THE OCCURRENCE OF ZINNWALDITE IN CORNWALL

Transcription

1 THE OCCURRENCE OF ZINNWALDITE IN CORNWALL By E. K. CUNDY, W. WINDLE and I. H. WARREN. Research Laboratories, English Clays Lovering Pochin & Co. Ltd., St. Austell, Cornwall. [Read 15th April, 1959] ABSTRACT A lithium mica extracted magnetically from china stone is compared chemically and by X-ray methods with known lithium micas. This comparison shows a strong resemblance between the Cornish mica and zinnwatdite from the Zinnwald. Most publications on the geology of the St. Austell granite have pointed out the occurrence of lithium-bearing micas in the district. De la Beche in 1850 described a lithium-bearing pegmatite vein in the Trelavour Downs near St. Dennis while Ussher, Barrow and MacAlister (1909) noted that some of the Cornish biotites contained lithium. Richardson (1923) went further and divided the St. Austell granite mass into lithionite and non-lithionite types. The granite, which is thought to have been intruded into Lower De, onian rocks during the Carboniferous period, has recently been described, with particular reference to the china-stone region between Nanpean, St. Stephens and St. Dennis, in a series of papers by Keeling (1954) and Keeling and Exley (1954). The china-stone type of granite, which is used in the potteries as a flux, is characterized by the absence of iron and boron compounds, particularly tourmaline, and by the presence of lithium and fluorine compounds. This portion of the granite is thought to be either of later date than the remainder or to be the area which crystallized last. Interspaced throughout the whole region in a random manner, however, and forming a considerable proportion of the whole is a granite, known locally as shellstone, which is kaolinized in some areas. While commercial china stone contains a mica of the muscovite type with very little iron or fluorine, this granite contains a characteristic golden brown mica which, in contrast to muscovite, contains iron and fluorine as well as lithium. The shellstone mica is a primary mica with crystals of average diameter mm, and if the granite is crushed to below this size it can be extracted by magnetic or flotation methods. Table 1 gives 151

2 152 E.K. CUNDY, W. WINDLE AND I. H. WARREN TABLE l---chemical analyses of Zinnwald and Cornish lithium micas. Zinnwald* Shellstone Trelavour SiO~ A03 Fe~O z FeO TiO~ MgO MnO CaO Li20 Na~O KzO 46 " ! "19 "22 0"37 3' '37 0"89 6"75 49"80 20 '70 0'90 5 "70 0"30 0"20 0"58 <0" 4" 0"26 "30 2"90 6 "80 38'90 22" "80 0"36 0"50 0"34 1' '33 9'32 4'58 4'00 Total --OforF 2" "64 2' Total 99"72 99" *After Winchell (1942). the chemical analysis of zinnwaldite (Winchell, 1942), lithium mica from shellstone and Trelavour mica, which is the best known of the Cornish lithium micas. The similarity between this shellstone mica and the German zinnwaldite is striking. The latter is a pneumatolytic mineral found in the cassiterite-and topaz-bearing pegmatites and granites of the Erzgebirge of Saxony at Zinnwald and Altenburg; a similar mineral is found in Greenland and Alaska. Although there are no known TABLE 2--X-ray powder diffraction data fm various lithium micas. Three-layer hexagonal Single-layer lepidolite* zinnwaldite* Shellstone mica d(a) I d(a) t d(a) 1 4" s 3.33 vs 3. s 2.93 m 2.71 vw 3-91 w 3-65 m 3-39 m 3'15 s 2.90 s 2-70 m 3-85 vw 3-63 w 3.33 wm 3.30 vs w 2.67 vw *After Grim, Bradley and Brown (1951).

3 ZINNWALDITE 153 metalliferous veins in this Cornish granite, there are traces of tin and wolfram and these metals have been worked in the district. Keeling and Exley (1954), following the classification of Winchell and Winchell (1951), of the lepidolites, identified the shellstone mica TABLE 3--X-ray powder diffraction data for various micas. Zinnwaldite from Zinnwald d(~) Zinnwaldite from Cornwall Lepidolite from South Africa d~a) Trelavour mica 9"82 4'92 4'51 4"36 3 '87 3 '63 3'34 3 ' lc "85 7'13 4"935 4"50 4' '51 9"87 4"965 4 "46 4"29 3"84 3"615 3"32 3'195 3"08 2"88 2"78 2" il 2" " " " " " " " " " ll as protolithionite and used this name to distinguish it from other micas in the St. Austell area. When this mica was first examined by X-ray diffraction methods its identification within the group was somewhat indefinite and reference to diagnostic data for lithium micas (Grim, Bradley and Brown, 1951) did not solve the problem. Table 2 indicates that there is a significant difference between the data for zinnwaldite and

4 154 E.K. CUNDY, W. WINDLE AND I. H. WARREN those for the shellstone mica. This applies particularly in the region between 3-39/~ and 3.15/~. A further study of the literature then available, and particularly of the paper by Levinson (1955), showed 3"324"9~ 65 9"87 B FIG. 1--Microdensitometer tracings for: A--lepidolite, South Africa; B--zinnwaldite, Zinnwald; C--zinnwaldite, Cornwall; D--the same, heated to 650~ a greater similarity between his 1M lepidolite and the shellstone mica. Eventually, a direct comparison with zinnwaldite from the Zinnwald and lepidolite from the Valley of Orange, South Africa,

5 ZINNWALDITE 155 was possible. The former is a typical sheet type of brown flaky mica, while the latter is inclined to be blocky and is light mauve in colour. X-ray examination of these two micas showed a very close resemblance between the Zinnwald mica and th", shellstone mica; consequently, the latter is now referred to as zinnwaldite. The X-ray data (Table 3) were deri~ed from films taken in a 19cm camera with filtered CoKa radiation, intensities being obtained from microdensitometer tracings. The latter show a larger difference between the major and minor intensities than would be expected from visual examination of the films. On the other hand, major differences exist between the zinnwaldites and Trelavour mica, which is structurally similar to biotite and should presumably be grouped with lithium-containing biotites. TABLE 4---X-ray powder diffraction data for Cornish zinnwaldite heated to 1,000~ d(/~,) I 4" " " " "60 8 2" " " "85 9 1" " "62 15 The microdensitometer records in Fig. 1 show more conclusively than the Tables how well the two samples of zinnwaldite are matched. Cornish zinnwaldite and the lepidolite, on the other hand, show significant differences. Apart from major differences in intensity level, the lepidolite does not show two spacings very close to one another in the 3-3 A region as do the zinnwaldites; moreover, the former has a weak line at A which does not occur with the zinnwaldites. Other differences are also apparent. Separate samples of the Cornish zinnwaldites were heated to 650~ 750~ 850~ and 1,000~ in order to determine the breakdown temperature. Comparison of the microdensitometer record from the 650~ sample with the original (Fig. 1) shows that the two are almost identical except for a slight overall increase in line intensity of the heated sample; the sample heated to 850~ showed a similar trend. At 1,000~ the zinnwaldite structure has completely disappeared and a new phase has developed. This gives a poor X-ray diffraction pattern from which the data in Table 4 have been

6 156 E.K. CUNDY, W. WINDLE AND I. H. WARREN obtained but it has not yet been possible to identify the structure of this phase. Acknowledgment.--The thanks of the authors are due to the Directors of English Clays Lovering Pochin & Co. Ltd., for permission to publish this paper. REFERENCES GRIM, R. E., BRADLEY, W. F., and BROWN, G., X-ray Identification and Crystal Structures of Clay Minerals (G. W. Brindley, editor). Mineralogical Society, London. Chapter 5, p LEVlNSON, A. A., Amer. Min., 40, 41. KEELING, P. S., Trans. Brit. Ceram. Soc., 53, 67. KEELING, P. S., and EXLEY, C., The British Ceramic Research Association Research Papers 238, 241, and 281. RICHARDSON, W. A., A.J.C.S., 79, 546. USSnER, W. A. E., BARROW, G., and MACALISTER, D. A., The Geology of the Country around Bodmin and St. Austell. Mere. geol. Surv., U.K. WINCnELL, A. N., Amer. Min. 27, 114. WINCHEr.L, A. N., and WINCHELL, H., Elements of Optical Mineralogy. Wiley, New York, 4th Ed., Part 2, p. 370.