Chemical composition of phosphorites of the Phosphoria Formation

The chemical composition, both major and minor constituents, of 60 samples of phosphorite from the Phosphoria Formation was determined. Major constituents of the average phosphorite are, by weight per cent: SiO₂, 11·9; Al₂O₃, 1·7; Fe₂O₃,1·1; MgO, 0·3; CaO, 44·0; Na₂O, 0·6; K₂O, 0·5; total H₂O, 2·2; H₂O⁻, 0·6; TiO₂, 0·1; P₂O₅, 30·5; CO₂, 2·2; SO₃, 1·8; F, 3·1; organic matter, 2·1; and oil, 0·2. Uranium averages 0·009 per cent.

The phosphate mineral is basically apatite, Ca₅(PO₄)₃F, with small but significant and variable substitutions—Na, Sr, U and Th for Ca, and CO₃ and SO₄ for PO₄.

Rare metals not associated with apatite are associated principally with the organic-matter component of the rocks. This group includes As, Ag, Cd, Cr, Cu, Mo, Ni, Sb, Se, V and Zn. Chromium is the most abundant, having a modal abundance of 0·1 per cent and a maximum concentration of 0·3 per cent.

The average phosphorite is composed of approximately 80 per cent apatite, 10 per cent quartz, 5 per cent muscovite-illite, 2 per cent organic matter, 1 per cent dolomite—calcite, 1 per cent iron oxide, and 1 per cent other components. It is texturally a medium-grained pellet phosphorite.