Brucite sheets are also seen in the clay minerals in a similar manner. But this group is more complicated than that. The members of this group differ not only by chemistry but by the number of olivine or silicate layers that are present between the brucite layers. There can be either one, two, three or four layers of the silicate layers between each brucite-like sheet. This has the effect of extending the unit cell of the minerals and affecting their symmetries.
Fluorine is also a common substitute for the hydroxyl in the brucite-like sheets.
An alternate generalized formula, shown below, does not reflect the structure as well as the formulas shown above but is often used as the formula for these minerals. Unfortunately the Humite Group members have similar characteristic and properties and are difficult to tell apart without optical tests. Below is a table expressing the formula, structure and symmetry class differences in the members of this group. The first four members shown were the original members of the Humite Group. The others members were added to this group due to their structural similarities.