Wednesday, 27 February 2013

In search of bentonite?

Can anyone help with this please?
"I'd like to obtain, if possible, some small hand specimens of bentonite (the soft smectite-rich clay with swelling properties, widely used in ground engineering and as an absorbent, and traditionally in our area for 'fulling' sheep's fleeces).
I could easily buy some cat litter bentonite from a pet shop, but this is ground down to gravelly particles - I'd ideally like to have specimens of the rock in its primary form as dug out from the ground.
I'm aware of the bentonite mine that once operated at Coombe Hay near Bath, but which has long been closed. Do you know anyone (e.g. the owner, local cavers) who has access into the mine? Id value details so I could contact them to see if I could get some specimens. There may also be somewhere where some of the bentonite has been left (e.g. an old storehouse near the mine) - this would be ideal as a source of specimens and would make unnecessary any potential visit into the mine."
If you can help, please email.


Ian said...

I'm not sure that Bentonite and Fullers' Earth are the same thing. Yes they are both similar clays, but I believe that under the electron microscope there are obvious differences. There's plenty of Fullers' Earth around Bath - I found some kilogram blocks by road workings a couple of years ago at Hinton Charterhouse. The road from Wellow had collapsed where it climbs the hill from the valley bottom, just west of the village of Hinton Charterhouse, This is because Fullers' Earth is very slippery, and roads crossing it are prone to subsidence.
Be aware too, that the Fuller's Earth Clay - which is what they were mining, is only a small level in the much larger Fuller's Earth Formation. When freshly exposed and moist it is a pale grey colour. In air, however this rapidly changes colour to greenish then yellow, like putty. This is because underground the iron in it, free from oxygen is in the Ferrous state, that is Fe++ or Fe (II)+ In air this rapidly "rusts" to the pale yellow Ferric state - Fe+++ or Iron (III) as it is known nowadays.
I have a tiny specimen of powdered Fullers' Earth, from one of the mills in Bath. It is grey, not yellow.
Talk to your local Highways Dept - they'll be sure to know where you can find some more - so frequently does it cause landslips!

Nick said...

My understanding of Bentonite is that it is a mudrock in which the most abundant constituent is clay from the smectite group (the latter includes montmorillonite), and which is produced from the diagenetic alteration of mafic volcanic ash. (See, for example, Maurice Tucker's 1991 Sedimentary Petrology introduction textbook p.105 and p. 226).
The stratigraphic unit once worked in the Coombe Hay mine is c. 2m thick (reaching 3m further east), with the lowermost 1m, having a slight greenish tinge when fresh from the mine, containing c. 80% montmorillonite (see Bristol University's 1977 Geological Excursions handbook p.124.) Past research on the Coombe Hay unit showed it to have a volcanic origin (see paper by Hallam & Sellwood 1968 in Nature volume 220). This unit lies within the Fuller's Earth Formation (c.35m thick in the Bath area), of Middle Jurassic age: only very subordinate parts (laterally and vertically) of which are bentonites, the remainder being a variety of other silicate mudrocks, with subordinate limestones, the name 'Fuller's Earth Formation' being a convenient label for these strata lying between the limestone-dominated formations immediately above and below. When the M4 was widened at Tormarton in June 2005, I examined the temporary exposures, these passing continuously through most of the Fuller's Earth Formation; most of the mudrocks I saw there were a blue-grey colour, darker towards the base, with some lighter grey colours coming in towards the top. I've recently found that in a working quarry in the north Cotswolds, there is a 3m thick bentonite band exposed in the Fuller's Earth Formation; the highest content of smectite occurs in the middle 1m. I'll be able to collect some from there!