56 pages, full colour.

 

UKJMM No. 25 Shipping: UK (second class) £9.00 Europe (air mail) £12.00 Rest of World (surface) £12.00 Rest of World (air mail) £14.00

 

Rare minerals exert a special fascination for some mineralogists. A truly rare species may be known from only one locality and in exceptional cases from just a single specimen. The minerals andyrobertsite and calcioandyrobertsite from Tsumeb in Namibia are good examples. They were described in 1999 on a unique specimen that had been misidentified as keyite for many years. Subsequent discoveries can of course reduce the perceived rarity of a mineral species. Orlymanite was considered to be about the world’s rarest minerals when it was described from a single specimen in 1990. When the author visited the Lyman Museum on the Island of Hawaii in 1993, the type specimen of the mineral, which is named for one of the museum’s founders, was displayed as a great rarity. However recent discoveries in South Africa mean that good hand specimens are now readily (and inexpensively) available to both museums and collectors.

As time passes the worldwide rarity of a mineral species becomes more established. Oxidised lead-bearing orebodies containing fluorite and galena are common across the world and they have been exploited for centuries. In all of that time only one small pocket containing large crystals of the rare lead fluoride-chloride matlockite has been discovered. Most collectors will be familiar with the story of the discovery of the matlockite pocket at Bage Mine, Wirksworth, Derbyshire and its subsequent rediscovery by the Victorian mineral dealer Bryce Wright. The Bage Mine matlockite was found in association with large crystals of phosgenite, which were also considered extremely rare when they were first found. Matlockite has remained excessively rare but phosgenite has subsequently been discovered at numerous worldwide locations, sometimes in much larger and finer crystals than the original Derbyshire specimens. The reason why matlockite is so rare is not at all clear. Lead, fluorine and chlorine are common elements in base metal ore deposits and matlockite appears to have a reasonably large stability field in the conditions that might be expected during the oxidation of orebodies.

Some simple chemical compounds which would seem to be good candidates to occur as natural minerals are yet to be found in nature. Unstable or highly reactive compounds would not generally be expected, but compounds such as the strontium analogue of fluorite are very reasonable possibilities (this was suggested by the late Michael Fleischer in a personal communication to R.A. Howie). Fluorite occurs with strontianite at many localities in Britain including Strontian in Scotland. Perhaps a strontium fluoride is lurking in one of these ore deposits, or is masquerading as fluorite in the dusty corner of a museum drawer.

Some minerals are rare because they are unstable in the conditions found in nature. Scotlandite was described as the world’s rarest mineral in a recent edition of the Guinness Book of Records. Diligent fieldwork has recently revealed minute crystals of the lead sulphite at half a dozen locations in the British Isles. Although it contains the elements lead, sulphur and oxygen, which are common in many ore deposits, scotlandite is a very rare mineral. This is because of the limited range of chemical conditions in which it is stable. Rare minerals can be indicators of unusual geochemical conditions and of considerable interest to professional mineralogists. Species such as scotlandite might be useful as a guide to the way important chemical processes such as the oxidation of galena begin.

In a more limited context, species such as stokesite, fluellite and rhabdophane are great prizes in any Cornish mineral collection, though they are readily available from worldwide locations. The reverse applies to liroconite which is reasonable easy to obtain from Cornwall (albeit at an ever-increasing cost) but which is not available from other worldwide localities. Liroconite appears to be a genuinely rare mineral.

This issue of the UK Journal of Mines and Minerals concentrates on rare minerals. Parkerite, ilvaite, olsacherite, philipsbornite and ewaldite are described for the first time in the British Isles. The discovery of a mineral new to a country such as Britain where the geology and mineralogy has been thoroughly investigated requires patience, diligence and luck. This is especially important for rare species (which may be known from only a few locations worldwide) as it puts information into the public domain. There is a very real possibility that this information might otherwise be lost. Unlike the well-known county record schemes, which operate in subjects such as botany and entomology, there is no equivalent recording scheme for British minerals. The only way to preserve mineralogical data with any certainty is to publish it in a journal. If, at the same time, specimens are lodged in a public museum and this is stated in the article, they will be available for inspection by future workers. This has been especially important in examining some of the British species claimed by Arthur Kingsbury. As a result a number of minerals have been removed from the British list.

First occurrences can turn up in the most unexpected places. Although ilvaite is well known in mafic igneous bodies, its occurrence in the Whin Sill, described in this volume, is something of a surprise, if only because the petrography of that body has been so well studied. The occurrence of ewaldite at Dolyhir Quarry is also rather surprising as no other rare earth minerals have been found there in the past, despite years of diligent searching. The fact that the philipsbornite described from Wheal Unity was found on a specimen collected in the 1970s shows that it is worthwhile re-examining older specimens. The minerals scotlandite, mattheddleite and chenite were all found in this way.

It is essential that an account of a rare or unusual mineral is backed up with the appropriate scientific data. In most cases confirmation of the identification of a rare species by X-ray diffractometry (XRD) is required before an editor will consider publication. In the case of complex mineral groups supporting data in the form of qualitative or quantitative chemical analyses may also be required. This can be difficult to obtain, but most national museums and a few universities offer identification services.

A few readers may be aware that a new book about the minerals of the British Isles is currently in preparation. All of the minerals currently known in our islands are to be described, although the occurrences of the common species will necessarily be abbreviated. For those who have found something new, unusual or remarkable there has never been a better time to report it. We hope that the next edition of the UKJMM will feature some of these discoveries and will be published before the end of 2005.

Photographs of three dimensional micro-minerals are difficult to produce using conventional single shot film-based techniques. Software which combines the sharply focussed areas of different digital photos offers considerable advantages at magnifications greater than life-size and is almost indispensable at high magnifications. To get the best possible results it is important to understand the limitations of the microscope, the camera and the eye. The depth of field and resolution of the microscope must be measured or calculated at different magnifications and the size of the camera sensor (in megapixels) must also be taken into account when deciding the most useful magnification. When the image is printed, the maximum reproduction ratio is determined by the amount of useful detail in the photo and the resolution of the eye. With a modern stereomicroscope and a high resolution objective it is possible to produce good quality images for printing at linear magnifications of up to 100x, for a more typical instrument, 30 to 50x is the limit.

14 pages.

A complex assemblage of minerals including arsenopyrite, cobaltite, chalcopyrite, scheelite, bismuth, erythrite, quartz, fluorite, stannite, bismuth, bismuthinite, aikinite, cosalite, gersdorffite, ikunolite and various bismuth-bearing supergene minerals is present on the dumps of the North Devon United Mine at Peter Tavy, Devon. The rare nickel bismuth sulphide parkerite (ideally Ni3Bi2S2) has been identified as grains up to 50 µm across in quartz-fluorite veinstone associated with sphalerite, ullmannite, bismuth, bismite, gersdorffite, an unknown lead copper sulphide selenide and cassiterite. Quantitative analyses of the parkerite record significant substitution of antimony for bismuth and include the most highly antimonian composition yet described, with 5.3 wt% Sb. This is the first report of parkerite in the British Isles.

4 pages.

The rare mineral ewaldite has been identified in mineralised tension fractures in Precambrian sedimentary rocks of the Yat Wood Formation at Dolyhir Quarry, Old Radnor, Powys. It crystallises as isolated pale yellow-brown translucent striated hexagonal pyramids in association with harmotome, witherite, calcite and quartz. This is the first record of ewaldite in the British Isles.

2 pages.

Drusy cavities in the granite at Lindsay’s Leap in the Mourne Mountains, Co. Down, contain a variety of minerals. They are lined with quartz and feldspar and commonly contain mica and chlorite group species. Beryl, calcite, chabazite-Ca, fluorite, heulandite-Ca, monazite-(Ce), saponite, stilbite-Ca and topaz have also been identified, together with the first Irish occurrence of the rare iron beryllium silicate sulphide danalite. Danalite occurs rarely as small translucent brown crystals and grains on joint planes and in small cavities with quartz, saponite and stilbite-Ca. It is later in the paragenesis than all of the other silicate species except the low temperature zeolite group minerals.

6 pages.

The compositions of the magnesium and iron borate minerals boracite, trembathite and congolite, which occur in Zechstein evaporites at Boulby Mine, Redcar and Cleveland are described. Brown blocky crystals which have been described in the literature as either iron-boracite or ericaite, are in fact the recently described mineral trembathite, or the rare iron borate congolite, or mixtures of both trembathite and congolite.

2 pages.

The rare lead selenate sulphate mineral olsacherite has been identified on a single minute specimen collected at Waterbank Mine, Ecton, Staffordshire. It was present as an inconspicuous pale brown spheroidal crystal aggregate composed of sharp bladed crystals up to about 0.1 mm in length. A copper and selenium-bearing supergene mineral was identified on the same specimen. The ore deposits at Waterbank Mine appear to contain more selenium than hitherto suspected. This is the first report of olsacherite at any locality in the British Isles

2 pages.

The supergene mineralisation at Greystone Quarry near Launceston, Cornwall is unusually diverse. It includes eighteen lead-bearing supergene minerals, four which were new to Cornwall when first reported and one which was new to Britain. The rare supergene lead copper sulphate hydroxide elyite has recently been identified, adding another rare species to an already remarkable list. Elyite occurs as minute pale violet to purple sprays of acicular crystals which appear to have formed by natural supergene oxidation. This is the first report of elyite in Cornwall.

2 pages.

The rare supergene mineral philipsbornite has been identified at Wheal Unity, Gwennap, Cornwall. It occurs in drusy crusts composed of minute trigonal crystals. Empirical formulae determined for two typical grains are in good agreement with theoretical expectations for members of the crandallite group. Iron, sulphate, phosphate and fluoride are the major chemical substitutions.

2 pages.

lvaite, a rare calcium-iron silicate, is present in the dolerite-pegmatite facies of the Whin Sill at Forcegarth Quarry near High Force in Teesdale, Co. Durham. It has a near stoichiometric composition in the range Ca0.98-1.00Fe2+1.95-2.00Fe3+Si1.96-1.98O8(OH). Ilvaite forms part of an association of opaque minerals that includes ilmenite, altered titanomagnetite, titanite and minor sulphides, notably pyrrhotite, chalcopyrite and sphalerite that is characteristic of ilvaite in mafic igneous rocks. Other than the presence of ilvaite and trace amounts of molybdenite there is nothing unusual about the sampled dolerite-pegmatite suggesting that ilvaite may be present in dolerite-pegmatites elsewhere in the Whin Sill. This is the first authenticated occurrence of ilvaite in the British Isles.

The opaque mineralogy of pink aplite from the Whin Sill at Ratcheugh Quarry near Alnwick comprises ilmenite, titanite and pyrite with minor amounts of pyrrhotite, chalcopyrite, iron-rich sphalerite and galena. Much of the sulphide assemblage and associated barite is later than the crystallisation of the aplite.

8 pages.