase relations between cymrite, BaAlSi3O8(OH), and celsian, BaAl2Si2O8: American Mineralogist, v. 49, p. 1407-1426.

---L. C. Hsu, Geochemist, and H. F. Bonham, Research Geologist