Eakle and Rogers — Wilkeite, a New Mineral. 265 



that the best results were obtained by its precipitation with 

 magnesia mixture in the solutions remaining after direct 

 precipitations of all calcium. The great excess of calcium in 

 the solution permits of a direct precipitation by ammonia of 

 the phosphorus, as Ca 3 P 2 8 , and a small amount of manganese 

 phosphate, presumably Mn 3 P 2 8 . The former goes into solu- 

 tion readily on addition of acetic acid, leaving the manganese 

 phosphate to be weighed as such. 



The average of the analyses gave as the composition of 

 wilkeite : 



Molecular ratios. 

 19-66 or 20 



CaO 54-44^ 0-972) 



MnO 0-7-7 0-011 ) 



P 2 O s ... 20-85 0-147 2-94 3 



SO s ... 12-28 0-157 3-14 3 



SiO s 9-62 0-160 3-20 3 



C0 2 2-10 0-048 0-96 1 



H„0 trace 



100-06 



The molecular ratios indicate that the chemical formula 

 of wilkeite is 20CaO.3P 2 O,.3SiO 2 .3SO 3 .CO„, which may be 

 written 3Ca 3 (PO 4 ) 2 .3Ca 2 SiO 4 .3CaSO 4 .Ca0O 3 .CaO. 



Relation to Apatite. — The physical properties suggest that 

 wilkeite belongs to the apatite group, but it contains only 20 

 per cent P 2 6 as against 41 or 42 per cent for apatite. The 

 formula may be written 30a 3 (PO 4 ) 2 .CaCO 3 .3Ca 3 [(SiO 4 )(SO 4 )]. 

 CaO, which shows its relation to the apatite group. In wilke- 

 ite the compound radical [(Si0 4 )(S0 4 )] vi is equivalent to 

 (P 2 8 ) vi . The replacement of fluorine in the formula 3Ca 3 (P0 4 ) 2 . 

 CaF„ by oxygen, O, or by the carbonate radical, C0 3 , may be 

 explained by mass-effect isomorphism ; but the replacement of 

 (P0 4 ) 2 by the compound radical [(Si0 4 )(S0 4 )] is explained by 

 the fact that they have nearly the same formula weights, 190 

 and 188'4, respectively, and the same valence (six). Wilkeite 

 has a molecular volume almost identical with that of apatite, 

 and this explains why compound molecules, which are so dis- 

 similar chemically, may form isomorphous mixtures. Whether 

 wilkeite is an isomorphous mixture of 3Ca 3 (P0 4 ) 2 .CaC0 3 and 

 30a 3 [(SiO 4 )(SO 4 )].CaO or 3Ca 3 (P0 4 ) 2 .CaO and 30a 8 [(SiO 4 )- 

 (S0 4 )].CaC0 3 or a double salt of these two compounds, it is 

 impossible to say. The ratios are fairly close to Ca 3 (P0 4 ) 2 : 

 Ca 2 SiO, : CaS0 4 = 1:1:1, but this agreement may be acci- 

 dental. 



There are a number of minerals with three radicals, such as 

 hanksite, 9Na a S0 4 .2Mg 9 CO a .KCl, sulfohalite, 2Na 2 S0 4 .NaCl. 



