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On stilpnomelane from North Wales1

Published online by Cambridge University Press:  14 March 2018

A. F. Hallimond
A. F. Hallimond*
Affiliation:
Museum of Practical Geology, London
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Extract

Pisolitic ironstones occur at various localities in North Wales, at about the horizon of Nemagraptus gracilis. The rocks have undergone considerable folding and the hard ironstones are often faulted and in many cases deformed. Greenstone dikes frequently intersect the beds, usually with little visible alteration of the ore, though the ironstones are generally hardened, with incipient re-crystallization of the chamosite. At Pen-y-rallt iron mine, situated half a mile east of Llanfrothen church near Penrhyndreudraeth in Merionethshire, a steeply inclined pisolitic ironstone bed rests upon a mass of igneous rock; and here the alteration of the ore is in a more advanced stage, for the ironstone is in places seamed with roughly parallel asbestiform veins (up to 1 inch) which contain a brittle dark-brown mineral strongly resembling biotite ; the centre of the vein is sometimes filled with chalybite.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 20 , Issue 104 , March 1924 , pp. 193 - 197
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1924

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Footnotes

With a chemical analysis by F. R. Ennos, B.A., B.Sc., A.I.C.

1

Communicated by permission of the Director, H.M. Geological Survey.

References

Page 193 note 2 See Mem. Geol. Survey, Special Reports on Mineral Resources, 1920, vol. 13.

Page 193 note 3 Discussion on paper by Sherlock, R. L., Quart. Journ. Geol. Soc., 1919, vol. 74, pp. 106–115 CrossRefGoogle Scholar.

Page 193 note 4 The occurrence of this mineral had been observed by Mr. G. J. Williams, of Bangor, and by Mr. Arthur Russell ; the latter very kindly put at the author's disposal a specimen of similar material in a vein about half an inch thick, between white vein dolomite and oolitic ironstone, from Cross Foxes mine near Dolgelly, Merionethshire. The colour is not quite so dark as in the material now described ; ω 1.66, ∊ 1.56 approx. Feathery growths of the same mineral, appear in the oolitic ore itself.

Page 194 note 1 This is possibly thuringite or an aluminous variety of stilpnomelane, but could not be isolated for analysis.

Page 194 note 2 E. S. Larsen (Bull. U.S. Geol. Survey, 1921, no. 679) records: ‘Stilpnomelane ’, Nassau, ω 1.69 dark brown, ∊ 1.60 yellowish. ‘Chalcodite’, Antwerp, N.Y., ω 1.76 dark red-brown, ∊1.63 pale yellowish. ‘Chalcodite’, North Carolina α 1.65 pale yellowish, β and γ 1.78 deep red-brown.

Page 197 note 1 Kretschmer, F., Neues Jahrb. Min., 1918, pp. 19–42 [Min. Abstr., vol. 1, p. 254]Google Scholar ; Centralblatt Min., 1905, pp. 195-204, 1906, pp. 298-811.

Page 197 note 2 See Min. Abstr., 1922, vol. 1, p. 264.