Almandine

"Almandin" redirects here. For the racehorse, see Almandin (horse).

Not to be confused with Amandine (disambiguation).

Almandine (/ˈælməndɪn/), 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 Minor. Almandine is an iron alumina garnet, of deep red color, inclining to purple. It is frequently cut with a convex face, or en cabochon, and is then known as carbuncle. Viewed through the spectroscope in a strong light, it generally shows three characteristic absorption bands.^[6]

Almandine
General
Category	Nesosilicate
Formula (repeating unit)	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/m 3 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]
Ultraviolet fluorescence	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 is one end-member of a mineral solid solution series, with the other end member being the garnet pyrope. The almandine crystal formula is: Fe₃Al₂(SiO₄)₃. Magnesium substitutes for the iron with increasingly pyrope-rich composition.

Almandine, Fe²⁺
₃Al
₂Si
₃O
₁₂, is the ferrous iron end member of the class of garnet minerals representing an important group of rock-forming silicates, which are the main constituents of the Earth's crust, upper mantle and transition zone. Almandine crystallizes in the cubic space group Ia3d, with unit-cell parameter a ≈ 11.512 Å at 100 K.^[7]

Almandine is antiferromagnetic with the Néel temperature of 7.5 K. It contains two equivalent magnetic sublattices.^[8]

Occurrence

A 19th-century almandine garnet brooch

Almandine occurs rather abundantly in the gem gravels of Sri 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 from Syriam, an ancient town of Pegu (now part of Myanmar). Large deposits of fine almandine-garnets were found, some years ago, in the Northern Territory of Australia, 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. Fine rhombic dodecahedra occur in the schistose rocks of the Zillertal, in Tyrol, and are sometimes cut and polished. An almandine in which the ferrous oxide is replaced partly by magnesia is found at Luisenfeld in German East Africa. In the United States there are many localities which yield almandine. Fine crystals of almandine embedded in mica-schist occur near Wrangell in Alaska. The coarse varieties of almandine are often crushed for use as an abrasive agent.

Cultural significance

Connecticut has designated almandine garnet as its state gemstone.^[9]

See also

• List of minerals

References

1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
2. Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
3. Mindat.org - Almandine
4. Webmineral.com - Almandine
5. "Handbook of Mineralogy - Almandine" (PDF). Archived from the original (PDF) on 2016-03-03. Retrieved 2013-03-01.
6.  One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Almandine". Encyclopædia Britannica. Vol. 1 (11th ed.). Cambridge University Press. p. 712.
7. Geiger, C.; Armbruster, Th.; Lager, G.; Jiang, K.; Lottermoser, W.; Amthauer, G. (1992). "A combined temperature dependent ⁵⁷Fe Mössbauer and single crystal X-ray diffraction study of synthetic almandine: evidence for the Gol'danskii–Karyagin effect". Physics and Chemistry of Minerals. 19 (2): 121–126. Bibcode:1992PCM....19..121G. doi:10.1007/BF00198609. S2CID 98610041.
8. Zherebetskyy, Danylo (2010). Quantum mechanical first principles calculations of the electronic and magnetic structure of Fe-bearing rock-forming silicates (PhD). Dissertation.com, Boca Raton, Florida. ISBN 978-1-59942-316-6.
9. "State of Connecticut – Sites, Seals and Symbols". State of Connecticut. Retrieved 2009-11-12.