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The unit-cell contents of anthophyllite

Published online by Cambridge University Press:  14 March 2018

G. H. Francis
G. H. Francis*
Affiliation:
Department of Mineralogy, British Museum
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Summary

Conclusions concerning the unit-cell contents and chemical formula of a mineral are all too often drawn from a small part of the available data. A procedure is outlined by which any chemical analysis for which a density is available can be utilized, provided X-ray data are available for a reasonable range of analysed specimens. The effects of possible errors in the determination of water, and in the assessment of essential and non-exsential water, are discussed both qualitatively and quantitatively.

A survey of all available data for anthophyllite has not disclosed any specimens in which the number of oxygen atoms per unit cell is significantly in excess of 96 with the possible exception of the Glen Urquhart gedrite and the Edwards (New York) material. But it is certain that the number of cations per unit cell is normally well in excess of 60 (partial occupation of the A lattice positions), and that, although ‘excess’ water in sonic fibrous anthophyllites may be adsorbed impurity, the number of hydroxyl groups in others is well in excess of 8 per unit cell; it is also fairly clear that the number of hydroxyl groups may fall below 8 per unit cell.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 31 , Issue 233 , June 1956 , pp. 173 - 186
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1956

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References

page 173 note 1 See, for example, R. E. Grim and W. F. Bradley. Journ. Amer. Ceram. Soc., 1940, vol. 23, p. 242 [M.A. 8-295]; S. Z. Ali and G. W. Brindley. Proc. Leeds Phil. Soc., 1948, vol. 5, p. 109 [M.A. 11 104]: G. W. Brindley and S. Z. Ali, Acta Cryst., 1950, vol. 3, p. 25 [M.A. 11-104]; W. F. Bradley and R. E. Grim, Amer. Min., 1951, vol. 36, p. 182 [M.A. 11-345]; W. P.Johns, Min. Mag., 1953, vol. 30, p. 186.

page 173 note 2 L. G. Sillen and B. Aurivillius, Zeits. Krist., 1939, vol. 101, p. 483 [M.A. 7-491]; L. C. Sillwn, Inaug. Diss., Stockhohn, 1940 [M.A. 9 98]; B. Auriviltius, Arkiv Kemi, Min. Geol., 1943, vol. 16A, no. 17 [M.A. 9 44].

page 173 note 3 E. Zintland A. Udgard, Zeits. anorg. Chem., 1939, vol. 240, p. 150 [M.A. 8 117].

page 173 note 4 G. H. Francis, Min. Mag., 1955, vol. 30, p. 709.

page 174 note 1 For many elenlents modern analytical methods are more rapid, more convenient, and more ',ulaptable to microchemical procedures, but do not appear to be inherently more accurate. Ox~ the other band. many old analyses are demonstrably incomplete, and for some elements (e.g. B, Cb, and Ta) satisfactory methods of analysis have only recently been developed.

page 174 note 2 Even if a complete dehydration curve is prepared, it is often impossible to fix with any certainty the point at which loss of absorhcd water ends and loss of essential water begins; often the two stages overlap inextricably. And, as has been pointed out by G. D. Nicholls and J. Zussman (Min. Mag., 1955, vol. 30, p. 717), the amount of adsorbed water may wtry sufficiently with the state of subdivision to call for caution in derivation of empirical unit-cell contents, owing to uncertainties in the correction of the observed density for adsorbed water.

page 175 note 1 M. H. Hey, Min. Mag., 1954, vol. 30, p. 493.

page 175 note 2 Here V is the volume of the unit cell in Å3, and S the summation of the analysis.

page 176 note 1 G. H. Francis, Min. Mag., 1955, vol. 30, p. 713 (footnote).

page 176 note 2 J. C. Rabbitt, Amer. Min., 1948, vol. 33, p. 263 [M.A. 10-416].

page 176 note 3 He actually lists II analyses as 'doubtful or discredited'. Of these, nine can certainly be excluded on the grounds he assigns; but no. 88, while indeed very doubtful on many grounds, including the high value for b calc, cannot be unequivocally rejected ; and no. 85 is also doubtful, but cannot be definitely excluded.

page 178 note 1 K. Johansson, Zeits. Krist., 19B0, vol. 73, p. 31 [M.A. 4-356].

page 178 note 2 B. E. Warren and D. I. Modell, ibid., 1930, vol. 75, p. 161 [M.A. 4-463].

page 178 note 3 E. T. Allen and J. K. Clement, Amer. Journ. Sci., 1908, ser. 4, vol. 26, p. 111 .

page 178 note 4 J. C. Rabbitt did not assess the probable accuracy of his chemical data, but it may probably be assumed that the errors are of the same order as those for the Glen Urquhart gedrite (column G, table II). It is possible that some of his water determinations may be a little low, in view of his experience with no. 30; if they are, the true values for Σ (O, OH,F) would be higher, and those for Σ (cations) lower than the values in table II.

page 181 note 1 M. H. Hey, Min. Mag., 1956, vol. 31, p. 69.

page 183 note 1 M. H. Hey, Min. Mag., 1956, vol. 31, p. 69.

page 184 note 1 J. J. H. Teall, Min. Mag., 1888, vol. 8, p. 116.

page 184 note 2 To prove that Σ (O,OH,F) for any particular analysis really differs from the normal, it is essential to show that the departure from normality exceeds the probable experimental error, however we assess the distribution of the water.

page 185 note 1 Another possible explanation of some of these low water contents is that the mineral might contain fluorine, which passed undetermined; this would have very nearly the same effect as an underestimation of the water.

page 185 note 2 The additional evidence of a dehydration curve suggests that the observed densities for nos. 43 and 79a may be in error, and that this material may have Σ (O,OH,F) a little in excess of 96 (see above).

page 185 note 3 Since the b-dimension of the unit cell was calculated from the composition by means of the above regression equation, a revision of this dimension was also made.