The optical anisotropy of 44 anhydrous carbonates were discussed in this paper. These minerals belong to uniaxial and biaxial (2V≤40°) crystals. Their refractivities, K ware calculated according to Gladstone-Dale’s formula. The refractivity of a carbonate mineral can be resolved into two components, for ordinary ray (Ko)and Ke for extraordinary ray. If the mineral is biaxial, positive,and and if the mineral is biaxial negative,where d is density. Two new conceptions, the birefractivity (BE) andthe optical anisotropic index (OAI), are proposed by the authors, BR = Ko-Ke, OAI = Ke/Ko. It will be convenient to discuss the relationship between optical amsotropy and structure of a carbonate mineral by using the two new conceptions. The conclusions may be drawn as follows:1. Bach component refractivities, Ko or Ke, of a complex carbonate can be directly calculated from that of the single carbonates. The results calculated are in good agreement with the experimental values. The largest deviation between them is only ±0.060, and average deviation only -0.001.2. The BR of carbonates having the same structural type are approximately a constant, that is 0.060 for calcite-type, 0.039 for aragonite-type (but 0.051 for arago-nite),-0.023 for vaterite-type (but-0.039 for vaterite) and so on.3. The OAI of carbonates in which all the oblate CO3 groups are not only parallel to each other, but also perpendicular to the unique axis, is equal to about 0.75. While the oblate CO3 groups in the carbonates are not parallel to each other, but parallel to the unique axis, the OAI is equal to 1.14 or so. If there exist two different orientations of CO3 groups in a carbonate mineral, its OAI is equal to about 0.90 instead of 0.75 or 1.14.4. For the first transitional elements the effect of the cation on refractivity depends upon their electronic configuration, especially on the number of d-electron.5. The authors infer that the CeCO3F having negative sign as calcite may probably be found in nature, its two refractivity components Ko and Ke will be 0.186 and 0.139, respectively.