Crystal-size distributions (CSDs) for clay minerals with depth were measured from the Salton Sea Geothermal Field (SSGF) as a test for the presence and meaning of theoretical crystal-size distributions in a natural system. The SSGF is a classic open hydrothermal system, and crystals are forming directly without apparent modification of early-formed crystals, over a wide range of temperature. Thus, the measured CSDs are the actual distributions for a single episode in which all crystals grew at the same time from solution at different temperatures and depths, rather than through modifications of shallower samples.

Some TEM images of ion-milled samples from a range of depths were used to measure the crystal thicknesses of illite, chlorite and biotite. Grain-size histograms flatten, broaden and shift to larger sizes with increasing depth. Values of α and β were calculated and used to verify that the measured distributions are log normal. Reduced grain-size distributions for illite in SSGF samples obey steady-state constraints.

The observations appear to be consistent with evolution of illite with increasing depth in the SSGF system by growth in an open system giving rise to log-normal distributions, followed by supply-controlled growth in an open system. Because crystals at different depths grew simultaneously under different temperature and fluid conditions as a function of depth, they do not represent different stages of a single evolving system. The relations imply that isochemical and isothermal systems which permit an evolving system to be sampled are rare or non-existent. The data for distributions for a given depth in the SSGF are consistent with growth in an open system. The collective relations therefore imply that caution should be used in interpreting conditions of crystal growth in natural systems even where CSDs give results which are necessary for, but not sufficient to prove, a given modeled mechanism.