The mineral pyrope is a member of the
garnet
Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...
group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for ''fire'' and ''eye''. Despite being less common than most garnets, it is a widely used
gemstone with numerous alternative names, some of which are
misnomers. ''Chrome pyrope'', and ''Bohemian garnet'' are two alternative names, the usage of the latter being discouraged by the
Gemological Institute of America
The Gemological Institute of America (GIA) is a nonprofit institute based in Carlsbad, California. It is dedicated to research and education in the field of gemology and the jewelry arts. Founded in 1931, GIA's mission is to protect buyers and se ...
.
[(Gia), Gemological. Gem Reference Guide. City: Gemological Institute of America (GIA), 1988. ] Misnomers include ''Colorado ruby'', ''Arizona ruby'', ''California ruby'', ''Rocky Mountain ruby'', ''Elie Ruby'', ''Bohemian carbuncle'', and ''Cape ruby''.
The composition of pure pyrope is Mg
3Al
2(SiO
4)
3, although typically other elements are present in at least minor proportions—these other elements include
Ca,
Cr,
Fe and
Mn. Pyrope forms a
solid solution
A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...
series with
almandine
Almandine (), also known as almandite, is a species of mineral belonging to the garnet group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia M ...
and
spessartine
Spessartine is a nesosilicate, manganese aluminium garnet species, Mn2+3Al2(SiO4)3.Gemological Institute of America, ''GIA Gem Reference Guide'' 1995, This mineral is sometimes mistakenly referred to as ''spessartite''.
Spessartine's name is a ...
, which are collectively known as the ''
pyralspite'' garnets (pyrope, almandine, spessartine). Iron and manganese substitute for the magnesium in the pyrope structure. The resultant, mixed composition garnets are defined according to their pyrope-almandine ratio. The semi-precious stone
rhodolite
Rhodolite is a varietal name for rose-pink to red mineral pyrope, a species in the garnet group. It was first described from Cowee Valley, Macon County, North Carolina. The name is derived from the Greek "rhodon" for "rose-like", in common wi ...
is a garnet of ~70% pyrope composition.
The origin of most pyrope is in
ultramafic
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...
rocks, typically
peridotite from the
Earth's mantle
Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up about 84% of Earth's volume. It is predominantly so ...
: these mantle-derived peridotites can be attributed both to
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
and
metamorphic
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
processes. Pyrope also occurs in ultrahigh-pressure (UHP) metamorphic rocks, as in the Dora-Maira massif in the western Alps. In that massif, nearly pure pyrope occurs in crystals to almost 12 cm in diameter; some of that pyrope has inclusions of
coesite
Coesite is a form ( polymorph) of silicon dioxide Si O2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the ...
, and some has inclusions of
enstatite and
sapphirine
Sapphirine is a rare mineral, a silicate of magnesium and aluminium with the chemical formula ( (with iron as a major impurity). Named for its sapphire-like colour, sapphirine is primarily of interest to researchers and collectors: well-formed c ...
.
Pyrope is common in
peridotite xenoliths from
kimberlite
Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known to be the main host matrix for diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond called the Star of S ...
pipes, some of which are
diamond
Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...
-bearing. Pyrope found in association with diamond commonly has a Cr
2O
3 content of 3–8%, which imparts a distinctive violet to deep purple colouration (often with a greenish tinge) and because of this is often used as a kimberlite indicator mineral in areas where erosive activity makes pin pointing the origin of the pipe difficult. These varieties are known as ''chrome-pyrope'', or ''G9''/''G10'' garnets.
Mineral identification
In hand specimens, pyrope is very tricky to distinguish from almandine; however, it is likely to display fewer flaws and inclusions. Other distinguishing criteria are listed in the adjacent table. Care should be taken when using these properties as many of those listed have been determined from synthetically grown, pure-composition pyrope. Others, such as pyrope's high specific gravity, may be of little use when studying a small crystal embedded in a matrix of other silicate minerals. In these cases, mineral association with other
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...
and
ultramafic
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...
minerals may be the best indication that the garnet you are studying is pyrope.
In
petrographic
Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The class ...
thin section
In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron ...
, the most distinguishing features of pyrope are those shared with the other common garnets: high relief and isotropy. Garnets tend to be less strongly coloured than other silicate minerals in thin section, although pyrope may show a pale pinkish-purple hue in plane-polarized light. The lack of cleavage, commonly euhedral crystal morphology, and mineral associations should also be used in identification of pyrope under the microscope.
References
{{Gemstones
Magnesium minerals
Aluminium minerals
Garnet gemstones
Garnet group
Cubic minerals
Minerals in space group 230