CHROMITE AND PICOTITK. 



183 



in looking at the table it can be seen that in the first 23 analyses the limits 

 of Crg O3 are 4.74 and 08. 12; while in these 23 analyses, there are to be 

 found of analyses of Mg O, one having no Mg ; four having less than 10 per 

 cent; fourteen carrying between 10 and 20 per cent; four carrying between 

 20 and 30 per cent; and none having any higher percentage. But in these 

 same 23 analyses we see that there are five (picotites) which carry between 

 50 and 60 per cent of Al^ O3. 



TABLE II. 



In Table II. there is given in the upper line the percentage limits for the 

 different constituents, and below, the number of analyses whose constituents 

 fall between those limits. Thus it will be seen that out of the 120 analyses, 

 there are seven containing less than 10 per cent of CrgOg, foar between 10 

 and 20 per cent, etc. ; while there are 81 analyses in which no Fe2 03 is re- 

 ported and one which has between 60 and 70 per cent of it. From the above 

 illustrations, the use of the tables will doubtless be readily understood. 



In reply to a question of the present writer as to his views concerning 

 the chemical relations of chromite and picotite, Dr. Genth states that, in 

 common with many others, he thinks " that all the aluminates, ferrates, etc. 

 of Mg, Zn, Mn, and Fe, which are isometric, are only varieties of the species 

 R R2 O4 — which so gradually change from one into the other, that it is often 

 difficult to say where one begins and the other ends. Of typical forms we have 

 only spinel (MgAl2 04), magnetite (Fe Fcg O4), and magnoferrite (MgFe2 04). 

 All the rest are mixtures. As for chromite, I have never seen a specimen 

 which did not contain magnesia and alumina, although some of the analyses 

 do not give any ; but there are a great many poor analyses." 



In the same manner, Rammelsberg classes together chromite and picotite, 

 not even placing the latter under spinel.* 



In the microscopic study of cliromites and j^icotites we see a reason tor 

 the variability in their chemical analj^ses, and the nearly constant presence 



* Handbuch der i\[incra!chcmio, 1875, pp. 141-144. 



