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A Staining Technique for Barium Silicates in Thin Section

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A Staining Technique for Barium Silicates in Thin Section MINERALOGICAL MAGAZINE, SEPTEMBER 1985, VOL. 49, PP. 614-615 A staining technique for barium silicates in thin section BARIUM alumino-silicates (celsian, hyalophane, tained celsian from the Aberfeldy deposit, one as a and rare cymrite) have recently been discovered in vein and four disseminated in metasediments. A association with the Aberfeldy Ba,Zn,Pb deposit barium-free plagioclase from Skye provided a check (Coats et al., 1981), which occurs in Middle that the stain only reacts with barian minerals. To Dalradian garnet-amphibolite facies rocks (Moles, determine whether mica merely takes up stain along 1983) in Perthshire, Scotland. The deposit is hosted the cleavage, a single thin section (to eliminate by a muscovite schist containing cryptic Ba enrich- variations in technique) was made from two ment in the form of barian muscovite. Celsian micaceous rocks; one from Glen Lyon, Perthshire, and hyalophane also occur in stratigraphically containing barian muscovite and hyalophane, the equivalent uneconomic mineralized horizons else- other a barium-free sample of Moine Schist. where in Scotland, e.g. in Glen Lyon, Perthshire Several crystals of the barium zeolite, harmotome (M. J. Gallagher, pers. comm., 1981). Such barium (BaA12Si6016-H20), from Strontian, Argyll- minerals may thus indicate more generally the shire, and various barium-free calcium zeolites of nearby presence of base metal sulphides and baryte. unknown provenance: chabazite, CaA12Si4012" A quick and inexpensive method of detecting Ba in 6H20; heulandite, (Ca,Na2)A12SivO18"6H20; feldspars, micas, and other silicates would therefore and stilbite, (Ca,Naz,K2)A12SiTO18-7H20, were be a useful tool in mineral exploration. mounted in epoxy resin and ground down to Staining techniques can be easily employed to provide an etchable surface. Three samples of distinguish between numerous white or near-white baryte (BaSO4) from Aberfeldy, Castleton (Loch minerals (Hosking, 1957), both in hand specimen Fyne, Argyllshire) and Leadhills, Lanarkshire, were and, more usually, in thin section. One of the included, although baryte is very difficult to etch. commonest is that used to distinguish plagioclase The identification of these barian minerals was from alkali feldspar, quartz, cordierite, and felds- confirmed by X-ray diffraction except in the case of pathoids (Hutchison, 1974). This method uses the the Glen Lyon sample, which was analysed by potassium or sodium salt of rhodizonic acid, which microprobe and found to contain about 10 % BaO stains plagioclase (NaAISiaO8-CaAI2Si2Os) bright (N. J. Fortey, pers. comm., 1983). The sections red, provided the (etched) rock has previously been were etched over 35~ hydrofluoric acid for immersed in barium chloride. This replaces the 15-30 seconds (60 for baryte) and then immersed in calcium in the plagioclase structure with barium, fresh rhodizonate reagent (a saturated solution and is effective if there is at least 3 ~o of the calcium of the sodium salt) for 5 minutes (10 for baryte). end-member present. It is therefore to be expected Surplus stain was then rinsed off with deionised that feldspars already containing Ba will stain if the water and the slide dried and covered in the chloride immersion stage is omitted, and that normal way. barium-free feldspars will not. An isomorphous The barium feldspars all stained as predicted; the solid solution series exists between orthoclase barium-free plagioclase did not stain. This method (KAISi3Os) and the barium feldspar celsian (BaAI 2 thus successfully distinguishes celsian and hyalo- Si208), with a composition gap between 65 and phane from other feldspars. The barium-containing 80~o celsian; feldspar with 15-65 ~ of the celsian half of the two-lithology mica section also stained. component is called hyalophane (Fortey and Care must be taken here because all micas will Beddoe-Stephens, 1982). By analogy, any feldspar adsorb the stain along the cleavage after prolonged with more than 3 ~ celsian component should immersion; truly barian mica stains between the stain, since Ba replaces Ca 1:1. Staining is a cleavage planes as well. The crystals of harmotome more sophisticated test than Portable Induced became a pale pink; the technique should therefore Fluorescence Analysis (Grout and Gallagher, 1980), also work on other barium silicates which were which only reveals the presence of Ba within a rock, unobtainable for this experiment, for instance the and does not identify its host mineral. hydrated barium silicate, cymrite (BaA12Si20 8 To test this idea that barian minerals should HzO). The calcium zeolites were unaffected. stain directly with rhodizonate reagent, eleven Apparent staining of the baryte was in fact due to uncovered thin sections were prepared. Five con- infilling of pits and cracks; etching problems make SHORT COMMUNICATIONS 615 this technique applicable only to more soluble while the author was in receipt of a NERC/IGS Case barium minerals. studentship. An attempt was also made to stain hand speci- mens, but this was less successful; the 15 ram-wide REFERENCES coarse-grained celsian vein etched well and became Coats, J. S., et al. (1981) Inst. Geol. Sci. Miner. Reconn. pink and friable when treated with the stain, but the Rep. No. 40. fine-grained rocks with disseminated celsian did Fortey, N. J., and Beddoe-Stephens, B. (1982) Mineral. not stain. This result, plus the hazard of carrying Mao. 46, 63-72. hydrofluoric acid in the field, makes the technique Grout, A., and Gallagher, M. J. (1980) Trans. Inst. Mining unsuitable as a portable field test, but a small mobile MetaU. B, 89, 130-3. laboratory could easily be set up for rapid identifica- Hosking, K. F. G. (1957) Camborne School of Mines Mayazine, 37, 5-16. tion of barium silicates. Hutchison, C. S. (1974) Laboratory Handbook of Petro- graphic Techniques, p. 18, John Wiley. Acknowledaements. Samples for this experiment were Moles, N. R. (1983) Mineral. Mag. 47, 487-500. kindly donated by B. R. Bell, N. J. Fortey, M. J. Gallagher, A. J. Hall, I. M. Samson, and R. C. R. Willan. Thin sec- [Manuscript received 16 November 1984] tions were expertly prepared by P. Wallace and the epoxy mounts by J. Gilleece. This work was done Copyright the Mineralogical Society KEYWORDS: barium silicates, staining. Department of ApPlied Geology, University of Strathclyde, Glasgow G1 1XJ S. FISK .
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