On the X-ray identification of amblygonite and montebrasite

Abstract

S.W. 7 SUM MARY. Previous investigations have been confined to individual members or to small compositional ranges of the amblygonite-montebrasite series. The present study is a survey based on twentytwo specimens in the range Amb~Monso to Amb~o.Mon8 (i.8-11.8 % fluorine), and indicates that the fluorine content may be related both qualitatively and quantitatively to differences in the X-ray powder patterns. Fully indexed powder data to d ~ 1.68 ,~ and derived cell parameters are given for four members of the series. There is no evidence of a break in the continuity of the series. The first British occurrence of both amblygonite and montebrasite is reported from a pegmatite vein in aplite at Meldon, Okehampton, Devonshire.I Y the course of an examination of a number of specimens of presumed amblygonite and montebrasite collected by the late Mr. A. W. G. Kingsbury at the old aplite quarry, Meldon, Devonshire, we noted marked differences between the X-ray powder patterns. It was by no means clear from published data that all the specimens belonged to the series, and variations in powder patterns of different specimens of montebrasite (Quensel, 196z ) were less than we observed. Other workers (Fisher, I958; Kittrick and Hope, I967) distinguish between amblygonite and montebrasite on the basis of spacing and intensity differences of the three strongest lines of the patterns, and on comparisons of single crystal diffraction patterns with powder patterns, but their reference samples are inadequately characterized and their data too scanty. The powder patterns figured by Pough and Henderson (1945) are indistinct, and are not supported by chemical analyses. Twenty-two specimens of amblygonite and montebrasite were selected for apparent homogeneity, and powder prepared from each was used for partial chemical analysis (table l) and both photographic and diffractometer records of the powder patterns. Careful examination of the photographic powder patterns showed that it is possible to distinguish four general types of pattern, here designated A, AB, B, and C ( fig. l). Diffractometer records, mentioned below, permit division of the AB pattern-type into two ( fig. z).For chemical analysis, between 15 and 7o mg of the powder were fused with five times their weight of sodium carbonate, extracted with 4o ml of I : I sulphuric acid, transferred to a distillation flask, and the fluorine steam-distilled. Fluorine was determined on the distillate by the zirconium-eriochrome-cyanine method of Megregian (t954). Alkalis were determined by flame photometry on a sulphuric acid solution.Comparison of the chemical and X-ray results shows that there is a correlation between pattern-type and fluorine content ( fig. 4)-The replacement of lithia by soda

Natromontebrasite, a Discredited Mineral Species

“…Although the potentiometric determination of F is relatively more difficult than a measurement of the Na or Li contents with an atomic absorption spectrometer, the new chemical data (Table 3) gave values for the fluorine content similar to the data of Schaller (1911a), and values for Na 2 O and Li 2 O strongly divergent from the results of that author. Compared with the chemical data of Heinrich & Corey (1955) (Table 3, column 2) and the results of a partial chemical analysis given by Moss et al (1969) (Table 3, column 3), the results obtained herein tend to show that Na and Li are not homogeneously distributed within the montebrasite -amblygonite samples from Canon City (Table 3). It also becomes evident that sample USNM 86843 is impure, as the P : Al : (Na+Li) : (F+OH) proportion, equal to 1 : 1 : 0.96 : 1.22, mainly shows a (F + OH) excess likely due to an excess of H 2 O.…”

“…Considering the fluctuation of the chemical data available at that time and the absence of a systematic and accurate X-ray investigation of specimens of the isomorphous series montebrasite -amblygonite, Moss et al (1969) proposed a variation of the crystallographic parameters of montebrasite -amblygonite minerals as a function of fluorine content. Although no specimen of true "natromontebrasite" was considered, one sample from Canon City was included with 3.2 wt.% Na 2 O.…”

“…No significant effect caused by Na was detected by these authors, who reported that the samples richest in F tend to have high Na contents, however. According to Moss et al (1969), such a relationship could be correlated with the geological context and not with a crystallochemical constraint.…”

A Raman Investigation of the Amblygonite-Montebrasite Series

“…Several physical criteria have been found to vary as a function of the F/(F + OH) value or of the F content and, thus, have been used to estimate rapidly and indirectly the F contents of any specimen belonging to the amblygonite-montebrasite solid-solution series. Among these criteria, we can report: the optical properties and density (Winchell 1926, Winchell & Winchell 1951, Černá et al 1973, Greiner & Bloss 1987; the position of selected peaks in the powder-diffraction pattern (Moss et al 1969, Dubois et al 1972, Černá et al 1973, Kallio 1978; the crystallographic parameters of the triclinic unit-cell (Černá et al 1973); the position of some absorption bands of the infrared spectra (Fransolet & Tarte 1977, Groat et al 1990; the position of the 27 Al peak on magic-angle-spinning nuclear-( = 1.9373 Å). Unit-cell parameters were calculated with the least-squares refi nement program LCLSQ 8.4 (Burnham 1991) on the basis of corrected d values; we used Pb(NO 3 ) 2 as an internal standard.…”

Characterisation of phosphate mineralogy in Montebras-en-Soumans Pegmatite, Massif Central, France

“…Figure 3. Fluorine determination in bulk amblygonite-montebrasite in different samples by infrared spectroscopy (Fransolet & Tarte, 1977); X-ray diffraction (Moss et al, 1969;Černá et al 1973;Kallio, 1978) and point analyses with EPMA.…”