Physical Properties of Garnet for Identification and Classification Purposes.
This site contains information on the Physical Properties and Chemical Composition of Minerals, gemstones, crystals, precious metals, and sedimentary, igneous and metamorphic rocks for crystallography, geology, identification of minerals, Jewelry and mineralogy. This includes cleavage, description of crystal formations, crystal structure, hardness, specific gravity, Mohs’ Scale, crystal features and crystal habits for identification and classification purposes.
The most popular color Garnet is known for is a deep maroon or dark red color but garnet is not only a single gem related to shades of red, it is a family of gems that come in many different colors (excluding blue) that, for the most part, all contain the same physical and chemical properties. Garnet gets it's name from the Latin word "granatum" meaning seed or grain. A red seed being associated with those of a pomegranate. The color and gem quality of the different types of garnet also can vary depending on the region from which they come. These color varieties are caused by impurities of manganese, iron, calcium, and other elements. Some garnets can range in transparency and some have even been known to naturally contain within them a mineral fiber star-shaped image. Throughout history some garnets have been mistaken for rubies because of their tendency to exhibit a different shade of color when viewed from different directions under certain light conditions. Garnet is a semiprecious stone that is brilliant in jewelry but it also has played a very important role in the abrasives industry because of the graininess in a particular type of garnet (used for smoothing or grinding other things). It has been the abrasive of choice because of it's cost efficiency, ease of clean up and ability to be recycled. Below you will find physical and chemical properties that are general to garnet and properties of the other members of the garnet family.
Physical
Properties of Garnet
As mentioned above, garnet can range in color form the common, yet
dazzling, deep reds to bright orange and lime greens. Different types
of garnet include Almandite, Andradite, Cachemirine, Grape,
Grossular, Imperial, Malaya, Mandarin, Mozambique, Pyrope, Rhondolite,
Spessartine and Umbalite. Garnets can be found in both igneous and
metamorphosed rock which are those produced under the conditions of
intense heat (such as volcanic or crystallization due to heat conducted
from magma). Scientists can actually calculate the amount of heat a host
rock has endured by studying the garnet from which it came. With a
specific gravity between 3.5 and 4.2 and a hardness ranging from 6.5 to
7.5, garnets has held the reputation of being both utile in the abrasive
industry and a brilliant gemstone. Garnet is known as being a very
efficient substitute for sand and mineral slag in blast cleaning
treatments. It's specific gravity and hardness contribute to the reduction
of dust and material usage. Although garnet itself is more expensive than
sand or slag, it's efficiency and the amount needed for treatments make a
garnet blast operation less expensive, not to mention cost effective
because of it's ability to be recycled and reused. Among the various types
of garnet their is a range of rarity and brilliance. Varying in
transparency and color saturation, the garnet family has exhibited it's
magnificence in the gemstone and jewelry industry as well.
Chemical
Properties of Garnet
Within the garnet family there is a complex variety of different
compounds and elements such as silica, aluminum, calcium, iron and
magnesium. The chemical symbol for garnet is also a little complex: [Ca,
Fe, Mg, Mn]3 [Al, Fe, Mn, Cr, Ti,V]2 [SiO4]3. Garnets can
possess any variety of the metals listed in the first bracket (that give
up 2 electrons) in conjunction with those of the second bracket (that give
up 3 electrons), while the third (silicate) bracket remains constant
throughout. This mixing and matching of elements is what gives the garnet
it's rainbow of coloration (a rainbow without blue, that is). The bonds
formed between these different types are the cause of it's various
hardness. Garnets have been found to crystallize in the isometric system
in octahedrons (eight faced axes), dodecahedrons (twelve faced
axes), trapezohedrons (twenty four faced axes) and even
hexoctahedrons (forty eight faced axes).
Properties of the Garnet Family
Almandite
or Almandine-(Fe)3 (Al)2 (SiO4)3
Almandite can range in color from black to darker shades of red and orange
to brown and it can sometimes have a star-shaped inclusion. It's color is
provided by Magnesium and Ferric Iron. Almandite has been known to be used
in early Victorian Jewelry. The name almandite has evolved from the name
of an Anotolian city (Indo-European tradition includes Hittite, Luwian,
Lycian and Lydian custom) in Turkey (Asia Minor) named Alabanda. The
specific gravity of almandite can be anywhere between 3.87 and 4.2.
Andradite-(Ca)3
(Fe)2 (SiO4)3
Andradite garnet contains calcium and iron elements and is generally
varieties of yellow, green and dark brown. It's specific gravity is
between 1.77 and 1.81 and can contain inclusions of byssolite in the shape
of a comet tail. Some transparent andradites are described as demantoid,
or diamond like, because of its prismatic abilities and it's similarity in
appearance aside from being yellow, green or brown. These also are
diamond-like in their rarity and value. The name for this extermely
lusterous garnet comes from the name of a mineralogist from Brazil named
J.B D'Andrada. Other andradites include Demantoid, Topazolite, and
Melanite.
Grossular-(Ca)3
(Al)2 (SiO4)3
Grossular garnets also contain calcium and iron but they range in shades
of red, yellow, orange and when lacking in impurities can be
completely clear. Some grossular garnets can have round and elliptical
inclusions. Hessonite is the name given to a form of grossular
garnet that is in the color range of deep yellow to brown. Tsavolite
or Tsavorite is a form of grossular garnet that comes in shades of green
mainly because of it's chromium and vanadium composition. It obtained it's
name from it's exclusive findings around the Tsavo National Park in
Tanzania, Africa. The name grossular comes from the Greek word for
gooseberry "grossularia." Other grossular garnets include Leuco,
Rosolite, Raspberry, Gooseberry, and Imperial. There even exist grossular
garnets that resemble jade and come in a variety of green and multicolored
shades. These garnets, known as African jades, have been used in tribal
carvings and statue fashioning.
Pyrope-(Mg)3
(Al)2 (SiO4)3
Pyrope is one of the most common forms of garnet. It's coloration is
commonly deep red to black as a result of the elemental presence of
chrome. Some pyrope contains tread-like inclusions which often tapper on
the ends and other pyrope garnets can be transparent. Pyrope gets it's
name from it's coloration: the Greek word "pyro" meaning fire.
Rhodolite-(Fe
and Mg)3 (Al)2 (SiO4)3
Rhodolite also has a name of Greek origin: "rhodo" meaning rose
is reminiscent of it's range in color from a light rosy pink to a
medium/dark violet. It has physical and chemical make-up that mirror that
of pyrope and almandite and therefore has been recognized as a mixture of
the two. There is a somewhat rare occurrence of one or more inclusions
which resemble stars with many rays. This type of garnet is among the more
expensive of the family.
Spessartite-(Mn)3
(Al)2 (SiO4)3
Spessatite has a range of coloration from brownish red, caused by iron
impurities, to (in some cases) orange. It's chemical make-up includes the
elements manganese and aluminum. This particular type of garnet is named
for the area of Bavaria in which it is mined: Spessart. It too is one of
the more expensive versions of garnet.
Uvarovite-(Ca)3
(Cr)2 (SiO4)3
Uvarovite is another green-colored garnet that occurs in fine crystal
clusters. It is named for a Russia noble Sergei Uvarov. Although uvarovite
comes in small sizes and is not frequently used in jewelry, uvarovites of
an emerald green hue are of increasing value.
The above listing is further summed up into two major groups: The Pyralspite Group and The Ugrandite Group. The Pyralspite Group consists of (as the name suggests) pyrope, almandite and spessartite. The Ugrandite Group consists of (also as the name suggests) uvarovite, grossular and andrite. Among this listing of the major types of garnet exist many combinations of the minerals found within each. Some of these include a deep purple colored garnet which is a combination of almandite and spessartite. There is also Mozambique which is a medium pink to muave/purple garnet, a combination of pyrope and almandite. Grape garnet is a mix of spessartine and almandite and are reddish purple to deep violet. Malaya garnet has a vivid orange to dark yellow coloration and is also exclusive (as of now) to the Tanzania/Kenya area of Africa. Within this mixed group falls a Mandarin, one of the newest additions to the garnet family. Found in Namibia, Africa, Mandarin is the most intense of the orange colored garnets.
Other Characteristics of Garnet
Crystal System: isotropic
Mohs' Scale Hardness: 6.5-7.5
Specific Gravity: 3.5-4.2
Index of Refraction: 1.73-1.89
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