TETRACALCIUM PHOSPHATE 195 



content of phosphoric acid will be low. When made from pigs 

 rich in phosphorus there may be a comparative deficiency of iron 

 in the slag, and in this case the content of tetrabasic calcium phos- 

 phate would be unusually high. 



It is found also that the content of iron in the slag varies 

 widely. In general, the greater the content of iron the harder 

 the slag and the more difficult to grind. If the pig iron contain 

 sulfur, as is often the case, this sulfur is found also in the slag in 

 combination with the lime, either as a sulfid or sulfate. 



No certain formula can therefore be assigned to basic slags and 

 the availability of each one must be judged by its chemical com- 

 position. 



165. Composition of Slag Phosphate. The slags produced by the 

 method above outlined may be amorphous or crystalline. When 

 large masses are slowly cooled the interior often discloses a crys- 

 talline composition. In some samples analyzed in the labora- 

 tory of the Division of Chemistry the crystals were found to be 

 of two forms, viz., acicular and tabular. 46 They had the follow- 

 ing composition : 



CALCULATED PER CENTS. AS 



CaO. Fe a O 3 . A1.O 3 . MgO. V 8 O S . P S O 5 . SiO z . 



Acicular crystals 42.69 20.98 3.71 0.49 0.18 27.06 4.96 



Tabular crystals 53.61 9.64 0.91 0.08 ... 33.92 1.75 



These data show that the two sets of crystals belong to two 

 distinct mineral forms. The presence of vanadium in one of the 

 samples is worthy of remark, and leads to the suggestion that in 

 the slags made of phosphoriferous pigs may be found any of the 

 rare metals which may exist in the ores from which the pigs were 

 made. The amorphous portions may have a widely varying com- 

 position and consequent variable content of phosphoric acid. In 

 all good slags, however, whether in crystalline form or as amor- 

 phous powder, the lime and phosphoric acid will be found com- 

 bined as tetracalcium phosphate (Ca 4 P 2 O 9 ). 



1 66. Molecular Structure of Tetracalcium Phosphate. Several 

 theories have been- advanced in respect of the atomic arrange- 

 ment of the elements contained in a molecule of tetracalcium 

 phosphate. It must be confessed that so little is known concern- 



48 Journal of Analytical and Applied Chemistry, 1891, 5 : 685. 



