Gossip meeting “Rocks and minerals”
Tuesday 13th August 2019
The Club President Joan Bingley opened this meeting in the PA135 Meeting Room in the Natural History Museum, attended by a good crowd of members, several of whom brought exhibits.
The meeting started with a talk by Kit Brownlee, introducing us to the basics of geology, rocks and minerals.
You can see Kit’s slides here:
Click the arrows to move through the slides; click the symbol at bottom right for a larger version
After the introduction, we were able to examine the range of rocks, minerals and slides that members had brought.
Lisa and Nigel Ashby brought a boxed set of 36 slides by Watson, Series 53.—The Prospector’s Set of Rock Slides. Unusually, the specimens are crushed rocks, not sections, and they were intended as an aid to identification in the field. Lisa and Nigel also brought a 1914-1915 Watson catalogue of microscopic objects and slides.
The Prospector’s Set of Rock Slides
Joan Bingley was recently asked to sell a Vickers polarising microscope (it will be at Microscopium) to raise money for the Civil Engineers Benevolent Fund, and in the bag with the paperwork she found some rock sections relating to the construction of the Mudhiq Dam in Saudi Arabia. Joan brought the slides so that we could examine them with one of the Club’s stereomicroscopes.
Robertson Research slides
Kit Brownlee’s exhibit
Kit Brownlee’s micromounts and minerals
Kit also showed 2 books, “A Field Guide in Colour to Minerals, Rocks and Precious Stones” by J. Bauer (easy to use because it starts from colour) and “A Colour Atlas of Rocks and Minerals in Thin Section” by W. S. MacKenzie & A. E. Adams (a classic text) and 2 Field Studies Council fold-out guides, Minerals Identification Chart and Rocks Chart.
Alan Wood with Kit’s exhibit [by Danny Ferri]
Danny Ferri showed some rocks that he had bought in a charity shop, some of which turned out to be interesting specimens.
Danny Ferri’s exhibit
Dennis Fullwood brought some museum-quality moldavites (tektites from a meteorite strike in southern Germany) from his collection. Dennis also brought a book, “Moldavites – The Czech Tektites” by Vladimir Bouska.
Pam Hamer took part in the North York Moors Microscopy Weekend in July, and during the visit to Runswick Bay she collected mudstone that had fallen from the cliff and contained fossils. She also found a sliver with a thin layer of brown and clear crystals; the brown substance was probably an iron compound, and after examining the clear crystals and estimating their RI and hardness Pam concluded that they were probably gypsum. Pam brought along photographs and notes, and her slides so that we could examine them under the Club’s Swift polarising microscope.
Pam Hamer’s exhibit
Pam has recently bought a copy of “Geology of the country around Hitchin” by the British Geological Survey to help find fossiliferous deposits within cycling distance of her home, and she brought some samples of chalk for us to examine.
Pam Hamer’s chalk samples
Nigel Ashby, Pam Hamer and Lisa Ashby
Stephen Parker used his Goren portable polarising microscope to show thin rock sections from six Japanese teaching sets that included small polished slabs of the rocks as well as the slides.
Stephen Parker’s exhibit
Stephen Parker’s slides
Stephen also brought a book, “Collins Discovery Guides: Rocks and Minerals of the World” by Adrian Jones, that includes photos and fact files for 220 common rocks, minerals and gemstones.
Mark Shephard’s exhibit
Mark Shephard’s slides
Paul Smith’s exhibit
Paul also showed a bright, tiltable LED light from his local pound shop.
360° Orb COB Robot Light
Alan Wood has only a few rock sections, all bought cheaply on eBay or at club meetings, and brought them all. None of them include the name of the mounter, but Pam Hamer believed that 4 of them were by C. H. Caffyn (the Club has an old database of its Caffyn collection).
Alan Wood’s exhibit
Thin rock sections by Charles Henry Caffyn
Many thin rock slides show nice colours under polarised light, so Alan brought his N.B.S. Polarspex and a small LED lamp. The Polarspex has crossed polarisers held in a card mount with a gap to insert a slide. It can be held up to a light, or used on the stage of a stereomicroscope or a compound microscope with low-power objectives.
The N.B.S. ‘POLARSPEX’
Most rock sections look dull using normal transmitted light, but crossed polarisers introduce all sorts of colours, and adding a retarder can add even more colours, as shown in these 3 photomicrographs of the same view of a thin section of the aluminosilicate rock sillimanite.
Thin section of sillimanite
Thin section of sillimanite with crossed polarisers
Thin section of sillimanite with crossed polarisers and a retarder
Alan also brought 2 rock samples, peacock ore (an important copper ore that tarnishes to iridescent blue and purple colours) and pyromorphite (lead chlorophosphate that forms green, yellow and brown crystals).
Peacock ore (bornite)
Report and most photographs by Alan Wood