Physical properties of selected illites, beidellites and mixed-layer illite–beidellites from southwestern Idaho, and their infrared spectra

Abstract

The silver–gold ore deposits of the De Lamar mine, the mines in Florida Mountain and the mines in War Eagle Mountain occur in conjunction with large seams of clay minerals, which consist mainly of mixed-layer illite–beidellite. When saturated with groundwater, these clay seams have caused slope failures during mining and processing of ore. The illite–beidellite clays have effective internal friction values, Ø e, as low as 4° and cementation C values of about 1.0 kg f/cm 2. The Fe-beidellite used for the clay core of the mill pond dam has an average Ø e value of 8.3° and C value of 0.79 kg f/cm 2, with an average permeability ( K-value) of 5.6×10 −7 cm/s. The fundamental OH spectra of beidellite, illite and muscovite are similar with beidellite having bands near 3635, 3654 and 3653 cm −1. Montmorillonite has one Al–OH libration band near 915 cm −1, whereas beidellite has a second band near 942 cm −1. In like manner, illites and muscovites have bands from 928 to 933 cm −1, as well as the Al–OH libration bands from 912 to 917 cm −1. The OH libration bands vary both in position and intensity with particle orientation and heating. Combination bands for Fe-bearing muscovite show moderate bands at 4451 cm −1 for ferrous muscovite and at 4471 cm −1 for ferric muscovite. These bands also are present for illites. The dominant Al–OH combination band wave numbers of muscovites form a linear relationship with the Al 2O 3 muscovite contents, but not so precisely for illites. Chemical analysis of illites indicates that the compositions are a continuum into muscovites. Illites from the Miller–Walters mine contain a rather large amount of certain trace elements, whereas the adjacent beidellites do not.