Chemical compositions of epidote from active and ancient geothermal systems were summarized. Iron and aluminum contents of epidote from geothermal systems vary widely, ranging from about 4 to 19 wt % for Fe
2O
3 and from 18 to 30 wt % for Al
2O
3. It was found that the iron content of epidote has roughly a positive correlation to the Fe
2O
3 content of original rocks, indicating that the Fe
2O
3 content of original rocks affects largely the variations in iron content of epidote. This correlation is consistent with the results of theoretical calculations on the water/rock interaction under the hydrothermal conditions which show that the epidote is formed under the rock-dominated conditions. Epidote associated with hematite has high iron content, while that associated with prehnite, pyrite and pyrrhotite has low iron content. These facts indicate that CO
2 and O
2 fugacities are also important factors controlling the compositional variations in epidote.
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