Inesite, a hydrous manganese, calcium silicate from Quinault, Washington, a new locality for that mineral, has been analyzed and the optical properties determined. For purposes of comparison, the optical properties of inesite from eight other localities have been redetermined and the chemical analyses compiled. These all show close agreement in optical properties, and chemical composition with the inesite from Quinault. The maximum variation of indices of refraction is 0.005, and agrees with the uniformity in chemical composition. The average indices of refraction of inesite are: α = 1.616, β = 1.640, γ=1.651.

The analyses do not agree with the commonly accepted simple formulas. Calcium is present in fixed quantity and does not vary reciprocally with manganese, as it does in rhodonite. No simple formula could be determined for inesite. The most probable formula seems to be the complex one 15SiO2·3CaO·11MnO·10H2O.

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