Article: A word to the wise: hydrothermal.

Most of us learned at a very early stage in our geologic education that there are three major types of rocks: igneous, sedimentary, and metamorphic. There is another group of rocks, however, that is important to mineral collectors, mineralogists, and economic geologists and that does not fit well into the defining characteristics of these three categories. This type of rock forms by the precipitation of minerals from water at elevated temperature (fig. 1). Such mineral deposits (or rocks) are known as hydrothermal (meaning "hot water": usually between 50[degrees] and 700[degrees]C) and call be considered a fourth major rock type. *

[FIGURE 1 OMITTED]

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