In this chapter we shall give an account of the more commonly occurring mineral phosphates. In Chapter V., where we discussed the position of phosphoric acid in agriculture, it was pointed out that mineral phosphates were very abundant, and that large deposits of them were found in many parts of the world.

Coprolites.

Reference may first be made to the so-called coprolites or phosphatic nodules which have been found in great abundance in the greensand formation, in the crag of the eastern counties, and in the chalk formation of the southern counties. These coprolites are rounded nodules, and are composed of the fossil excrements and remains of ancient animals. They are found in large quantities in Cambridgeshire, and were discovered by Dr Buckland many years ago. The history of their discovery is not a little curious. The manurial properties of road-scrapings in parts of Cambridgeshire were noticed, and on being examined were found to be in part composed of phosphate of lime, derived from phosphatic nodules dug out of the underlying greensand, and used for the purpose of repairing roads. Professor Henslow first drew attention to them at a meeting of the British Association held in Cambridge in 1845, and pointed out that they contained about 60 per cent of phosphate of lime. They were also found in enormous quantities in Suffolk, Norfolk, Bedfordshire, and Essex, and were for a long time largely used in the manufacture of superphosphate, but of late years have not been used to anything like the same extent, owing to the fact that there are richer and cheaper sources of phosphate of lime available. In 1887 about 20,000 tons of coprolites were raised. The richest were those obtained in Cambridge, while those got from Bedfordshire were about the poorest. Deposits have also been found in France and other countries. The average amount of phosphate of lime in English coprolites is between 50 and 60 per cent, while the French contain about 45 per cent.

Canadian Apatite or Phosphorite.

We have already referred in Chapter V. to large deposits of apatite or phosphorite found in Canada. The Canadian mines commenced to be worked about fifteen years ago, and the output now amounts to nearly 25,000 tons per annum.[221] A portion of this goes to the United States; the rest, amounting to about 20,000 tons, being shipped to England, whence it is again exported to Hamburg and other places.[222] It contains from 70 to 80 per cent of phosphate. Deposits are also found at Estremadura in Spain, and in Norway.

Estremadura or Spanish Phosphates.

Large deposits of phosphate have long been known to exist at Estremadura in Spain, and the mines at Caceres have been worked on a large scale for seventeen years, and about half a million tons have been raised. In 1882 the imports into this country amounted to over 56,000 tons; but latterly they have only been about a fourth of this amount. Dr Dauberry visited the deposits in 1843, and wrote a most interesting account of them. They do not seem, however, to have been imported for purposes of superphosphate manufacture till a number of years afterwards. Of Estremadura phosphate there are three classes, containing respectively 50, 60, and 70 per cent of phosphate of lime, the lowest quality being the commonest.[223]

Norwegian Apatite.

This apatite has ceased to be imported of late years, owing to a duty on exportation.

Charleston or South Carolina Phosphate.

For a number of years these deposits have formed the chief source of phosphate of lime used in the manufacture of mineral superphosphates in this country (in fact they have furnished two-thirds of our phosphate supply during recent years). Discovered twenty-five years ago, some four to five million tons have already been shipped. About half a million tons were raised in 1886 from these mines, which are the most abundant in the world. There are two kinds—the so-called "land" and "river" phosphates. The former contains more oxide of iron and alumina, and is therefore less pure than the latter, in which the iron and alumina do not exceed 2 per cent. The river phosphate is dredged from the Bull, Coosaw, and Beaufort rivers. Of phosphate of lime it contains from 50 to 60 per cent. It is generally sold in three grades—50 to 52 per cent, 55 to 56 per cent, and 58 to 60 per cent of phosphate of lime. It will thus be seen to be incapable of producing very high-class superphosphates —i.e., containing more than 30 per cent "soluble" phosphate. This point will be more intelligible when we describe the manufacture of superphosphate. The demand for these phosphates in the United States has increased enormously in recent years, owing to the increase in the quantity of manure used.

Belgian Phosphate.

Another very important source of mineral phosphates are deposits discovered some years ago in Belgium near Mons. These phosphates are of different qualities, and are found, some in layers near the surface in pockets forming the richest class, and containing from 45 to 65 per cent of phosphate, and some in the form of a friable phosphatic rock, the so-called craie-grise (phosphatic chalk), containing from 25 to 35 per cent of phosphate of lime. The higher quality of Belgian phosphate is pretty well exhausted, and it is the second class that forms the bulk of the ordinary Belgian phosphate at present exported. The commercial article contains about 35 to 40 per cent of phosphate, and about 45 per cent of carbonate of lime. The fact of its poor quality, together with the large percentage of carbonate of lime it contains, renders its adoption alone in the manufacture of superphosphate unsuitable. Attempts have been made to get rid of a portion of this carbonate of lime and to raise the percentage of phosphate. For this purpose the phosphate has been calcined, but this was soon found to be a great mistake. Other means have been adopted, with the result that the percentage has been increased to 50 per cent. It is consequently used in small quantities as a drier, for which it is peculiarly suited on account of its carbonaceous nature, along with the higher-class phosphates. In the year 1886 about 145,000 tons of this phosphate were raised, of which about 45,000 tons were imported into the United Kingdom.

Somme Phosphate.

Still more recently a discovery of phosphate deposits has been made in the Somme and Pas de Calais departments in the north of France, adjoining, and similar in character to, the Belgian deposits. The only difference between Belgian and French phosphates is, that the latter is of a higher quality, and contains from 50 to 80 per cent of phosphate of lime. A very large demand for these phosphates sprang up, and in 1888, although they had only been worked for some two years, no less than 150,000 tons had been raised, of which about one-half contained from 70 to upwards of 75 per cent. There are four grades in the market, containing 55 to 60, 60 to 65, 70 to 75, and 75 to 80 per cent of phosphate of lime. The highest quality furnishes the chief material for the manufacture of high-grade superphosphates.

Florida Phosphate.[224]

During the last few years large quantities of phosphates have been imported from Florida. These are of different qualities, the land rocks now imported containing from 70 to 80 per cent of phosphate of lime, and the river phosphate about 60 per cent. The latter class are similar in composition to the best South Carolina river-phosphates, which they much resemble.

Lahn Phosphate.

Phosphate deposits were found at Nassau in Germany in 1864; but as the phosphate contained a considerable proportion of iron and alumina, they are not used in this country now, although they are in Germany for double superphosphate manufacture.

Bordeaux or French Phosphate.

Similar in quality to Lahn phosphate is that obtained in the neighbourhood of Bordeaux.

Algerian Phosphate.

Excellent phosphates are now being sent from Algeria—some cargoes being as rich as 70 per cent.

Crust Guanos.

We have already referred to the guanos in the chapter on Guano. They are also known under the name of Caribbean phosphates, and come from the West India Islands. The chief kinds are Aruba, CuraÇao, Sombrero, and Navassa, the Great Cayman, Redonda, and Alta Vela. Most of them are of high quality, containing from 60 to 80 per cent of phosphate, and are thus suited for the manufacture of high-class superphosphates. Some of them, however, contain a considerable proportion of iron and alumina, and are not suitable for this purpose. The Redonda and Alta Vela phosphates consist chiefly of phosphate of alumina.

Value of Mineral Phosphates as a Manure.

While it is commonly regarded as unadvisable to use mineral phosphates directly as phosphatic manures, it may well be questioned how far such an opinion is warranted by actual experience. Professor Jamieson of Aberdeen, in his interesting and valuable experiments, has drawn attention to the fact that coprolites in a fine state of division are an extremely valuable source of phosphoric acid for crops, and are a more quickly available source than is commonly supposed. Experiments conducted elsewhere with ground coprolites and other mineral phosphates corroborate Professor Jamieson's conclusions. The successful use of Thomas-phosphate has drawn attention to the possibility of profitably applying undissolved mineral phosphate to the soil; and no doubt the practice may in future years be increased. At present, however, with the exception of Thomas-phosphate, mineral phosphates alone are used for conversion into superphosphate.

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