672 WATSON AND STEIGERI SPINELLITE FROM VIRGINIA 



The results given in the table above indicate the differences 

 in composition of the spinel from the three widely separated 

 localities. By way of comparison attention need only be di- 

 rected to the closer similarity in composition of the spinel from 

 Virginia and Norway than that from New York. Upon further 

 comparison with analyses of spinel from other localities, the Vir- 

 ginia mineral is remarkable for its comparatively low MgO and 

 high Fe203. 



According to the analysis given in table 2, the Virginia spinel 

 corresponds to the formula (Fe, Mg)0.(Al, Fe)203, in which FeO 

 is molecularly greater than MgO and Fe203 is slightly more than 

 one-eighth of the AI2O3. In the comparatively high Fe203 the 

 mineral shows similarity to chlorosp'nel, while in the protoxide 

 ratio (FeO molecularly greater than MgO) approach toward 

 hercynite is indicated. It is, however, apparently more closely 

 allied to pleonaste to which it is referred and in which Fe203 on 

 chemical grounds is shown in this case to be isomorphous with 

 AI2O3. 



The spinel in the New York and Norway rocks is also referred 

 to the variety pleonaste by Rogers- and by Vogt.^ Williams^ previ- 

 ously concluded that from low MgO (about 9 per cent) shown on 

 analysis of the New York spinel, the results of which were un- 

 fortunately lost, the mineral closely approached hercynite, al- 

 though on a later page (197) he remarks, in commenting on other 

 analyses, that in "a very large proportion, if not all, of the New 

 York spinel, the samples consisted of pleonaste." 



Magnetite, the second most important mineral quantitatively, 

 is usually much less abundant than spinel. It is developed in 

 grains that often show a strong tendency toward crystal outline, 

 although good crystals are rare. The grains will average slightly 

 less in size than those of spinel and are scattered among the 

 latter with which they are intergrown or interlocked with sharp 

 angular boundaries, and are also formed as inclusions in both 

 pleonaste and corundum. 



2 Rogers, G. S., Ann. N. Y. Acad. Sci. 21: 69. 1911. 



3 VoGT, J. H. L., Zeitschr. Prakt. Geol., p. 237, 1900. 

 * Williams, G. H., Amer. Journ. Sci. 33: 195. 1887. 



