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MINERAL NEWS
Mick Wolfe • Mark Oddy • Roy Starkey
SADDLEBACK MINE
Peter Briscoe • David McCallum • Paul Nicholson
STEREOMICROSCOPES FOR MINERALOGY
David Green • Mark Oddy • Nigel Hoppé
NORTH DEVON UNITED MINE
Mike Rumsey • Mike Savage

Front cover of UKJMM No. 29:  Golden yellow barite crystal 55 mm tall.
64 pages, full colour.

UKJMM No. 29
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Editorial:
Stereomicroscopes

David Green

The late Neil Yedlin, whose micromounting column in The Mineralogical Record may be familiar to older readers, would exhort collectors to buy and use a good mineral book, advice which remains as good today as it did thirty years ago. It also provides the subject of this editorial in a rather oblique manner, borrowing the words ‘buy and use’ to which is added (with an ironic flourish as Yedlin was a supreme micromounter) ‘a good stereomicroscope’. Modern stereomicroscopes provide superb optical quality and in relative terms they are less expensive than at any time in history. In other areas of microscopy, complex specimen preparation is the norm, but a stereomicroscope simply requires the user to sit down at a bench and look. Specimens are revealed in breathtaking detail, opening up a whole new world…

Assembling a mineral collection usually involves both scientific and aesthetic decisions. Why else would factors such as rarity and damage be so important? There is no single item of equipment that is more useful when investigating the scientific and aesthetic qualities of a specimen. A stereomicroscope invites closer investigation, and this almost automatically leads to a deeper understanding of mineralogy.

Occasionally collectors take the view that stereomicroscopes are purely for those who collect micromounts. Nothing could be further from the truth! It’s hard to imagine a collector who wouldn’t benefit from investigating his or her specimens using a stereomicroscope. Unusual assemblages and crystal morphologies are revealed, rare species and associations can be investigated, and any attempts at fakery are quickly apparent. The number of completely new species which have been discovered in this way is considerable: the mineral ‘scrutinyite’ even records the process formally in a name, which alludes to the careful investigation of the specimen that was required to find and characterise it.

Many collectors are content simply to use their microscope for visual observation, but with the addition of a few accessories it can do much more. It is relatively easy, for example, to do a variety of micro-chemical tests. The simple chemical tests for copper, lead and carbonate minerals should be part of every mineralogist’s toolkit. Add polarising filters and simple optical investigations become possible. These almost inevitably lead into the chemistry and physics of minerals and the process of scientific investigation.

Improvements in manufacturing technique and lens design in the last two decades together with the competition brought about by manufacture in the Far East have made good quality stereomicroscopes relatively cheap. The rise in the expectations of the semiconductor and biotechnology industries has produced considerable improvements in capabilities of high-end instruments and these have fed into the collector market. A good stereomicroscope can be purchased new for less than a thousand pounds and a second-hand instrument (if it is well looked after) for perhaps a little over half the new price. The author recently purchased a small, light stereomicroscope to take on field trips for a little under £50 (second-hand), it produces a crisp image at a single magnification of 20x and cost about the same as the petrol required to pick it up.

Twenty five years ago the cost of a good stereomicroscope was perhaps five to ten times that of a ‘fine British mineral specimen’, today they are about equal. In relative terms, microscopes, like cameras, computers and other manufactured goods, have become less expensive and more capable. You get an awful lot of bang for your buck! So at the conclusion to this piece I would strongly recommend any reader who does not already have one to buy and use a good stereomicroscope. It will add immeasurably to your enjoyment of mineralogy.

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Mineral News

Mick Wolfe
Mark Oddy
Roy Starkey

A remarkable number of new discoveries were made in 2007. These include world class botallackite from Cligga Mine; aragonite from Port Quin; rhodochrosite from Durnford Quarry; and smithsonite from Middlesmoor Mine. Two excellent new galleries in London and Glasgow are also reviewed.
19 pages.
Left: An unusual stalactitic smithsonite specimen, 45 mm tall, which has been cut and polished to show an internal greenish-yellow to grey colour zoning.
Right: Deep green bladed botallackite up to about 3 mm long forming a rosette 10 mm across on microcrystalline connellite. Cligga Head, Perranporth, Cornwall.

Left: White well terminated aragonite crystals on a specimen 35 mm across from Port Quin, Cornwall.
Right: Botryoidal pink rhodochrosite 45 mm across with black acicular manganite and occasional overgrowths of white botryoidal barite. Chris Finch collection.

Double page spreads from Mineral News

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Mineralisation in the
Upper Glenderamackin Valley, Mungrisdale, Cumbria

Peter Briscoe
David McCallum
Paul Nicholson

Primary lead and copper sulphide mineralisation is present in quartz-barite veins in Ordovician mudstone rocks of the Skiddaw Group at Saddleback Old Mine in the upper Glenderamackin valley, Mungrisdale, Cumbria. At the shaft dump, a sparse supergene assemblage comprising brochantite, caledonite, cerussite, chenite, connellite, cuprite, goethite, leadhillite, mimetite, malachite, pyromorphite and pseudomalachite has developed in cavities near partially oxidised lead and copper sulphides. In cavities that are spatially separated from the sulphides, pyromorphite and pseudomalachite are the dominant supergene minerals, the latter occasionally occurring in rich micro-crystalline crusts.

About 100 m upstream from the shaft dump a vein crops out in the river bank. Lead mineralisation is conspicuous at this point and it displays the commonly observed galena-cerussite-pyromorphite oxidation sequence.

At the head of the Glenderamackin valley, about half a kilometre beyond the mine site, manganese-rich quartz veins (containing lithiophorite and hollandite) host small quantities of supergene lead mineralisation. Hinsdalite and corkite pseudomorphs after pyromorphite are abundant and plumbogummite and plumbojarosite are also present.

An examination of the supergene mineralisation suggests that the leaching of copper and lead into the environment is buffered by phosphate. The stable and insoluble phosphates pyromorphite and pseudomalachite predominate at the shaft dump and a well defined galena-cerussite-pyromorphite oxidation sequence has developed at the nearby vein exposure. The supergene lead mineralisation at the head of the valley is dominated by another phosphate, hinsdalite. These assemblages probably reflect enhanced phosphate concentrations in the Skiddaw Group sediments.
12 pages.

Left. An unusual hexagonal cerussite prism 0.5 mm long. Paul Nicholson collection.
Right. Bright green hexagonal pyromorphite crystals up to 0.7 mm across with paler outer zones. Peter Briscoe collection.


Double page spreads from the article.

 

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Choosing and Using a
Stereomicroscope for Mineralogy

David Green
Mark Oddy
Nigel Hoppé


A stereomicroscope is made by arranging two low-power microscopes side by side so that they produce identical magnified images of the same area, viewed from a slightly different angle. The two images are fused into a three-dimensional representation by the observer and it is this three-dimensional effect that distinguishes stereomicroscopes from conventional compound microscopes. Advances in manufacturing techniques and optical design over the last decade have produced significant improvements in the zoom range, resolution and ergonomics of the best instruments at a cost that has decreased in real terms. Stereomicroscopes are ideally suited to the detailed examination of irregular objects such as mineral specimens. They are widely used by collectors and have resulted in the discovery of a considerable number of new and unusual minerals. An appreciation of the link between resolution and depth of field and of the optical limitations generated by the interplay between these parameters is useful to anyone who wants to get the best out of their instrument.
12 pages.

Left. A core-bit twin harmotome crystal 9 mm long perched on hexagonal brown calcite. The detailed morphology of the core-bit twin and the tiny pink ancylite crystal on the left hand face of the main calcite crystal were only evident after careful scrutiny with a microscope. David McCallum collection.
Right. Bright yellow boltwoodite crystals several millimetres in length from South Africa. Bruce Charlier collection.

Left. Radiating fans of chalcosiderite to 1 mm with smaller yellow cyrilovite crystals from Gunheath Pit, Cornwall.
Right. Pale blue trigonal susannite 2 mm tall on quartz from Red Gill Mine, Caldbeck Fells, Cumbria. Peter Briscoe collection.

Double page spreads from the article.

 

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The First British Occurrence of Feitknechtite at
Benallt Manganese Mine, Rhiw, Pen Llyn, Gwynedd, Wales

Tom Cotterell

The rare manganese oxide hydroxide mineral feitknechtite has been discovered intergrown with hausmannite in material collected from Benallt manganese mine during its last phase of working in the 1940s. The Benallt material is identical to the mixture ‘hydrohausmannite’ described from Franklin, New Jersey, U.S.A., and Långban and Pajsberg, Värmland, Sweden. This is the first record of feitknechtite in the British Isles. It occurs in massive manganese ore which also contains either iwakiite or jacobsite, rhodochrosite, caryopilite, fluorapatite and pennantite.
2 pages.

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Mineralisation at North Devon United Mine,
Peter Tavy, Devon

Michael S. Rumsey
Michael A. Savage

Several distinct mineral assemblages are present on the dumps of North Devon United Mine, Peter Tavy, Devon. The mine, which lies within the metamorphic aureole of the Dartmoor Granite, is of special scientific interest because of its complex and diverse ore mineralogy. Tin-tungsten-arsenic mineralisation in vein quartz is almost certainly part of the high temperature mineralisation which occurs in east-west trending veins throughout Cornwall and Devon. Scheelite, rather than the more usual wolframite, is the primary tungsten-bearing mineral. A complex polymetallic chlorite-dominated assemblage contains a wide variety of lead, copper, zinc and bismuth bearing minerals including the first British parkerite and ikunolite. It has considerable mineralogical affinities with the polymetallic chlorite lodes that are present at shallow levels in many mines at or near the margins of granitic bodies in Cornwall.
4 pages.

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