Staff Keeper of Natural History, E.G. Hancock, BSc, FMA, FRES; Assistant Keeper with responsi- bility for geology, A.H. Gunning, BSc, AMA. Altogether there are five curatorial staff and four conservatiodtechnical staff within the Department, covering all aspects of natural history.

Other geological resources in close proximity: Hunterian Museum and Department of Geology, University of Glasgow; Department of Applied Ge- ology, University of Strathclyde.

Compiler: A.H. Gunning.

Minerals explained 5: Calcite Calc i te is perhaps the most common carbonate mineral in the Earth’s crust and is found in many environments. Frequently occurring as a rock- forming mineral, enormous thicknesses of it exist as limestone, chalk, tufa and travertine. Metamorphic processes may convert these into purer calcite-marble. Calcite may also act as a cementing medium to are- naceous sediments, and it may be produced by igneous activity (especially alkaline) to form flows, pipe structures and dykes of calcite-dominated carbonatite. Distinction should be made, however, between biogenetic calcite, formed by life processes, and abiogenetic calcite, formed by processes of diagenesis, hydrothermal activity, igneous or meta- morphic processes. It is the latter, the true mineral calcite, which is described here.

The forms and habits calcite may adopt verge on the infinite and, because of that fact alone, become the sole object of attention to the so-called single-species collector. The name of the mineral is derived from the Latin caix, ‘lime’, or calcis, ‘limestone’, and replaces the German Kalkspat and Kalkstein. There are many other synonyms.

Varieties of calcite Agaric mineral the Latin agaricum, ‘fungus’. Aphrite

- A soft, white, friable variety; from

- A white froth-like variety; from the Greek ‘foam’, in allusion to its appearance.

- A lamellar variety possessing a sil- very-white pearly lustre; from the Latin argenturn, ‘silver’, in allusion to the colour.

- A fibrous variety restricted to vein- like bodies in sedimentary rocks and frequently associated with cone-in- cone structures. So named because of its resemblance to the meat. Ori- ginally a quarryman’s term.

Argentine

Beef

Fig. 2. The combination of prism and negative rhombohedron in a calcite group from Greenside Mine, Glenridding, Cumbria. ‘Nailhead Spar’. The specimen measures 126 x 85 mm. (Natn. Mus. Wales 83.41G.M5114.)

tufa: a chemical sediment- ary rock consisting of cal- cium carbonate, deposited from water that has passed through limestone.

travertine: a harder, denser variety of tufa.

arenaceous: said of a sedimentkedimentary rock having the texture and appearance of sand.

diagenesis: the low temper- ature and pressure processes affecting a sediment and its contained organic matter on burial.

Fig. 1. A scalenohedron (104 mm long) from hber Mine, Matlock, Derbyshire. ‘Dog tooth Spar’. (Natn. Mus. Wales 83.41G.M5092.)

Cave Pearls -

Dog-tooth - Spar

Hislopite -

Iceland Spar -

Mountain or - Rock Milk Nail-head - Spar

Onyx -

Papierspath -

Sand Crystal -

Satin Spar -

Schieferspath - or Slate Spar Stalactite -

Pearl-like spheroids of calcite, usually in nest-like accumulations formed in hollows by dripping carbonated water. A term used to describe the acute scalenohedral form, but now embrac- ing all scalenohedra (Fig. 1). A green, readily cleavable variety containing up to 14% of glauconite and restricted to Central India. Perfectly transparent and colourless calcite present as masses and crystals in the cavities of basalt at Helgustadir in Iceland. A synonym of agaric.

A descriptive term indicating the presence of the hexagonal prism and the positive or negative rhom- bohedron (Fig. 2). An unfortunate application used in the lapidary trade to describe onyx (SiOz)-like stalagmitic calcite. From the German, to describe very thin tabular crystals. Crystallized calcite carrying an admixture of up to 50% sand grains, the most famous occurrence being the loosely cemented sandstones at Fontainebleu, near Paris, in France, but known at many other localities. A fibrous variety with a satin-like or chatoyant lustre, closely resembling gypsum, said to be sometimes pseudomorphous after the latter. A variety adopting a lamellar habit, often with a pearly lustre. Cone-like masses of calcite, often with an annular radiate internal

IZOIGEOLOGY TODAY July-August I986

Stalagmite -

Stinkkalk- - Stinkstein or Foetid Calcite

structure, found hanging from the roofs of caves, etc. Similar to stalactite, but growing upwards from the floor of caves. A variety which emits a bituminous odour when struck.

Chemical composition Most calcite is relatively free from additional ions and is therefore close to the ideal composition of CaC03. Ionic compatibilities exist, however, and many diva- lent cations may replace the Ca to varying degrees. The most common are those of Mg, Mn, Fe and, more rarely, Zn. Sr may substitute for Ca to a minor degree, but the large ionic radius of Sr is more acceptable in the orthorhombic structure and is more likely to be present in aragonite (orthorhombic CaC03). Calcite is the stable form of CaC03 but may change to aragonite at temperatures below - 60”C, or when subjected to long periods of grinding at room temperatures. Other elements may be present, such as Co, Ba and Pb, but are rare and given other species names. Owing to its ready solubility in weak acids, calcite is frequently pseudomorphed by other mine- rals.

Structure The structure of calcite is that adopted by carbonates and nitrates possessing small cations. Calcite crystals obey trigonal symmetry and belong to the rhom- bohedral class of that system. Traditionally this is typified by the cleavage rhombohedron { l O i 1) , the true unit cell being a tall, thin rhombohedron which, however, contains two formula units which are more difficult to define. Calcite shows a variety of forms, the essential ones being: (1) the unit rhombohedron (lOT1); (2) the flat rhombohedron (01i2); ( 3 ) the steep rhombohedron (4041); (4) the scalenohedron (2131); and (5) the prism (lOi0).

Crystals show an enormous variety of combinations of these forms, and it is this apparent infinity and beauty of combination which inspires the single-spe- cies collector. To appreciate the scale it is only neces- sary to observe the possibilities, as shown in 1901 by M.F. Heddle in his observations on the habits adopted by Scottish calcites (29 plates with 224 crystal drawings). There are other imitative shapes such as tuberose, coralloidal, stalactitic, coarse-to-fine granu- lar, oolitic or pisolitic.

There are two principal twinning laws recognised in calcite: (a) twin plane on the pinacoid {OOOl}, and (b) twin plane on the negative or flat rhombohedron {0112} which is usually lamellar. There are other rarer planes and frequent random intergrowths, but perhaps the most dramatic twins are those from west Cumbria and Derbyshire, the so-called butterfly twins (Fig. 3 ) .

A feature of great importance in the examination of calcite is the dramatic phenomenon of double refrac- tion. This was first noted in Iceland Spar by the Danish surgeon Erasmus Bartholin in 1669. This ability to split an incident ray of light into two refrac- ting rays was noted by William Nicol, a Scottish professor. Nicol is perhaps remembered best for his

use of calcite prisms in the production of polarised light - the nicol prisms - which revolutionised the study of petrology.

Other physical features Calcite has perfect and easily demonstrable cleavage on the unit rhombohedron jlOT1). When pure, its density is 2.711 x lo3 kg m- , but this rises with the addition of substitutional ions. The hardness on Mohs’ scale is 3, but may vary slightly from face to face of a crystal. Most calcite is colourless and trans- parent, but it may be white and then translucent or opaque. Such elements as Fe, Co and Mn may pro- duce a change of colour. The streak, however, remains white or light grey. Up to 7% of MnC03 can produce a pale violet colour and the mineral becomes strongly fluorescent under ultraviolet light.

Calcite is one of three common colourless or light- coloured minerals which will effervesce in cold, dilute hydrochloric acid. Calcite may be specifically stained by organic dyes to provide a simple method of dif- ferentiation from other associated carbonates.

Where calcite may be found The mineral is ubiquitous and will form wherever a supply of carbonate is available. Britain is rich in fine calcites. Thus it is common in limestone regions in the form of calcrete, travertine, tufa, stalagmitic deposits or as veins and vughs -. the product of lateral secre- tion. The limestones of .Devonshire, the Mendips and South Wales, ranging in age from Devonian to Meso- zoic, all produce excellent crystals from such veins which cut the limestones or exist as vughs within them.

Where solutions of calcium carbonate percolate through loose arenaceous rocks, fine crystals or ball- like masses with a radiate interior may develop. These sand crystals, carrying high proportions of included sand grains, are well known in British Permo-Triassic sediments, especially above unconformities, and there are notable localities in Devon, Leicestershire and Cheshire.

The low-temperature metasomatism of limestones may produce well-developed crystals, as in the Missis- sippi Valley-type deposits. Superb crystallizations occur in the west Cumbrian and Lancashire iron mines. The large ‘flats’ which flank the north Pennine vein systems are renowned for the beauty and com- plexity of habit of their calcites. Their association

vugh: a relatively large and irregular void in a rock, in this case one containing calcite.

metasomatism: a form of metamorphic change in which some of the minerals in a rack are altered by the action of fluids from exter- nal sources.

Fig. 3. A ‘Butterfly Twin’ from Gillfoot Mine, Egremont, Cumbria. Twinned on { 101 1) the positive rhombohedron. The specimen is 75 mm across. (Natn. Mus. Wales 83.41(i.M5213.)

GEOLOGY TODAY July-August 19861121

with fluorite, baryte and sulphides make them accepted as classics worldwide.

Calcite frequently occurs in the later stages of hyd- rothermal activity and may crystallize in vughs in the lode systems. Many have produced beautifully crys- tallized calcites almost characteristic of localised areas. From the Land’s End area of Cornwall, such mines as Botallack, Levant and Geevor have produced superb groups resembling those from the Harz Moun- tains of Germany, with the strong development of the pinacoid. Mid-Wales (from such mines as the Van) and North Wales (from such mines as Parc, near Llanrwst, and Trecastell, near Conway) have both produced characteristic crystal groups. The large rakes of Derbyshire have produced excellent material, especially scalenohedra, from mines like Millclose, near South Darley. In Derbyshire, by far the most interesting crystals have come from ‘pipe structures’, neptunean dykes and cave fills, resulting from the modification of the rakes, from mines like the Gol- conda, near Brassington. Bi-pyramids of rare beauty have grown in clay-filled vughs. The hydrothermal veins of the Lake District have produced fine crystals, as have the mining districts of Scotland, especially in the Leadhills-Wanlockhead area.

Calcite is found frequently in the amygdales of basic igneous rocks, where they are associated with zeolites, as in Iceland. Complex pseudocubic crystals occur in several Scottish localities, in the basaltic rocks of Skye, or in similar rocks in Northern Ireland, and with zeolites in Leicestershire. The decay of calc silicates in igneous rocks seldom produces macrosco- pic crystals; neither do good crystals occur in carbona- tites.

The uses of calcite Following Nicol’s development of his prism, a thriv- ing industry developed in the exploitation of perfectly transparent colourless calcite. The industry was cen- tred in Iceland, but other sources such as Montana in the USA offered some competition. Following the introduction of the plastic, herapathite ( ‘Polaroid’), the nicol prism industry collapsed and only minor amounts are now produced for demonstration and other purposes. The only other use for calcite senso strict0 is ornamental. The so-called ‘calcite onyx’ is mined extensively in such countries as Argentina, Mexico and France, and a little fibrous calcite is turned on lathes for the tourist industry. In Britain, crushed calcite (‘spar’ in the trade), a by-product from

the milling of sulphide ores, is used for ‘pebble dashing’ on the exterior walls of buildings.

The curation of calcite Calcite is soft and subject to bruising. Great care should be exercised in its extraction and transporta- tion. It is possible, however, to reduce an unsightly bruise by carefully dabbing the immediate area of the bruise with a size 00 brush dipped in dilute hydroch- loric acid. Some calcites are naturally etched and they should remain so. Any attempt to ‘brighten’ the crys- tals results in the development of a hideous and completely atypical lustre. Similarly, the removal of associated species from crystal faces is unethical and should not be contemplated by the collector.

Care should be exercised when collecting wet cal- cite from clay-filled vughs. It should not be stored wet in polythene bags for long periods, but allowed to dry slowly in air. Long periods of wet storage can promote continued growth, not in megacrystal continuity but as a frosting effect.

Calcite should be stored in cabinets or drawers as dust-free as possible. Once a calcite has become dusty, some of its natural lustre is lost. Providing there are no metastable associates, calcite will survive in a wide range of environments, but should never be subjected to violent changes of temperature. British calcites are becoming highly desirable in foreign mar- kets. The collector should not be tempted. They belong in Britain.

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

Boyd. Bishop, A.C. 1967. A n Outline of Crystal Morphology.

Hutchinson. Correns, C.W. 1969. A n Introduction to Mineralogy.

George Allen & Unwin. Deer, W.A., Howie, R.A. & Zussman, J. 1966. An

Introduction to the Rock-forming minerals. Longman. Hartshorne, N.H. & Stuart, A. 1970. Crystals and the

Polarizing Microscope. Edward Arnold. Heddle, M.F. 1901. Mineralogy of Scotland, v.1,

pp. 114 -136. David Douglas, Edinburgh. Palache, H., Berman, H. & Frondel, C. 1951. Dana’s

System of Mineralogy (7th edn), v.2. Wiley & Sons.

R.J. KING Department of Geology

National Museum of Wales

Book Reviews A Handbook of Sand by Gordon Osborn, 1984. ISBN 0 9505530 2 E6.95 (paperback). (Available from the author at 38 West Street, Olney, Bucks MK46 5HR.) 76 pp.

This little book on sand was obviously a labour of love for the author. It is a natural outlet for the data on sand gathered by Gordon Osborn over many years and from many parts of the world. He is obviously immensely proud of the more than 2500 sand samples

from 100 countries which he has curated for the Northamptonshire Natural History Society, a collec- tion of sands which it is claimed is the best in the United Kingdom.

The book is divided into 13 chapters clearly laid out, and contains 30 figures and 20 plates (both colour and black and white). The author discusses such conventional aspects of sand as provenance, mineral- ogy and transport structures, and more esoteric aspects such as ‘singing’. He also briefly describes

122lGEOLOGY T O D A Y July-August 1986