Conclusion We are accumulating a wealth of data on Recent reefs which relates to the ecology and distribu- tion of the organisms and to their influence on the patterns of carbonate generation and accre- tion. The data show that ‘present day’ growth is no older than about 7000 years and that the total duchess of limestone formed in that time is only about 10 metres. They also question our simple models of the way in which reefs grow and emphasise deposition of loose sediment within the energy shadow down-current from the reef frame. This contrasts with the textbook model of coarse debris accumulating on the reef front. However, although we can apply these principles of growth and development to fossil reef systems, it has become clear that they are not sufficient to explain the total character of Recent reefs and that we must also consider the foundations of these structures. This we will do in a future article, where we will look at their deep structures in relation to sea-level change and plate tectonics.

Suggestions for further reading Chave, K. E., Smith, S . V. & Roy, K. J. 1972.

Carbonate production by coral reefs. Marine Geol. v. 12, pp. 123-140.

Graus, R. R., Chamberlain, J . A. Jr. & Boker, A. M. 1977. Structural modhcations of corals in relation to waves and currents. In: Reefs and related carbonates: Ecology and Sediment- ology. Frost, s. H., Weiss, M. P. & Saunders, J. B. (eds). American Association of Petroleum Geologists, Studies in Geology 4, pp. 135-153.

Land, L. S . & Moore, C. J. Jr. 1977. Deep fore- reef and upper island slope, North Jamaica. In: Reefs and related carbonates: Ecology and Sedimentology. Frost, S . H., Weiss, M. P. & Saunders, J. B. (eds). American Association of Petroleum Geologm, Studies in Geology 4, pp. 53-65 .

Macintyre, I. G. & Glynn, P. W. 1976. Evdu- tion of Modern Caribbean Fringing Reef Galeta Point, Panama. Bulletin of the American Association of Petroleum Geologists, v. 60, pp. 1054-1072.

Rosen, B. R. 1981. The tropical high diversity enigma - the corals’-eye view. In: The evolving Biosphere. Forey, P. L., (ed). British Museum (Natural History), Cambridge University Press. pp. 103-129.

Stehli, F. G. & Wells, J. W. 1971. Diversity and age patterns in hermatype corals. Systematic Zoology. v. 20, pp. 115-126.

Colin Braithwaite is Senior Lecturer in Geology, Dundee University

Minerals explained 6: Baryte w h i l e there are relatively few naturally occurring barium compounds in the Earth’s crust, barium sul- phate in the form of the mineral bartye is common and geographcally widespread. But although com- mon, oftensin the form of massive orebodies, it is frequently found in the form of beautiful crystals and in a range of pastel shades of colour. Most often found in medium-to-low-temperature hydrothermal deposits, where it occurs as a gangue mineral associ- ated with such ores as lead and zinc, it may also be present in sedimentary rocks where it may acr as a cementing agent. It may also be produced by pro- cesses of metasomatism, especially of limestones. Owing to its insolubility, it often forms residual strata-bound deposits following the erosion of such limestones. Beautifully transparent crystals may also be found in the vesicles of some igneous rocks.

The name baryte is derived from the Greek barys, ‘heavy’, alluding to its density. The many synonyms and variations of the spelling have produced a confus- ing picture. In 1868 Dana, in an attempt to rationalise mineralogical nomenclature, modified the Greek to produce the name barite, a spelling st i l l in common use in America. The International Commission on Mineralogical Nomenclature has decreed, however, that Karsten’s derivative of 1800, baryr, should have

preference. The names barytes, barytine and barytite are obsolete, as are the many names of Latin deriva- tion. Other names still in use include (1) Bologna Stone (Spar) - a locality term; (2) Brain Stone- a tight grouping of crystals which, when cut and polished, resembles the convolutions of the surface of a brain; (3) Cawk (Cauk, Caulk, Calk) - a descriptive term used by Derbyshire miners to describe an earthy, massive variety; (4) Heavy Spar - a miner’s term; ( 5 ) Hepatite - a fetid variety arising from the presence of oily compounds; and (6 ) Sand crystals - crystallised baryte containing a proportion of sand grains. There are famous localities in the United States (e.g. Okla- homa and Kansas) where these last are known as desert roses.

Chemical composition Baryte is essentially barium sulphate (BaS04). Being isostructural with celestine (SrS04) and anglesite (PbS04), elemental substitution of the barium by strontium, and less commonly by lead, is possible. Although there is no series between barium and lead, the substitution being in the order of Pb:Ba = 1:4, a complete series may exist between barium and stron- tium. There may be minor substitution of calcium for barium, but there is no isosuuctural relationship

GEOLOGY TODAY Jam~y-Febncory 1987121

Although beautiful specimens have come from such mines as the Ale and Cakes, near St Day in Cornwall, virtually nothing may be collected in Cornwall today at the surface. The collector has to rely on dealers who may have acquired Cornish baryte from an old collec- tion. However, good crystallised material may be found in Devon, from localities in the Christow area or in the quarries about Brixham and Torquay. Material of a similar habit may be found in the quarries at Cannington, near Bridgwater in Somerset. In the same county, a variety closely resembling the Derbyshire variety, cawk, may readily be found in the Mendip Hills, and well crystallised baryte occurs in the Dolomitic Conglomerate (Triassic) at such locali- ties as Dulcote Hill near Wells.

Fine crystal groups associated with sprays of tiny svontianite crystals occur from time to time in the quarries about Portland in Dorset. Beautiful groups of crystals in the form of rosettes occur in the septa- rim nodules of the London Clay (Eocene) on the Isle of Sheppey in Kent (Fig. 3). Large masses of baryte occur in the course of working the Lower Cretaceous Fuller’s Earth of Nutfield in Surrey. Cavities in these masses often contain h e , transparent, golden- coloured, highly modified crystals. Sand crystals, white in colour and in large groups, occur in the Lower Cretaceous Woburn (glass) Sands of the Leighton Buzzard area of Bedfordshire, but they are rare and diligent searching is necessary.

The mines in the Shelve and Church Pulverbatch areas of Shropshire have been important producers of baryte. Well-crystallised material may still be found on the old mine dumps there. Unique pisolitic aggregations of spheroids occur in the neptunean dykes cutting the Dinantian limestones of north Leicestershire. Baryte is also abundant in Derbyshire, and its habits are as diverse as its many localities. Most commonly adopting the cockscomb habit, fine crystal groups occur, especially in the mine dumps and quarries of the Sheldon and Castleton areas. The collector should do well in Derbyshre. The unique dark-brown stalactitic material, formerly found at Arbor Low, near Middleton-by-Youlgreave, may still be found cut and polished in a characteristic manner in the many ‘rock shops’ in the county. It is sold under the trade name of ‘oak stone’ (Fig. 2).

Occasionally, good specimens of baryte may be found in the Triassic sandstones of the Cheshire Basin - for example, at Alderley Edge. Sand crystals in the form of spherical aggregations of platey crystals are abundant in Perm+Triassic Sandstones in the sand pits of Nottinghamshire, especially in the neighbour- hood of Bramcote where they almost constitute an ore body. Formerly, the counties of northern England were internationally famous for the abundance, beauty of colour, and perfection of their crystallised barytes. The names of many mines in west Cumbria, the northern Pennines and Weardale became house- hold words. Sadly, such names as the Mowbray Mine, near Frizingtoa (Fig. l(b)), the Silverband Mine, near Knock (Fig. l(a)), in which area some crysds found in clay-filled cavities attained 247 kg in weight, and the Haggs Mine, near Nenthead, where baryte pseudomorphous after witherite made attractive coral-like masses, have all become history.

Although it may be possible to acquire cleavage masses and the rare crystal group on old mine dumps, the collector’s best hope is to visit the dealers as far as

Fig. 3. Baryte on calcite in septarian nodule, Minster, Isle of Sheppey, Kent. The rosette is 21 mmindiameter. (Natn. Mus. Wales No. 83.41G.M87S8.)

the north of England is concerned. Only there may he or she be able to acquire something comparable to that seen in the museums of the world. While relatively common in some of the old mining districts of Wales, the most likely oppormnity to collect well- crystallised baryte is in the working quarries of South Wales. Quarries working in the ‘South Crop’ of the Dinantian limestones which flank the South Wales Coalfield frequently produce fine crystal groups.

Scotland, hitheno a large producer of baryte (with the exception of the Leadhills-Wanlockhead area) has not been noted for the excellence of well-crystallised or beautifully-coloured baryte. Nevertheless, the col- lector may st i l l be successful at such localities as the Glen Sannox Mine on the Isle of Arran and from old mine dumps elsewhere, especially in the Leadhills area of the Southern Uplands. The same may be said of the Isle of Man and Ireland. On the former, the Foxdale mines have produced rare groups; and in the latter, localities such as Glendalough in County Wick- low and old mine sites in County Cork may still reward a keen eye.

The uses of baryte As the world’s reserves of witherite dwindle, baryte is increasingly sought as an economic source of barium and its compounds. It is the chief source of the metailic element. With the increasing awareness of the toxicity of lead, baryte is now much used in the manufacture of paints, as a filler for paper, in the production of floor coverings and some textiles, and in better-quality wallpapers. It is also used extensively in the sugar industry. The ‘barium meal’ used to render the alimentary canal opaque to X-rays is composed of finely-ground barium sulphate. By far the largest consumer of baryte is, however, the oil industry, which uses it as the principal constituent of drilling mud for oil and gas wells.

The curation of baryte Baryte, being relatively heavy, soft and possessing perfect cleavage, needs careful handling. Its weight is disarming and can catch the unwary. Owing to its softness it is easily bruised, and the bruises cannot be removed because of the mineral’s insolubility. Should a specimen with well-developed cleavage be dropped or placed down abruptly, the cleavage may develop

GEOLOGY TODA Y January-Februnry 1987123

Mantle: the portion of the Earth’s interior between the base of the crust and the top of the core, volumetrically the largest component of the Earrh, and comprising m a d y olivine and pyroxene.

Picrite: an ulvamafic rack with accessory plagioclase; an olivine-rich basalt.

and the specimen be destroyed. For the same reason, well-cleaved baryte should not be subject to cleaning by ultrasonic means. Baryte will submit, without ill effects, to extremes of relative humidity but not to extremes of temperature, and it should never be stored or vansported without insulation in sub-zero temperatures. Any moisture trapped within the cleav- ages or as fluid inclusions will freeze and burst the specimen.

Baryte from some localities is notoriously meta- stable under strong lighting, the o n p a l colour fading or becoming lost ‘altogether. Such photosensitivity was taken advantage of by west Cumbrian iron miners, who would expose normally yellow-coloured baryte to strong sunlight to produce a blue coloura- tion which could be sold at a higher price to collec- tors. Coloured baryte placed in a showcase under strong lighting should be carefully watched for the onset of colour loss or change.

Providing there are no metastable or delicate associ- ates, such as pyrite or acicular aragonite, the appear- ance of baryte may be improved by gentle washing in

solutions of soap in lukewarm water. I t is a beautiful mineral and worth good care. There is a high pre- mium on crystallised British baryte. It cannot be replaced.

Suggestions for further reading Battey, M.H. 1972. Mineralogy for Students. Oliver

and Boyd. Bishop, A.C. 1967. An Outline of Crystal Morphology.

Hutchinson. Collins, R.S. 1972. Barium Minerals. Mineral Dossier

No. 2, Institute of Geological Sciences (HMSO). Dines, H.G. 1922. Barytes and Witherite. Institute of

Geological Sciences (HMSO). Palache, H., Berman, H. and Frondel, C. 1951. Dana’s System of Mineralogy (7th edn), v. 2. John Wiley.

R.J. KING Department of Geology

National Museum of Wales

The latest on Oceanic basalts T o many geologists basalts are rather nondescript, uninteresting rocks, but to the igneous petrologist they are revealing some fascinating insights into how the Earth works. No longer are basalts merely ‘fine- grained, basic igneous rocks’ which often remain obscure to the most ardent microscopist. Extremely subtle variations in their chemistry, detectable only by high-precision analytical instruments, can now be related to large-scale events which occurred within the Earth millions, if not billions, of years ago. The story that is beginning to emerge is an exciting one that involves global recycling of the outer layers of the Earth throughout long periods of geological time.

Diversity of eruptive settings Basaltic magmas are the main product of melting of the mantle. The exact nature of the melt, or magma, that is initially produced is still disputed, even after decades of experimental petrological research; but it is essentially basaltic or picritic. Basalts are erupted in virtually every tectonic setting, but they are particu- larly common in the ocean basins - at mid-ocean ridges, within subduction-related island arcs and back-arc basins, and on intra-plate islands.

Mid-ocean ridge basalu ( M O R B ) , the most volumi- nous basalt type, form the half-kilometre-thick cara- pace of sheet flows and pillow basalts of the uppermost oceanic crust (Fig. 1). They are erupted in response to crustal extension, the basaltic magmas oozing from fissures or s m a l l volcanoes sited at abyssal depths on the mid-ocean ridge system. MORB effec- tively underlie all of the abyssal plains.

Island arcs (for example, the Mariana Islands in the western Pacific) are more localised, being associated with subduction zones, and basaltic rocks are an im- portant product of their volcanic activity. Most intra- oceanic islands arcs are sited on oceanic crust which

may have formed by the process known as back-arc spreading. Back-arc basalrs are formed in an exten- sional setting similar to MORB, but their chemistry is often intermediate between MORB and island arc basalts.

Basalts are also ubiquitous on oceanic islands, which are not usually sited on the ridge axis and can be considered as ‘within-plate’ or ‘intra-plate’. Here, magmatism has been sufficiently prolific and pro- longed for the volcanic pile to emerge from the sea. Emergence is aided by the doming of the oceanic plate above buoyant mantle, a phenomenon which only rarely occurs on mid-ocean ridges - for example, Iceland, on the mid-Atlantic Ridge. In many cases of off-axis activity, the volcanism is insufficient for an island to emerge, in which case a seamounf will result. (Seamounts or guyots may also develop when islands subside as the associated thermal bulge dies away.) A recent estimate suggests that some 20% of the Pacific Ocean crust is made of seamount-related basalt - much more than is generally appreciated.

The very existence of an oceanic island suggests that unusual conditions exist locally within the mantle. The doming of the lithosphere and the asso- ciated volcanic activity have led to the suggestion that islands are underlain by mantle ‘hot spots’. Deep- seated hot spots are also termed ‘plumes’ or ‘blobs’, depending on how vertically continuous they are thought to be. The concept of plumes has been given a recent boost by seismic tomography, essentially a three-dimensional whole-Earth body-scan using seis- mic waves instead of ultrasound. This shows columns of anomalous, hotter mantle extending from beneath oceanic islands into the deep mantle. There are still several unsolved problems, however. How do the plumes remain intact in a convecting mantle and not get dissipated? Is the mantle where the plumes origi- nate different from the overlying mantle? How can we

2WGEOLOGY TODAY January-February 1987