Now here’s something unusual. Very strange black crystal structures embedded in boulders on the beach at the base of Houns-tout near Chapmans Pool in Dorset, England. Specifically, they were in boulders at Egmont Rocks which is between Chapmans Pool and Egmont Bight. The rocks have fallen from high in the cliff of Houns-tout – which used to be more of a hill than a cliff but repeated rock falls have reduced it.
I have done a bit of research and found references to crystals that have been found in the local dolomite beds where the dolomite has been replaced locally by calcite pseudomorphs forming black nodules (Melville & Freshney 1982). Some of these nodules are said to contain crystals of celestite, However, images of celestite on the internet are very different in structure to the type of crystal shown in my own photographs. This makes me think that what I saw were probably more like the calcite pseudomorphs than the celestite.
Further investigation led to another reference which described an “interesting curiosity of poikilotopic black calcite crystals in the Dolomite Beds” of the Portland Sand (West, 2013). The illustration on this website shows something similar to my find but the crystals appear to be totally embedded in the stone and not as ‘free’ as the examples that I noticed where, maybe, weathering has removed the surrounding matrix(?) Townson (1975) attributed these structures to the de-dolomitisation of the rock, and also notes the presence of some celestite.
According to the Oxford Dictionary of Earth Sciences, poikilotopic is a term applied to the fabric of a sedimentary rock in which coarse crystals of cement enclose a number of smaller, detrital grains. I am unsure whether the term should apply to the particular structures that I observed. This is a topic on which I would really welcome input.
Melville, R. V. and Freshney, E. C. (1982) The Hampshire Basin and adjoining areas, Fourth Edition, British Regional Geology Series, Institute of Geological Sciences, Natural Environment Research Council, London, HMSO.
Townson, W. G. (1975) Lithostratigraphy and deposition of the type Portlandian, J. Geol. Soc. London, Vol. 131, Pt. 6, pp619 -638.
West, Ian M. 2013. Chapman’s Pool (Chapmans Pool), Houns-tout and Egmont Bight, Kimmeridge region, Dorset; Geology of the Wessex Coast (Jurassic Coast, World Heritage Site) of southern England. Internet site. http://www.southampton.ac.uk/~imw/Chapmans-Pool.htm. Ian M. West, Romsey, Hampshire. Version: 14th December 2013.
I have recently been contacted (8th March 2015) by Geoff Townson about these black crystals. He says “I can confirm what you say about celestite and dedolomite – the best samples I collected whilst doing my doctorate at Oxford (1968-71) came from Dungy Head. The presence of celestite was confirmed by microscopic and X-ray diffraction analysis.”
Geoff Townson’s thesis of 1971: Facies Analysis of Portland Beds. D.Phil. Thesis, Univ. Oxford 284pp is available either from his website in the “About the Geologist” page or via the link http://ora.ox.ac.uk/objects/uuid:653f8ef8-7cea-4413-b23a-25b9a618bc54 where there are two pdf files that can be downloaded.
He sent two extracts from this thesis, relevant to the black crystals, noting that he had used the term poikilitic rather than the more correct, these days, poikilotopic:
(ii) Lithofacies B and associated Rock Types: Dolomites Composition: the diagnostic feature of this facies is that the rock contains ˃80% finely crystallised dolomite rhombohedra or mosaic, with a ˂ 10% subangular coarse silt or v. f. quartz sand, and ˂ 10% clay (Plate 5). The associated sand-grade minerals are as described for Facies A, and because Facies B follows Facies A in the Portland Sand Formation, this is the natural upward continuation of the presence of sand.Because the dolomite is finely crystallised it is suggested that this was originally a lime mud. The rhombohedra are often euhedral with spaces between indicative of the 13% reduction in volume which occurs when dolomitisation takes place after compaction.
A typical thin section of the rock consists of dolomite rhombs or mosaic, scattered quartz grains and occasional glauconite, but sometimes rare fragments of calcitic shells may occur as well as later, cross-cutting, ferroan-calcite veins. Dedolomitisation has taken place in certain horizons producing poikilitic calcite-cemented black nodules (the “Black calcite nodules of Arkell, 1935, etc.). These consist of calcite crystals 5 – 10 mm across, which replaced mosaic patches of dolomite but the original micro-mosaic pattern and occasional rhombs is still visible (Plates 6, 17).
(a) Dungy Head: The Beds here are very similar to those at Gad Cliff, in spite of being over half the thickness. The rock is dolomite (Facies B), consisting of a mosaic of a mean crystal size of 20µ, with much less than 1% v. F. quartz sand and silt. There are a few high angle veins of secondary ferroan-calcite. At a distance of 0.7 and 1.0 m from the top are two horizons of dedolomitisation nodules of poikilitic calcite, as seen in the Isle of Purbeck Black Dolomite Beds. However, some of these nodules, or vugs, contain euhedral crystals up to 15 mm long of a mineral identified optically and chemically as celestite (SrSO4) (See Appendix). This is the first record of celestite from the Portland Group.
Geoff Townson is not only a highly professional geologist but also a well known artist. His brightly coloured paintings reflect and celebrate his deep understanding of what constitutes and underpins our landscape.
Geoff obviously believes in keeping busy because apart from painting, he has for the last 3-4 years been running the “Lyme Regis Heritage Coast U3A” Geology Course which includes field trips between Budleigh Salterton and Lulworth – and has so far has introduced 60 “mature students” to the geology and scenery of the Mesozoic Coast. He helps out at Charmouth Heritage Coast Centre with fossil walks, usually school trips – sometimes also on desk duty – where they had over 90,000 visitors last year including some 10,000 school-trip children. He also contributes geology articles to Charmouth’s free magazine “Shoreline” which includes items on marine biology by a couple of other residents. The magazine can be downloaded as pdfs from http://www.charmouth.org/charmouth_village/shoreline_magazine.php#.VQFjxrAfxjo
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