8 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 
CALCIUM FLUORIDE. 
Fluorides are present in almost all phosphate rock. Some samples 
contain as high as 8 per cent of calcium fluoride (CaF 2 ). The amor- 
phous phosphates as a rule contain smaller quantities of this com- 
pound than apatite. 
Calcium fluoride reacts with sulphuric acid, giving gaseous hydro- 
fluoric acid (HF) and calcium sulphate, thus: 
Calcium fluoride. Sulphuric acid. Hydrofluoric acid. Calcium sulphate. 
CaF 2 + H 2 S0 4 = " 2HF + CaS0 4 
But hydrofluoric acid (HF) acts upon the silica or silicates present 
in the mass, producing gaseous silicon, tetrafluoride (SiF 4 ), and water 
or steam, thus : 
Hydrofluoric acid. Silica. Silicon tetrafluoride. Water. 
4HF + Si0 2 = SiF 4 + 2H 2 
Silicon tetrafluoride in turn is decomposed by water with the 
formation of hydrofluosihcic acid (H 2 SiF 6 ) and precipitation of pure 
silica (Si0 2 ), thus: 
Silicon tetrafluoride. "Water or steam. Hydro fluosilicic acid. Silica. 
3SiF 4 + 2H 2 = 2H 2 SiF 6 + Si0 2 
Before this last reaction takes place, however, much of the silicon 
tetrafluoride escapes from the mass and can be detected by its pene- 
trating odor and smarting effect on the eyes and nose. 
Veiy high grade acid phosphate can be made from rock containing 
large amounts of fluorine, because, as pointed out above, many of the 
products formed during the process escape as gases or vapors, leaving 
the mass correspondingly richer in phosphoric acid. These gases 
also, in forcing their way out of the acid phosphate, tend to render it 
porous and more readily dried. The product, therefore, can be easily 
broken up and mixed with other ingredients to make a complete 
fertilizer. 
The main objections to using phosphates high in fluorides are, first, 
the increased quantity of sulphuric acid necessary to decompose these 
compounds, and, second, the noxious and even poisonous nature of 
the gases evolved during their decomposition. 
COMPOUNDS OF IRON AND ALUMINUM. 
Iron and aluminum oxides, either in the free state or combined as 
phosphates, are the most objectionable of the impurities found in 
phosphate rock. These substances even when present in very small 
quantities cause a certain amount of "reversion" in the superphos- 
phate, and when present in large quantities are likely to produce a 
sticky acid phosphate unfit for commercial purposes. 
The phosphate of iron in natural occurrences may conveniently be 
represented by the formula FeP0 4 , although actually it is probably 
