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Physical Properties of Garnet

The garnet mineral family is a group of silicate minerals consisting of two different metallic elements bound to their silicate (SiO4) ions in a specific ratio. Common garnets include pyrope (magnesium aluminum silicate), almandine (iron aluminum silicate), spessartine (manganese aluminum silicate), grossular (calcium aluminum silicate), andradite (calcium iron silicate), and uvarovite (calcium chromium silicate). While garnets are sometimes used as gemstones, they are significantly used in industry as abrasives.

Crystal Habit
- Crystal habit refers to the general shape of the mineral crystals. Garnet crystals form isometric dodecahedral and trioctahedral shapes. Isometric refers to the crystals growing in all directions at the same rate, forming crystal faces that are equidistant from the center of the crystal. Dodecahedrons are 12-sided polyhedrons, and trioctahedrons are 24-sided polyhedrons resembling an octahedron (eight-sided polyhedron) with each face divided into three faces.
Cleavage
- Cleavage describes a mineral breaking along internal planes within the crystal. Garnets do not exhibit cleavage but, rather, fractures that result in a range of smooth, regular surfaces to rough, uneven surfaces.
Hardness
- Hardness relates a mineral's ability to scratch and to resist scratching of its surface by another mineral. A commonly used reference scale is the Mohs Hardness Scale. On this scale, minerals are rated on a scale of one to 10 with a value of 1 for talc and a value of 10 for diamond. On this scale, most garnets range from 6.5 to 7.5 in hardness, similar to the hardness of quartz.
Tenacity
- Tenacity describes a mineral's resistance to bending or breaking. Garnets are brittle as they break apart rather easily.
Specific Gravity
- Specific gravity compares a mineral's density to the density of water. A specific gravity of 1 means the mineral has the same density (ratio of mass to volume) as water. A specific gravity of 2 means a mineral is twice as dense as water. Garnets have specific gravity ranging from 3.5 to 4.3.
Color
- The color of garnets varies greatly to include most colors with the notable exception of blue. The color range of individual members of the garnet family can be specific as in the uvavorites, characteristically emerald green to extremely broad, such as the grossulars, with a color range that includes white, pink, yellow, brown, orange, green and, even, colorless.
Other Physical Properties
- Garnets usually possess a glassy to brilliant non-metallic luster, leave a colorless streak on a streak plate and range in appearance from transparent to opaque.

Rocks & Minerals