The Humite Group of Minerals

The Humite Group of minerals form a morphotropic series with the mineral olivine and brucite. Members of this group contain brucite-like sheets in their structures. Brucite is composed of Mg(OH)2 tetrahedrons linked into sheets. The Humite Group is also structurally related to olivine, (Mg, Fe)2SiO4 (and the related mineral tephroite, Mn2SiO4) in that the members of this group contain layers of the olivine structure that alternate with layers of the brucite-like sheets.

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.

The generalized group formula therefore can be written as one of the following:

    • A2SiO4 - A(OH, F)2 - with one silicate layer
    • (A2SiO4)2 - A(OH, F)2 - with two silicate layers
    • (A2SiO4)3 - A(OH, F)2 - with three silicate layers
    • (A2SiO4)4 - A(OH, F)2 - with four silicate layers

The A is either manganese, magnesium and/or to a lesser extent, iron. Therefore not all the brucite-like sheets are composed of Mg(OH)2 as iron and manganese substitute for the magnesium in several members of this group. 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.

    • A(1+x)(SiO4)x(OH, F)2 - were x = the number of silicate layers

Nearly all members of this group are metamorphic or hydrothermal in origin. 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.

Mineral Name:

Chemistry: Silicate Layers: Symmetry Class:

Mg2SiO4 -
Mg(F, OH)2

1 Orthorhombic

((Mg, Fe)2SiO4)2 -
Mg(F, OH)2

2 Monoclinic

((Mg, Fe)2SiO4)3 -
Mg(F, OH)2

3 Orthorhombic

(Mg2SiO4)4 -
Mg(F, OH)2

4 Monoclinic
2 Monoclinic
3 Monoclinic
Mn(OH, F)2
4 Monoclinic
4 Orthorhombic
RIBBEITE ((Mn, Mg)2SiO4)2 -
(Mn, Mg)(OH)2
2 Orthorhombic
(Mn, Mg)(OH)2
3 Orthorhombic

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. Note that the same number of silicate layers does not necessarily produce the same symmetry.

Popular Members of the Silicates Class


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