| Almandine | |
|---|---|
 | |
| General | |
| Category | Nesosilicate |
| Formula | Fe2+ 3Al 2Si 3O 12 |
| IMA symbol | Alm[1] |
| Strunz classification | 9.AD.25 |
| Crystal system | Cubic |
| Crystal class | Hexoctahedral (m3m) H–M symbol: (4/m3 2/m) |
| Space group | Ia3d |
| Identification | |
| Color | reddish orange to red, slightly purplish red to reddish purple and usually dark in tone |
| Cleavage | none |
| Fracture | conchoidal[2] |
| Mohs scale hardness | 7.0–7.5 |
| Luster | greasy to vitreous |
| Streak | white |
| Specific gravity | 4.05+0.25 −0.12[2] |
| Polish luster | vitreous to subadamantine[2] |
| Optical properties | Single refractive, and often anomalous double refractive[2] |
| Refractive index | 1.790±0.030[2] |
| Birefringence | none |
| Pleochroism | none |
| Dispersion | 0.024[2] |
| Ultravioletfluorescence | inert |
| Absorption spectra | usually at 504, 520, and 573 nm, may also have faint lines at 423, 460, 610 and 680–690 nm[2] |
| References | [3][4][5] |
Almandine (/ˈælməndɪn/), also known asalmandite, is a species of mineral belonging to thegarnet group. The name is a corruption of alabandicus, which is the name applied byPliny the Elder to a stone found or worked atAlabanda, a town inCaria inAsia Minor. Almandine is an iron alumina garnet, of deep red color, inclining to purple. It is frequently cut with a convex face, or encabochon, and is then known ascarbuncle. Viewed through thespectroscope in a strong light, it generally shows three characteristicabsorption bands.[6]
Almandine is oneend-member of a mineralsolid solution series, with the other end member being the garnetpyrope. The almandine crystal formula is: Fe3Al2(SiO4)3.Magnesium substitutes for theiron with increasingly pyrope-rich composition.
Almandine,Fe2+
3Al
2Si
3O
12, is theferrous iron end member of the class of garnet minerals representing an important group of rock-formingsilicates, which are the main constituents of the Earth'scrust,upper mantle andtransition zone. Almandine crystallizes in the cubicspace groupIa3d, with unit-cell parametera ≈ 11.512 Å at 100 K.[7]
Almandine isantiferromagnetic with theNéel temperature of 7.5 K. It contains two equivalent magnetic sublattices.[8]
Almandine occurs rather abundantly in the gem gravels ofSri Lanka, whence it has sometimes been called "Ceylon ruby". When the color inclines to a violet tint, the stone is often called Syriam garnet, a name said to be taken fromSyriam, an ancient town ofPegu (now part ofMyanmar). Large deposits of fine almandine-garnets were found, some years ago, in theNorthern Territory ofAustralia, and were at first taken for rubies and thus they were known in trade for some time afterwards as Australian rubies.
Almandine is widely distributed. Finerhombic dodecahedra occur in theschistose rocks of theZillertal, inTyrol, and are sometimes cut and polished. An almandine in which the ferrous oxide is replaced partly bymagnesia is found at Luisenfeld inGerman East Africa. In theUnited States there are many localities which yield almandine. Fine crystals of almandine embedded inmica-schist occur nearWrangell inAlaska. The coarse varieties of almandine are often crushed for use as anabrasive agent.
Connecticut has designated almandine garnet as its state gemstone.[9]
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Almandine".Encyclopædia Britannica. Vol. 1 (11th ed.). Cambridge University Press. p. 712.Gemmological classifications by E. Ya. Kievlenko (1980), updated | |||||||||
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