Accessory apatite from hybrid granitoid rocks of the southern Snake Range, Nevada

Analytical data, optical properties, and unit-cell parameters are presented for 24 samples of accessory apatites recovered from hybrid granitoid rocks of the southern Snake Range, Nev.  A complete chemical analysis is given for one. In the Snake Creek-Williams Canyon
outcrop area, where the hybrid rocks grade from granodiorite with 63 percent SiO₂ to a quartz monzonite with 76 percent SiO₂ within a
horizontal distance of about 3 miles, abundance and crystal habit of the apatite change with rock chemistry. The apatite probably is slightly
more than 90 mole percent fluorapatite in the most felsic rocks, slightly less in the most mafic. The range of F->OH substitution in apatite from this outcrop area is much smaller than in the coexisting biotites. Except for manganese, strontium, sodium, and the rare earths, there is very limited substitution for calcium in the crystal structure of these apatites. Apatites from the more mafic rocks tend to contain a lighter,
more basic assemblage of rare earths, in agreement with results obtained for coexisting allanites, monazites, and zircons.