Refractive indexes for the end-member garnets and hydrogarnets were calculated from the unit-cell dimensions and the ionic refractivities, using the Lorentz-Lorenz relation. Values for the ionic refractivities of Fe (super 3+) , Cr (super 3+) , Si, Al, and H 4 were obtained. The calculated and observed values of n are compared for 75 chemically analyzed garnets. Some situations where the calculated n exceeds the observed n are best explained as the result of replacement of SiO 4 by H 4 O 4 .

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