There is urgent need for more accurate chemical and optical data on common rock-forming minerals. Mineralogists have naturally been more interested in seeking and describing the rare, remarkable and beautiful among minerals, and have thus neglected somewhat the everyday minerals with which the petrologist is concerned. A comprehensive investigation to collect such data was begun by the writers in 1934. The present paper is the second article to be published as a result of this plan and a number of others were under way at the time of Professor Phillips’ death.1 These will be completed and published from time to time in the future. In order to call attention to the continuity of the series they will be designated as the “Princeton Investigations of Rock-forming Minerals.”

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