Garnet: The mineral Garnet information and pictures

About

Garnet is not a single mineral, but a group contains closely related, isomorphous minerals that form a series with each other. The Garnet members form intermediary minerals between each member, and may even intergrow within a single crystal. The Garnets vary only slightly in physical properties, and some of the members may be so similar that they are indistinguishable from one another without x-ray analysis.

The common Garnets can be divided into two subgroups:
Group 1: Garnets containing aluminum (Al) as their second element.
These include Pyrope, Almandine, and Spessartine. ("Pyralspite")

Group 2: Garnets containing calcium (Ca) as their first element.
These include Uvarovite, Grossular, and Andradite. ("Ugrandite")

The members of each group freely intermingle among one another. For example, the magnesium in Pyrope may be partially replaced by some iron from Almandine or by some manganese from Spessartine. However, between the two groups of Garnets, it is much rarer for them to intermingle.

Identification

Chemical Formula: The Garnet Group is composed of several minerals with related chemical formulas. The generic formula for the common Garnets is:
X²⁺₃Y³⁺₂Si₃O₁₂
X represents Ca, Fe²⁺, Mn, or Mg
Y represents Al, Cr, or Fe³⁺

A more comprehensive list can be found in The chemical formula of Garnet.

The chemical formula for the main individual members of the Garnet group are:
Pyrope: Mg₃Al₂Si₃O₁₂
Almandine: Fe²⁺₃Al₂Si₃O₁₂
Spessartine: Mn₃Al₂Si₃O₁₂
Grossular: Ca₃Al₂Si₃O₁₂
Andradite: Ca₃Fe³⁺₂Si₃O₁₂Uvarovite: Ca₃Cr₂Si₃O₁₂
Composition: Pyrope: Magnesium aluminum silicate
Almandine: Iron aluminum silicate
Spessartine: Manganese aluminum silicate
Grossular: Calcium aluminum silicate
Andradite: Calcium iron silicate
Uvarovite: Calcium chromium silicate
Color: Red, brown, black, green, yellow, orange, pink, white, and colorless. (Garnets come in all colors with the exception of bluish shades.) Massive specimens from certain localities can be multicolored white, pink, and/or light green.
Streak: Colorless
Hardness: 6.5–8.0
Crystal System: Isometric
Luster: Vitreous, adamantine, dull
Cleavage: None. May exhibit parting.
Fracture: Conchoidal to uneven
Tenacity: Brittle
Transparency: Transparent to opaque
Specific Gravity: 3.5–4.3
Group: Silicates; Nesosilicates; Garnet Group
Rock Type: Igneous, Sedimentary, Metamorphic

Rankings

Popularity (1–4): 1
Prevalence (1–3): 1
Demand (1–3): 1

Crystal Forms & Aggregates

In well-formed, distinct, dodecahedral and trapezohedral crystals. Also in compact crystal groupings, grainy, massive, as rounded crystals, and as groups of small crystals.

Striking Features

Crystal forms and hardness

Uses

The Garnets are important minerals, and are primarily known for their gemstone uses. The transparent varieties are used in all forms of jewelry, with the most extensively used color being dark red. Other Garnet forms such as the reddish-orange Spessartine, yellow Grossular, and the green Tsavorite and Demantoid also make fine gemstones.

See the gemstone section on Garnet for more detailed information, as well as the individual Garnet gemstone variety pages on Almandine, Pyrope, Rhodolite, Spessartite, Grossular, Tsavorite, Andradite, and Uvarovite for more detailed gemstone information on each Garnet gemstone form.

Garnets with a high hardness are used as an abrasive, and can be made into sandpaper known as Garnet paper. Well formed crystals and interesting aggregates very popular among mineral collectors.