Mineral Alamandine

Chemical Composition: Fe3Al2(SiO4)3, Iron Aluminum Silicate

Belongs to the Silicates mineral class

Alamandine mineral is usually found in garnet schists, a metamorphic type of rock mostly of mica components. Alamandine is one of the iron aluminum garnets, along with the pyrope and spessartine, which is known as the pyralspite group. It belongs to the silica groups with (SiO4) being isolated, not linked in chains or sheets common in most silicate minerals. A pure Alamandine mineral is very rare. The only distinction between similar minerals is their change in densities Crystal formation may vary from transparent to translucent. It has an isometric crystal structure visible with the aid of polarized light microscopy with typical rhombic dodecahedron inclusion. They may also appear in 24- sided trapezohedron. These formations are more common compared to the formations showing faces of hexoctahedron, a 48-sided crystal habit that is also rarely seen by itself. When all these forms combine, a very attractive and complex multifaceted crystal is formed splendidly exhibited under polarizing microscopes. They may occur in granular and sometimes in irregular massive form. Crystals are sometimes embedded and isolated in metamorphic rocks.

This mineral when viewed under a petrographic microscope, usually exhibits a typical red-brown color. Alamandine is commonly known to be an iron alumina garnet possessing a deep red color with an inclination to purple. Color variations exhibited, when viewed with polarized light microscopy, is from deep red and violet red through brownish red to almost black. These rare minerals sometimes exhibit a little tinge of purple in appearance and a deep enough red to appear black. The transparent red varieties are most commonly used as gems. However, these minerals are more often dark and opaque in appearance making it more valuable to use as an abrasive, which is of course on account to its superior hardness. It shows vitreous luster in reflected light of using techniques in polarized light microscopy. They appear to be having no cleavage at any angle but may show parallel partings. Fracture shown is conchoidal or commonly irregular. Hardness is found to be of range 6.5 – 7.5 giving an average hardness of 7. Specific gravity is approximately 4.3 g/cm³, which is considered above average for translucent minerals and highest density of all garnets. It usually shows streak of white.

Almandine occasionally appears in orange hue when viewed under a reflected light of a polarizing petrographic microscope. It may also show strong absorption spectrum containing pronounced yellow bands, which accounts for the purple hue of some other stones. It has commonly rough surface exhibiting high relief when viewed under petrographic microscopes. Birefringence shows several variations often dark between crossed nicols of a polarizing petrographic microscope but some may exhibit weak double refraction. Areas of birefringence are often arranged in sectors and zones.

Alamandine is a common garnet in most mica schists and gneisses. It is also a common constituent of pegmatites, granites, and other felsic volcanic rocks. These Alamandine minerals are very widely distributed. Splendidly cut and polished crystals of Alamandine mineral in fine rhombic dodecahedron formation commonly occur in schist rocks of the Zillertal, Tyrol. A variety of this mineral with chemical analysis in which the ferrous oxide is replaced partly by magnesium occur a Luisenfeld, German East Africa. The one of a kind Alamandine crystal, which is splendidly embedded in mica-schist, is found in Fort Wrangell, Alaska, USA. This Alamandine mineral occurrence is of great abundance in the gem-gravels of Sri Lanka.