Garnet Group: The Colors and Varieties of Garnet

Garnet is not a single mineral, but describes a group of several closely related minerals. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition.  

Garnets are nesosilicates having the general formula X₃Y₂(SiO₄)₃. The X site is usually occupied by divalent cations (Ca, Mg, Fe, Mn)²⁺ and the Y site by trivalent cations (Al, Fe, Cr)³⁺ in an octahedral/tetrahedral framework with [SiO₄]⁴⁻ occupying the tetrahedra. Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit.

Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs scale of about 6.5 to 7.5.

Chemical composition

According the whether the Y or the X component in the chemical composition of the species is constant, we can divide the members of the garnet family into two groups.

• Pyralspites (Pyrope, Almandine, Spessartite) aluminium in Y site
• Ugrandites (Uvarovite, Grossular, Andradite) calcium in X site

Each member of these two groups has an "ideal" chemical makeup.

• Pyrope (magnesium aluminum silicate)
• Almandine (iron aluminum silicate)
• Spessartite (manganese aluminum silicate)
• Uvarovite (calcium chromium silicate)
• Grossular (calcium aluminum silicate)
• Andradite (calcium iron silicate)

In the pyralspite list above, the first element mentioned changes from member to member. However, in nature one will never find a pyrope garnet with such a pure chemical makeup. Instead, during growth other elements -- present in the melt from which the pyrope grew -- were mixed in.

When iron (as seen in almandine, above) is mixed in during the growth of a pyrope garnet, one would get a magnesium-iron aluminum silicate. Part of the magnesium content in the pyrope was replaced by iron.

It is the amount of magnesium/iron content that defines the distinguishing line between pyrope and almandine. When there is an abundance of magnesium in the chemical makeup, it is a pyrope. When iron is dominant, it is almandine garnet.

When the mix is somewhere in between, we name it pyrope-almandine garnet. (These mixtures may have some exotic names, such as rhodolite, that are not of importance here). It should be noted that the boundaries where it is a pyrope or where it is an almandine is under heavy debate.

Garnet Group 

Almandine 

Almandine Garnet Crystal with Muscovite crystals on the side. From Gilgit, Gilgit District, Northern Areas, Pakistan. Photo: mardani Fine Minerals

Almandine is the most common Garnet. The almandine crystal formula is: Fe₃Al₂(SiO₄)₃. Magnesium substitutes for the iron with increasingly pyrope-rich composition. Almandine range in color from Dark red to black. t is frequently cut with a convex face, or en cabochon, and is then known as carbuncle.

Pyrope 

Rough Pyrope

Pyrope is the only member of the garnet family to always display red colouration in natural samples. Pyrope is especially noted for its transparency and frequent lack of flaws or inclusions. The composition of pure pyrope is Ca₃Fe₂(SiO₄)₃, although typically other elements are present in at least minor proportions—these other elements include Ca, Cr, Fe and Mn. The origin of most pyrope is in ultramafic rocks, typically peridotite from the Earth's mantle: these mantle-derived peridotites can be attributed both to igneous and metamorphic processes.

Varieties of Pyrop:
Rhodolite is the purple variety of pyrope-almandine. The name and color come from the flower of the rhodondendron. It is often regarded as a variety of Pyrope.
Malaia garnet (or malaya) is the red-orange variety of pyrope-spessartite discovered in the 1970's in Kenia as a by product of rhodolite.

Spessartine

Rough and cut Spessartite Garnet. Photo: Gran Gem (@grangemltd)

Spessartite is an orange to red form of the gemstone Garnet. Spessartite is a nesosilicate, manganese aluminium garnet species, Mn²⁺₃Al₂(SiO₄)₃. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites. Spessartine, like the other garnets, always occurs as a blend with other species. Gems with high spessartine content tend toward a light orange hue, while almandine prevalence induces red or brownish hues.

Andradite

Demantoid Andradite Garnet on matrix. Photo: Ali (@deygallery)

Andradite is a calcium-iron garnet, Ca₃Fe₂(SiO₄)₃, is of variable composition and may be red, yellow, brown, green or black. Andradite is the most  is the most lustrous of the Garnets and has several gemstone varieties. 

Andradite includes three varieties:

• Melanite: Lustrous, opaque black or very dark red in color, referred to as "titanian andradite".
• Demantoid: Olive-green to emerald-green in color, one of the most valuable and rare stones in the gemological world.
• Topazolite: Yellow to brownish-yellow in color and sometimes of high enough quality to be cut into a faceted gemstone, it is rarer than demantoid.

Andradite occurs in skarns developed in contact metamorphosed impure limestones or calcic igneous rocks; in chlorite schists and serpentinites and in alkalic igneous rocks (typically titaniferous).

Grossular

Grossular is the most varicolored of the Garnets. It has the chemical formula of Ca₃Al₂(SiO₄)₃  but the calcium may, in part, be replaced by ferrous iron and the aluminium by ferric iron. Pure Grossular is colorless; the wide range of colors in this Garnet is caused by various impurities. Some Grossular types have their own unique variety or trade names, and are only called by these names in the gemstone market. The orange-brown Hessonite and deep green Tsavorite varieties are the best known varieties.

Hydrogrossular from Jeffrey Mine Asbestos, Quebec Canada

Varieties of Grossular:
Hessonite Orange to orange-brown, transparent variety of Grossular Garnet.
Hydrogrossular garnet is the green, pink, grayish-white to bluish variety of grossular.

Tsavorite garnets on graphite matrix!From Merelani Mine, Arusha, Tanzania.
Credit: Anton Watzl

Chrome grossular is a green variety of grossular.
Tsavorite (or tsavolite) is a grossular that, like the Meralani mint, is green in colour. It is coloured by chromium and/or vanadium. The distinction between a 'regular' green grossular and tsavorite is commonly thought to be a function of saturation and tone. However, according to the Bridges' family (tsavorite was discovered by the late Campbell Bridges), the distinction is chemical. The chromophore for green grossular is iron, as opposed to chromium and/or vanadium.
Mali is a yellow-green variety of grossular with a slight overlap in the grossular-andradite series.

Uvarovite

Gorgeous Green uvarovite crystals on chromite matrix from Ural, Russia. Photo: R. Tanaka

Uvarovite is the rarest of the familiar Garnets, and is seldom used as a gem. Uvarovite is a calcium chromium garnet with the formula Ca₃Cr₂(SiO₄)₃. The color of Uvarovite is a deep chrome-green, and Uvarovite is a unique Garnet being that it only comes in one color.

Knorringite

Knorringite is a magnesium-chromium garnet species with the formula Mg₃Cr₂(SiO₄)₃. Pure endmember knorringite never occurs in nature. Pyrope rich in the knorringite component is only formed under high pressure and is often found in kimberlites. It is used as an indicator mineral in the search for diamonds.

See also: 

How Does a Trapiche Emerald Form?

How Does Opal Get Its Color?