Refractory clay, derived from hornblende chlorite schist that was hydrothermally altered along nearly vertical fractures and faults, occurs near Guanajuato, Mexico, a few kilometers south and east of the famous monument, El Cristo Rey. Part of the chlorite is expanding, i.e. probably mixed-layer vermiculitic chlorite or biotite. The most refractory clay is comprised chiefly of kaolinite, with variable amounts of finely crystalline quartz. Huge, thick “gossans” of silica have been deposited as surface-covers over the fractures; such silica gossans are fairly common over Mexican hydrothermal clay deposits and may be used as probable field guides to argillation.

The trend of mineralogical and chemical changes (analyses tabulated in the paper) during argillation of the Guanajuato material is traced in overlapping sequence, as follows:

1. (a) bleaching of green ferro-ferric silicates accompanied by oxidation of part of the iron to “limonite”.

2. (b) leaching of alkali and alkaline earths accompanied by

3. (c) partial desilication of parent silicate minerals yielding kaolinite whose flakes are commonly oriented parallel with the chlorite of the schist, mobile silica, and finely divided quartz intermixed with the kaolinite

4. (d) refinement (“purification”) of the clay by further kaolinization and removal of silica minerals and some iron oxide.

The final clay may run higher in silica than does its parent rock.