The mineral andradite

Chemical Formula

Ca₃Fe³⁺₂Si₃O₁₂

Composition

Calcium iron silicate, often with some aluminum or chromium replacing some iron

Variable Formula

Ca₃(Fe³⁺,Al,Cr,Ti)₂Si₃O₁₂

Color

Yellow, green, orange, reddish-brown, brown, gray, black; rarely colorless or white

Streak

Colorless

Hardness

6½ - 7½

Crystal Forms
and Aggregates

(Isometric) Usually as dodecahedrons, occasionally as trapezohedrons. Crystals may be single, but are usually packed together densely. Also occurs grainy and massive.

Transparency

Transparent to opaque

Specific Gravity

3.8 - 3.9

Luster

Adamantine

Cleavage

None

Fracture

Conchoidal to uneven

Tenacity

Brittle

Varieties

Demantoid - Emerald-green to green, transparent variety

Topazolite - Lemon-yellow to yellow-green variety

Melanite - Black, opaque variety

Schorlomite - Titanium-rich variety. Schorlomite is scientifically classified as a separate mineral.

In Group

Silicates ; Nesosilicates ; Garnet group

All About

Andradite is a Garnet, belonging to the Garnet group. The Garnet group is a small group of closely related minerals. The members of the Garnet group are isomorphous, and some of them freely intermingle. They vary only slightly in physical properties, and some of them may be so similar that they are indistinguishable from one another without x-ray analysis. The most common members are:
Pyrope, Almandine, Spessartine, Grossular, Andradite and Uvarovite.

The minerals in the Garnet group are called "Garnets". All Garnets are hard and many are fit for gem use. Unless articulate, most mineral and gem dealers don't refer to garnets by their true name (i.e. Andradite), but by the name "Garnet". The garnets as a group occur in all colors (except for blue).
For more information about garnets, see the Garnet group.

Andradite is a rare Garnet, and its gem varieties are highly regarded. The most valuable gem form of Garnet is the rare Demantoid variety of Andradite. The less valuable emerald-green Grossular Garnet ("Tsavorite") appears identical to Demantoid, but lacks its adamantine luster.

Uses

Andradite has the most powerful luster of any garnet, and this makes it a reputable gem. The yellow, orange, and brown varieties are all faceted, but the green Demantoid variety is the main gem form of this garnet (and is the most valuable Garnet). The opaque, black Melanite variety is sometimes worn as mourning jewelry.
Andradite specimens are also highly sought after by mineral collectors, who marvel at the well-formed crystals and aggregates.

See the gemstone section on garnet

Striking Features

Crystal forms, color, hardness, and luster

Popularity (1-4)

Prevalence (1-3)

Demand (1-3)

Distinguishing
Similar Minerals

Almandine, Spessartine, and Grossular - although Andradite has a more powerful luster, it is very difficult to distinguish it from these other garnets.
Uvarovite - usually occurs in deeper colored green crystals and in aggregates of tiny crystals
Dioptase - softer, usually occurs in deeper colored green crystals

Commonly
Occurs With

Albite, Orthoclase, Calcite, Wollastonite, Biotite, Chlorite, Hedenbergite, Magnetite, Tremolite, Diopside

Noteworthy
Localities

Occurrences of this mineral include Val Malenco, Sondrio, and Monte Somma, Vesuvius, Italy; Serifos, Greece; and the Ural Mountains of Russia. It occurs in Mexico in Veracruz and in Canada at Mont Saint Hilaire, Rouville Co., Quebec.
In the U.S., it is found in Franklin and Ogdensburg, Sussex Co., New Jersey; in Henderson Co., North Carolina; and in Erskine Creek, Kern Co., California. Melanite occurs in Magnet Cove, Hot Spring Co., Arkansas and in the Gem Mine in San Benito Co., California. Dark olive-green crystals were found in the French Creek Mine, St. Peters , Chester Co., Pennsylvania. Topazolite comes from Stanley Butte, Graham Co., Arizona and the New Idria district, San Benito Co., California.

Picture Icon
Links

Picture Links

1. Topazolite crystal cluster
2. Black Melanite crystals