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Mineralogical ClassificationLizardite: Lizard peninsula, Cornwall- England.
30th Aug 2014 23:22 UTCGeorge deWit Chaney
George deWit.
31st Aug 2014 00:40 UTCJolyon Ralph Founder
31st Aug 2014 12:31 UTCPaul De Bondt Manager
Happy to hear you are in Cornwall. Wish I was there.
I hope you have better wether than on the mainland.
Jolyon is right. I found serpentine on a lot of localities in Cornwall. Never a pure specimen of lizardite tough.
This could be a good place to look : http://www.mindat.org/loc-1355.html but assure yourself that the locality is not on private land and/or collecting is allowed. Seems to be a rich specimen.
What you can do too is going to the Lizard point. There are some serpentine garving workshops. As the Cornish people are gentle and helpfull, they could probably pass you a specimen as they are worthless for garving.
Meanwhile, take a look around, it's fantastic up there.
Good luck.
Take care and best regards.
Paul.
5th Sep 2014 09:05 UTCPeter Haas
First of all, it is important to know that the outcrops of vein serpentine are not as vividly coloured as in the specimen Paul referred to above. Thus, if you go only for the bright greens, you will miss the wood for the trees and most likely not find anything at all. In fact, vein serpentine is abundant in serpentinites all over the Lizard peninsula, but the outcrops are typically of a pale green or white colour, due to weathering and subsequent chemical breakdown of the minerals. It's those veins you have to look for, and if you break them up, you may find attractive material of a nice green colour inside.
This having said, Kennack Cove is as good as any other place along the coast to look for vein serpentine. While the serpentinite quarries inland are long abandoned, as are the numerous small workings along the coast (they were for soapstone, later for talc, and many of them date back to the early 18th century), and considering that all the localities are frequently visited by collectors, one actually would not expect to find productive outcrops. However, serpentinite is a notoriously unstable rock and the walls tend to collapse, which is more likely to happen on the coast than inland. In addition, the sea does the job of breaking up the cliff fall material for you, and many good specimens are found in the beach pebbles.
Second, this pretty green material is still a mixture of chrysotile and lizardite, possibly with an excess of chrysotile . As has been documented for vein serpentines from a number of localities (not only Cornish), they consist of short chrysotile fibers with mostly random orientations (this is why the material does not look fibrous in the hand specimen) and a variable amount of lizardite plates. The variation in colours is caused by variable molar fractions of trace iron, but iron is present in both species and therefore not indicative of either mineral.
Pure lizardite has been found at Kennack Cove, but it looks much different. The following text is from the documentation of the find by H.G. Midgley et al.in Mineralogical Magazine 29 (1951):
"A white soapy mineral which could not easily be identified in the hand-specimen was collected from a vein in the serpentine rock at Kennack Cove, Lizard, in April 1949. From a preliminary optical examination the mineral was thought to be saponite, and therefore of interest to clay mineralogists <...> The mineral occurs in a vertical vein cutting the serpentine rock on the north-east side of Kennack Sands. The vein is about 2-3 inches wide and shows up very white against the dark serpentine rock. The mineral is pure white in colour, soapy to the touch, and soft (H. 2). It is an irregular aggregate of small crystals, which are for the most part hexagonal plates."
(This paper predates the type description of lizardite; it can be downloaded here: http://www.minersoc.org/pages/Archive-MM/Volume_29/29-212-526.htm )
In other words, if you want a lizardite-only specimen for your collection, you'll have to search white veins in the serpentinites for a white waxy material resembling saponite - and then send it for analysis, since there is no way to confirm it as lizardite by visual means.
At Kennack, the serpentinites are mostly in the cliffs at the northeastern end of the beach. In the southwestern part, the base of the cliff consists of gneiss and the top of peridotite (this is one of the precursor rocks of the serpentinite). There is a photo in the following document that will help you to discern these rocks (scroll to the bottom of page 6): http://projects.exeter.ac.uk/geomincentre/06The%20Lizard.pdf
In addition, there is an outcrop of gabbro which is located in the area where the basic dyke is indicated on the photo. This rock is almost black (you can see it at the righthand edge of the photo). Both the peridotite and the gabbro do also contain white vein(let)s. In the gabbro, which is a basic rock, the vein material is calcite; though, in the peridotite, which is an ultrabasic rock, the vein material is dolomite. Both vein types contain fragments of both serpentinite (the rock) and vein serpentine (the transparent chrysotile-lizardite mixture). Since the fragments are small and the veins very thin, trying to find specimens there is probably not worth the effort. However, they would look very nice, as they would have the green serpentine on a light ground mass. If you want to give it a try, don't hammer the cliff face but search in the beach pebbles instead: they have all of the rocks and come in handy sizes.
Finally, you can also find vein serpentine in the beach pebbles at the eastern cliff. When you search the pebbles, however, you should NOT look for the white stuff. This is because the white material that is so abundant in the outcrops in the cliff serpentinites does not withstand the action of the sea; it easily breaks apart and is carried away. The pebbles preserve the unaltered green vein serpentine instead. Unfortunately, the rock easily breaks along the veins and you'll very rarely (ever ?) find pebbles with serpentine veins still running through them. Instead, many pebbles preserve thin patches of transparent green serpentine on their outer surface. While this stuff is relatively easy to find, the serpentine patches are small and lack a contrast between the serpentine and matrix.
5th Sep 2014 11:12 UTCDale Foster Manager
-------------------------------------------------------
I found serpentine on a lot of localities in Cornwall.
Out of curiosity do you mean a lot of localities on the Lizard peninsula, or generally through Cornwall?
I have noticed that serpentine bearing aggregates have been taken to a lot of places and dumped as hardcore for roads and paths, but it isn't naturally found in any other part of Cornwall than the Lizard.
5th Sep 2014 16:28 UTCDavid Baldwin
The quarry is North of the first beach at Kennack. If you head East onto the second beach, the East cliff at the far end of the beach is a good place to explore. I visited this year and found a vein of hematite, and also some of what I think is white prehnite in crystalline sheets. Last year I found nodule of suspected green prehnite in the same place. http://www.mindat.org/forum.php?read,11,303082,303240#msg-303240
5th Sep 2014 17:44 UTCPaul De Bondt Manager
Peter, thank you for the detailed information. As always, it's with a 100% accuracy.
Next time in Cornwall, probably next year, I will look up the places you mention.
It will be an excellent geological journey.
Thank you.
Dale, I found the serpentine only on the Lizard. Sorry if I have not been accurate enough.
BTW, I like your cassiterites from Cornwall.
David, I will look around your spot too. Seems interesting.
Take care and best regards.
Paul.
5th Sep 2014 20:07 UTCDavid Baldwin
Having read your comments about Lizardite, is it likely that my specimen is not what I think? http://www.mindat.org/photo-559851.html
The vein where I collected this was a few cm across and consitently yellow/green in colour.
5th Sep 2014 21:08 UTCPeter Haas
Err ... what ?? I think the right answer to the double negation is 'No' ... :-S
Well, it's better to name it Lizardite + Chrysotile (and add a short note to the description, saying that it is a mixture of both). While it contains lizardite, this is not what you see - you only see the mixture. Leaving it as it is, it will only lead others into thinking that the green is lizardite. They will put this on their labels, swap specimens, etc. ... and in the end this will result in the diffusion of information, which, even if admittedly not outright wrong, is also not quite right.
> The vein where I collected this was a few cm
> across and consitently yellow/green in colour.
Can happen in a fresh outcrop. You were lucky!. Could also be that another collector felt equally lucky when he opened up the vein before and left it standing for collectors to come. When I make good finds, I always leave a pile of representative material in a conspicuous place. After all, where's the point in carrying (n+1) specimens home when it's essentially the same material ?
6th Sep 2014 11:02 UTCDavid Baldwin
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Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: April 17, 2024 22:03:29