Childrenite in South-West England

R. S. W. Braithwaite and B. V. Cooper
Chemistry Department, University of Manchester Institute of Science and Technology, Manchester M60 1QD
Torquay Natural History Museum, Babbacombe Road, Torquay TQ1 1HG

Synopsis: Childrenite (Fe,Mn)AlPO4,(OH)2.2H2O, the iron-rich end-member of the childrenite-eosphorite diadochic series, was first discovered by Brooke (1823) on specimens from near Tavistock in Devon. Subsequently it has been recorded from a small number of other British localities, mostly in the Tavistock area, and also from near St. Austell in Cornwall, and from one locality in Cumbria. The Sir Arthur Russell collection, now in the British Museum (Nat. Hist.), has specimens from a few additional localities, all near Tavistock, and the Geological Museum, Institute of Geological Sciences, has a specimen from Wheal Jane, near Truro.

Some specimens found in July 1973 and sub-sequenfly, on old mine dumps in the Callington-Gunnislake area in Cornwall, a few miles to the west of Tavistock, proved to be of childrenite. These finds prompted a survey of the area between Callington and Dartmoor in order to determine the distribution of childrenite. The results of this survey indicate that childrenite is more widely distributed than formerly supposed, a considerable number of new localities being discovered. Previously recorded localities in the area were also re-examined, including the Tavistock Canal Tunnel, reputed to be the original locality.

Francolite (carbonate-fluorapatite) is a paragenetically related phosphate mineral also classically from this area, and its distribution and paragenetic relationship to childrenite have also been examined.

During the course of this investigation various associated minerals have been observed, the most interesting being chalcoalumite from South Wheal Crebor, near Tavistock. This is the first record of chalcoalumite from Britain, it being recorded previously only from Arizona and recently from Belgium. This find is intended to be the subject of a further paper.

Details of all these occurrences are to be found in the Miniprint section of this paper, and the distribution of childrenite and apatite localities in this area is shown in fig. 1.

Childrenite crystals from four scattered localities in SW England have been analysed by electron probe microanalysis. These analyses are in agreement with older published analyses of childrenite from the area, and are reasonably constant in Fe and Mn values and ratios, independent of locality or country rock. Our analyses of the childrenites all give values between Ch81Eo19 and Ch89Eo11, the older analyses between Ch85Eo15 and Ch90Eo10 with one exception of dubious accuracy. P and Al values are near theoretical, leaving no room for diadocbic replacement, e.g. with arsenic, common in the main phase of mineralization in the area. Dr M. H. Hey has analysed chemically a number of childrenites from the area, distinguishing between oxidation states, and has found very variable and often considerable proportions of Fe(III), which can even exceed the Fe(II) content. This Fe(III) is almost certainly the result of super-gene oxidation.

The infrared spectra of childrenite from several localities, including analysed samples, have been measured and compared with those of eosphorites, one of which was analysed by electron probe microanalysis. The spectra are characteristic and useful for identification purposes, but the differences between the spectra of childrenite and of eosphorite are not sufficient to determine the position of a sample in the series with any appreciable accuracy. The observed splitting of spectroscopic degeneracy of PO4 absorptions is consistent with a phosphate anion site symmetry of Cs, in agreement with published structural data from X-ray diffraction measurements.

The childrenite is always found close to the granite margins. This distribution, which also cuts across the sediments, coupled with the paragenesis and the arsenic-free and constant composition, independent of locality, indicates that it is the result of hydrothermal phosphatization of the metasediments, connected with the intrusion of the granites, but later than the main mineralization. Francolite may take the place of childrenite in this stage of mineralization if conditions are more suitable for its preferential formation.

Mineralogical Magazine; March 1982 v. 46; no. 338; p. 119-126; DOI: 10.1180/minmag.1982.046.338.18
© 1982, The Mineralogical Society
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