What is a diatom?	Diatomite - uses in industry	Diatomite in Scotland

What are Diatoms?
Diatoms (Bacillariophyceae) are microscopic unicellular algae found in both freshwater and marine environments. The structure of a diatom is similar to that of a pillbox. The siliceous shell or ‘test’ is made of opaline silica (SiO₂.nH₂O) and consists of two interlocking, box-like halves, one larger than the other (the thecae). The thecae each consist of a structure called the valve (the main element of the shell) and several connected bands attached to it (the girdle). Although this is the basic structure of a diatom, they do vary considerably in shape and size (Canter-Lund & Lund, 1995).  Diatoms are found in fresh, salt or brackish water and light is essential for the organisms to grow, as they are photosynthetic. For this reason, many species are found floating in the surface layers of the water, although the majority are benthic, ‘floor dwelling’species.

The silica which makes up the shell remains unaltered on the death of the diatom, and is almost indestructible, except in strongly alkaline conditions (Taylor, 1929). As this outer covering is both porous and resistant to acids and heat, these unassuming little plants are very useful in industry (D. Mann, pers. comm.).

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Industrial Applications
The tests of diatoms are microscopic and, being composed of silica (the major constituent of glass), they are chemically inert. These two properties make them particularly suited for use as industrial filters. Diatomite is used in a variety of food production applications including :

• Clarification and filtration of beer and wines.
• Refining sugar and sweeteners.
• Filtering fruit juices, oils and syrups.

• Pharmaceuticals, serums and other biotechnology applications.
• Chemicals, including liquid acids and other liquid wastes.

Diatomite is also used as a filler, to ‘bulk out’ a number of products, as it is non-reactive.  These include paints, lacquers, rubber, plastic, polishes, agricultural chemicals, insulation, anti-caking agent, cement, concrete, animal feeds, fertilisers, catalyst carriers and dynamite! In the case of the latter, certainly it is the fact that diatom tests are relatively inert that makes them useful. Nitro-glycerine (the explosive component of dynamite) sits in a matrix of diatom tests, keeping it well spread out, such that it needs a mini-detonation or fuse to touch the explosive off (Roskill website; D. Mann, pers. comm.).

Other uses of diatomite include the manufacture of :

• Structural Materials: Such as heat and fire resistant products, sound proofing materials, polishes such as ‘Tripoli Powder’, porcelain, drain pipes, tiles, non-conducting materials and artificial stone.
• Cosmetic products: ‘Sozodont’ tooth powder, soap, deodorants, and abrasives such as toothpaste.

Usage and World Reserves of Diatomite
The percentages of use for diatomite in filtration and filler applications is approximately 73% and 14%, respectively. Although worldwide diatomite reserves amount to something like 2 billion tons, diatomite filters are being gradually replaced by membrane filters, and alternative fillers for paints are being explored as there is a growing concern about crystalline silica (Roskill website).

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Scottish Diatomite Deposits
There are a number of diatomite deposits in Scotland which have been commercially exploited in the past, and some more minor ones which have been noted and, in most cases, studied by scientists.

The deposits are typically found beside existing lochs and are of recent (Quaternary) origin. The lochs neighbouring the deposits were originally larger and, having receded, left the diatomite deposits behind. Over time, a layer of peat tends to form over the deposits, which had to be cut through in order to extract the mineral itself.

The diatomite deposits in Scotland are variable in their quality (according to the amount of organic matter and gravel adulterating the diatom tests). In any site, as well, there is considerable variation throughout, with samples from the centres of deposits tending to be more pure.

Skye
The most significant known Scottish deposits of diatomite, certainly in terms of human exploitation, are those on Skye. Eight of the Skye sites are in the Trotternish area and one is found on Waternish, to the West. The table below summarises some of the properties of the Skye diatomite deposits.

Quality is measured in percentage diatomaceous content and the ‘worked’ column refers to whether or not the deposits have been exploited commercially ( - represents missing data).
 

SITE	QUALITY	VOLUME	WORKED
Loch Cuithir	79 - 88 %	350 000m3	Yes
Loch Valerain	81 %	Worked Out	Yes
Loch Mealt	-	> 10 000m3	-
Eilean Callum Chille	57 - 76 %	-	-
Loch Sneosdal	78 %	1 000 000m3	-
Glen Uig	77 %	-	-

There are a further three sites on Skye; Loch Cleat and Staffin Bay (Trotternish) and Score Horan (Waternish).

The deposits at Loch Cuithir and Loch Valerain were worked during the last two decades of the 19th and first two decades of the 20th Centuries. Between 1899 and 1914, more than 2000 Imperial tons of diatomite was removed from these two sites, exhausting the one at Loch Valerain. In the late 1930s, the works at Loch Cuithir were reopened and exploited by the Skye Mineral Syndicate Company, but this did not last particularly long. The Diatomite was carried from Loch Valerain by aerial ropeway to Staffin bay, and from Loch Cuithir by tram to nearby Invertote. Here the diatomite was kiln dried, ground and then calcined (roasted to reduce the particle size and get rid of organic matter) (Haldane et al., 1940; Skye Data Atlas website).

Aberdeenshire
Near the village of Dinnet, Aberdeenshire, there are a number of diatomite deposits under moorland, typically covered by a thick layer of peat. The sites are: Black Moss, Ordie Moss, Kinnord Moss, Drum Moss, Auchnerran and Presswhin. They are thought to have been laid down when a large body of water receded, leaving these deposits exposed for peat to form on the top.

The deposits at Kinnord Moss, Black Moss and Ordie Moss are particularly close together and were all worked during the 1910s, to provide diatomite for dynamite, silicaceous paints and an Ultramarine substitute (Ultramarine is a pigment made from ground lapis lazuli). The diatomite works at Dinnet were dismantled during the end of the decade (late 1910s) but an excellent set of archival photographs still survives (Haldane et al., 1940).

Other Sites
There are three further sites of particular interest (mainly in a scientific rather than commercial context).

• On the Isle of Lewis, some fifteen miles North of Stornoway, is a deposit called the North Tolsta deposit. This site was worked on a minor scale, yielding quite good quality diatomite and there is estimated to be around 2000 Imperial tons left.
• At Kinross in Fife there is a thin layer of diatomite which is thought to have been deposited by Loch Leven as it changed its boundaries.
• Near Balerno, Edinburgh, at a place called Dalmahoy, there is a diatomite deposit which was discovered by one J. W. Lunn in 1924, and the diatomite from it was extensively analysed by researchers in Dundee. Recent attempts to study the deposit further met with problems as there is rumoured to be an unexploded bomb from the Second World War lying somewhere in the diatomite! (Henderson, 1925; D. Mann, pers. comm.).

Recent Commercial Possibilities
Scottish diatomite is not currently being exploited, and studies  which have looked into the possibility of a revival generally suggest it would be unfeasible. In 1969 a survey was carried out on the Loch Cuithir site but it seemed uneconomical to attempt the work, even with government help. More recently, the Highlands and Islands development board suggested that only a highly mechanised, low workforce operation would be able to see a suitable return from the same deposits.

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References

• Canter-Lund, H. & Lund, J.W.G. (1995), Freshwater Algae Their Microscopic World Explored.  Biopress, Bristol.
• Haldane, D., Eyles, V. A. & Davidson, C. F. (1940). Wartime Pamphlet No. 5: Diatomite. Geological Survey of Great Britain, South Kensington.
• Henderson, E. B. (1925). Notes on a diatomaceous deposit at Dalmahoy, Edinburgh. Transactions of the Botanical Society of Edinburgh. 29: 10 – 143.
• Mann, D. & Stickle, A. J. (1997). Sporadic evolution of dorsoventrality in raphid diatoms, with special reference to Lyrella amphoroides sp. nov. Nova Hedwigia 65: 59 - 77.
• Stoermer, F. & Smol, J.P. (1991). The Diatoms: Applications for the Environmental and Earth Sciences.  Cambridge University Press, Cambridge.
• Taylor, F.B. (1929). Notes on Diatoms.  Guardian Press, Bournemouth.
• Van Den Hoek, C., Mann, D.G. & Jahns, H.M. (1995). Algae an Introduction to Phycology.  Cambridge University Press, Cambridge.
• West, G.S. & Fritsch, F.E. (1932). A Treatise on the British Freshwater Algae.  Cambridge University Press, Cambridge.

Website References:

• http://www.bambi.demon.co.uk/skyedata/minerals.htm
• http://www.roskill.co.uk/diatom.html
• http://www.worldminerals.com/celite/index.htm