The Humite Group of Minerals

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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.

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Mineral Name:	Chemistry:	Silicate Layers:	Symmetry Class:
NORBERGITE	Mg2SiO4 - Mg(F, OH)2	1	Orthorhombic
CHONDRODITE	((Mg, Fe)2SiO4)2 - Mg(F, OH)2	2	Monoclinic
HUMITE	((Mg, Fe)2SiO4)3 - Mg(F, OH)2	3	Orthorhombic
CLINOHUMITE	(Mg2SiO4)4 - Mg(F, OH)2	4	Monoclinic
ALLEGHANYITE	(Mn2SiO4)2 - Mn(OH)2	2	Monoclinic
LEUCOPHOENICITE	(Mn2SiO4)3 - Mn(OH)2	3	Monoclinic
SONOLITE	(Mn2SiO4)4 - Mn(OH, F)2	4	Monoclinic
JERRYGIBBSITE	(Mn2SiO4)4 - Mn(OH)2	4	Orthorhombic
RIBBEITE	((Mn, Mg)2SiO4)2 - (Mn, Mg)(OH)2	2	Orthorhombic
MANGANHUMITE	((Mn, Mg)2SiO4)3 - (Mn, Mg)(OH)2	3	Orthorhombic

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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.