Pyrope-Almandine-Spessartine Series — The Pyralspite Garnet Triangle
Pyrope-Almandine-Spessartine Series — The Pyralspite Garnet Triangle
A continuous three-component solid solution that includes most gem garnets in the trade
The pyrope-almandine-spessartine series is a continuous three-component solid-solution among three garnet end-members sharing the same crystal structure but differing in the divalent cation occupying the dodecahedral site: pyrope (Mg3Al2Si3O12), almandine (Fe3Al2Si3O12), and spessartine (Mn3Al2Si3O12). Collectively the three end-members and their mixed compositions are known as the pyralspite series, a contraction formed from the first letters of the three names. The pyralspite triangle includes most gem garnets traded today and represents one of the most chemically diverse mineral series in the gem trade.
Compositional space
The three end-members occupy the corners of a triangular composition diagram, with mixed compositions plotting on the edges and in the interior of the triangle. Natural garnets rarely sit at any single corner; most are mixtures of two or all three components, with the relative proportions controlled by the host rock chemistry, the temperature and pressure of formation, and the availability of the substituting elements. Magnesium-rich pyrope-dominant garnets form preferentially in mantle-derived rocks and high-pressure metamorphic environments; iron-rich almandine-dominant garnets characterise medium-grade metasediments; manganese-rich spessartine-dominant garnets form in granitic pegmatites, manganese-rich skarns, and some hydrothermal systems.
Trade names within the series correspond to particular composition ranges. Rhodolite refers to balanced pyrope-almandine compositions with characteristic purplish-red colour. Malaia (or malaya) refers to pyrope-spessartine compositions with peach to orange colour, first described from the Umba Valley of Tanzania in the 1960s. Colour-change garnets, exhibiting a daylight-to-incandescent colour shift, often plot in the pyrope-spessartine field with appreciable vanadium or chromium content. Almandine, pyrope, and spessartine occupy their respective corners.
Properties across the series
Refractive index, specific gravity, and absorption spectra all vary continuously across the pyralspite triangle. Refractive indices range from about 1.71 (pure pyrope) to 1.83 (pure almandine) and 1.80 (pure spessartine). Specific gravities range from about 3.65 (pyrope) to 4.32 (almandine) and 4.18 (spessartine). The relationship between refractive index, density, and composition has been studied in detail and allows compositional inference from physical-property measurement, though direct elemental analysis by X-ray fluorescence or microprobe is now standard for any composition determination of trade significance.
Hardness is approximately constant across the series at 7 to 7.5, and all members share the same isometric crystal symmetry, perfect octahedral or rhombic dodecahedral crystal habit, and absence of cleavage that characterise the garnet group as a whole.
Distinction from the ugrandite series
The pyralspite series is one of two main garnet solid-solution groups; the other is the ugrandite series (uvarovite, grossular, and andradite), which substitutes calcium in the dodecahedral site. The two series share the same garnet structure but occupy distinct compositional spaces and rarely show extensive mixing between groups. Limited substitution between pyralspite and ugrandite garnets does occur — particularly between almandine and grossular at high pressures, and between pyrope and grossular in some metamorphic environments — but the bulk of natural garnet compositions plot clearly within one series or the other.
In the trade
The pyralspite series accounts for most red, purple, orange, and pink garnets in the gem trade, with the ugrandite series contributing green tsavorite and demantoid plus the more ornamental hessonite. Trade nomenclature within the pyralspite series is somewhat fluid, with the same physical material sometimes traded under different names depending on regional convention and marketing practice. Buyers should pay attention to colour and quality rather than relying solely on trade names, which do not always map cleanly onto compositional categories. See also pyrope, almandine, spessartine, rhodolite, and malaia garnet.