GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION
Zirconium occurs in nature in commercial quantities as a mixed oxide and silicate known as baddeleyite, sometimes called brazilite, and as the silicate, zircon. Baddeleyite, as supplied to the trade, usually carries 80 to 85 per cent. ZrO2. The silicate, zircon, carries about 65 per cent. ZrO2. The oxide deposits, containing as they do a higher percentage of zirconium and the ore being more pure and easier to reduce, will in all probability become the principal source of zirconium.
The principal known deposits of zirconium ores are in Brazil, India and the United States, the countries being named in the relative order of their commercial importance.
The Natural Oxide.
—The natural oxide, baddeleyite, or brazilite, occurs only in Brazil in commercial quantities and is described in Mineral Foote-Notes by Meyer,[99] who writes as follows:
[99] Meyer, H. C., Mineral Foote-Notes, November, 1916, p. 29.
There are but few commercial deposits of the unusual ores which present more interesting geologic as well as economic features than do the deposits of natural zirconium oxide in Brazil. The Caldas region (visited in 1915 by the writer) in which these zirconia deposits occur, is situated partly in the State of Minas Geraes and partly in the State of Sao Paulo, approximately 130 miles north of the city of Sao Paulo. It is a mountainous plateau, the main elevation of which is about 3,600 feet. The surface is undulating, presenting differences in level of from 300 to 600 feet.
The whole area is bounded on all sides by ridges rising abruptly from 600 to 1,200 feet above the general level and forming a roughly elliptical enclosure with a major axis of approximately 20 miles in length and a minor axis of 15 miles. This peculiar arrangement of the higher ridges is very significant when coupled with the fact that the predominant rock of the plateau is a phonolite and the presence of highly mineralized thermal water of considerable medicinal value.[100] No thorough geological survey has been made of this area with a view to determining the origin of the zirconia. The character of the ore, however, and the formation, seems to point to pneumatolitic agencies. A careful study of the relationship of the large masses of coarsely crystalline nephelite-syenite in this area, with pronounced segregations of eudialyte, might throw some light upon this subject.
[100] Derby, O. A., “Nepheline-rock in Brazil:” Quart. Jour. Geol. Soc., August, 1887.
Zirconia ore can be roughly divided into two classes:
First, alluvial pebbles ranging in size from one-half inch to three inches in diameter, generally carrying about 90 per cent. to 93 per cent. zirconium oxide. These pebbles, known as favas and having a specific gravity ranging from 4.8 to 5.2, are found along small stream beds and on the talus slopes of low ridges.
Second, zirconia ore proper, or zirkite (a trade name), which ranges in shade from a light gray to a blue black, the lighter colored material carrying a higher percentage of zirconium silicate, as evidenced by analyses, which in some cases show a minimum of 73 per cent. zirconium oxide. The blue-black ore generally carries from 80 per cent. to 85 per cent. zirconium oxide. By careful sorting, however, a uniform grade carrying about 80 per cent. is produced.
Prior to the investigations of Derby and Lee, this ore was considered identical with baddeleyite. It has now been shown, however, that it is a mechanical mixture of three minerals; namely, brazilite, zircon, and a new and unnamed zirconium silicate carrying about 75 per cent. zirconium oxide. This new mineral has the same crystal form as zircon (67 per cent. ZrO2) but is readily soluble in hydrofluoric acid, while zircon is not affected, this being a characteristic differential test. The finely powdered mineral, on being treated with a weak solution of hydrofluoric acid, leaves a residue of minute, perfect, pyramidal crystals of zircon, the brazilite and new zirconium silicate going into solution. Several large outcrops of the ore occur on the extreme westerly edge of the plateau, one or two isolated boulders weighing as much as thirty tons. This very cursory examination of the zirconia deposits makes it unsafe to venture any conjecture as to the quantity of ore available. Suffice it to say, however, that the deposits have been traced for a distance of fifteen miles between Gascata and Caldas, and if surface indications are of any significance, are of vast extent.
The oxides have also been found in the State of Montana, and in Ceylon, Sweden, and Italy, but none of these occurrences are of commercial importance.
The Silicate.
—The simple silicate, zircon, is found in seashore and river concentrations of monazite sands, associated with ilmenite, garnet, rutile, and various other heavy minerals usually found in such places.
An important concentration of zircon occurs along the coast of Brazil, in the states of Bahia, Espirito Santo, and Rio de Janeiro, where cusps of the beaches are protected on the north by granite headlands and bordered by Tertiary bluffs, which are cut by various streams and lagoons that constantly furnish fresh material for the concentrating action of the tides and waves.
Probably the next most important occurrence of zircon is in India, in the beach sands of the province or state of Travancore at the extreme southwestern end of the Hindustan peninsula.
Next in importance is an occurrence in the United States at Pablo Beach, Florida, where not only the beach sands, but the dunes bordering them, contain appreciable quantities of the following minerals in their relative order of abundance: ilmenite, garnet, epidote, zircon, rutile, and other heavy minerals including monazite.[101]
[101] Hess, Frank L., Letter of May 1, 1918.
Other occurrences in this country, most of which are of academic interest only, are in Colorado, at St. Peter’s Dome, near Pike’s Peak; in Idaho, in the Clearwater region and elsewhere in black sands and in certain granitic rocks; in Sussex County, New Jersey, at the Williams mine, where zircon occurs abundantly in magnetite; in New York at Lyon Mountain, Clinton County; at a few places near Crown Point; abundantly in pegmatite at Old Red Mines, Mineville, Essex County, and in numerous places in Orange County on the south, and St Lawrence County on the north. In North Carolina, in Burke, McDowell, and Rutherford counties, zircon occurs in monazite sands, and also in Henderson County near Zirconia and in Fredell County near Sterling.
Large crystals of zircon occur in a small pegmatite area in Comanche County, Oklahoma, in the southwestern portion of the Wichita National Forest. Oregon has many localities in which the presence of zircon has been noted, chiefly in black sands, old and present beaches, placer gravels, etc., and the same is true of Washington. In Virginia, zircon is found in pegmatite in Aurelia County, and in sandstone near Ashland, not far from Richmond.
Zircon is also found in Norway, the Ural Mountains, Ceylon, Australia, and British South Africa.