MINERAL PHOSPHATES. In mineralogy, those varieties of native calcium phosphate which are not distinctly crystallized, like apatite (q.v.), but occur in fibrous, compact or earthy masses, often nodular, and more or less impure, are included under the general term phosphorite. The name seems to have been given originally to the Spanish phosphorite, probably because it phos phoresced when heated. This mineral, known as Estremadura phosphate, occurs at Logrossan and Caceres, where it forms an important deposit in slate. It may contain from 55 to 62% of calcium phosphate, with about 7% of magnesium phosphate. Dahllite is a Norwegian phosphorite, containing calcium carbon ate, named in 1888 by W. C. Brogger and H. Backstrom after the Norwegian geologists T. and J. Dahll.
Phosphorite, when occurring in large deposits, is a mineral of much economic value for conversion into the superphosphate largely used as a fertilizing agent. Many of the impure substances thus utilized are not strictly phosphorite, but pass under such names as "rock-phosphate," or, when nodular, as "coprolite" (q.v.), even if not of true coprolitic origin. The ultimate source of these mineral phosphates may be referred to the apatite widely distributed in crystalline rocks. Being soluble in water containing carbonic acid or organic acids it may be removed in solution, and may thus furnish plants and animals with the phosphates required in their structures. On the decay of these structures the phos phates are returned to the inorganic world, thus completing the cycle.
There are three sources of phosphates which are of importance geologically. They occur (a) in crystalline igneous and meta morphic rocks as an original constituent, (b) in veins associated with igneous rocks, and (c) in sedimentary rocks either as organic fragments or in secondary concretionary forms.
The first mode of occurrence is of little significance practically, for the crystalline rocks generally contain too little phosphate to be valuable, though occasionally an igneous rock may contain enough apatite to form an inferior fertilizing agent, e.g., the trachyte of Cabo de Gata in south-east Spain, which contains 12-15% of phosphoric acid. Many deposits of iron ores found in connection with igneous or metamorphic rocks, and the oolitic iron ores, such as those of the Jurassic system in England and Lorraine, contain from 1 to 2% of phosphorus, which passes into the slags in the basic process of steel-making, and forms a very important source of phosphatic manures ("basic slag").
Another group of phosphatic deposits connected with igneous rocks comprises the apatite veins of south Norway, Ottawa and other districts in Canada. These are of pneumatolytic origin (see PNEUMATOLYSIS), and have been formed by the action of vapours emanating from cooling bodies of basic eruptive rock. They once formed an important source of phosphate, but are now worked out.
The phosphatic rocks which occur among the sedimentary strata are the principal sources of phosphates for commerce and agri culture. They are found in formations of all ages from the Cam brian to those which are accumulating at the present day. Of the latter the best known is guano. (See MANURES and MANURING.) Where guano-beds are exposed to rain their soluble constituents are removed and the insoluble matters left behind. The soluble phosphates washed out of the guano may become fixed by entering into combination with the elements of the rock beneath. Many of the oceanic islets are composed of coral limestone, which in this way becomes phosphatized; others are igneous, consisting of trachyte or basalt, and these rocks are also phosphatized on their surfaces but are not so valuable, inasmuch as the presence of iron or alumina in any quantity renders them unsuited for the prepara tion of artificial manures.
The leached guanos and phosphatized rocks, which are grouped with them for commercial purposes, have been obtained in great quantities in many islands of the Pacific Ocean (such as Baker, Howland, Jarvis and McKean Islands) between long. 150° to 180° W. and lat. 10° N. to 10° S. In the West Indies from Vene zuela to the Bahamas and in the Caribbean Sea many islands yield supplies of leached guanos ; the following are important in this respect : Sombrero, Navassa, Ayes, Aruba, Curacao. Christmas Island has been a great source of phosphates of this type ; also Jaluit Island in the Maldive Archipelago, Banaba or Ocean Island, and Nauru or Pleasant Island. On Christmas Island the phosphate has been quarried to depths of ioo feet. To these leached guanos and phosphatized limestones the name sombrerite has been given. It has been estimatedthat soo,000 tons of phosphate were obtained in Aruba, 1,000,000 tons from Curacao since the deposits were discovered in 1870, and Christmas Island in 1925 yielded 110,000 tons.