Garnet group minerals can be divided into two series: almandine series: (Mg, Fe, Mn) 3Al2 [SiO4] 3; Andradite series: Ca3 (Al, Fe, Cr, Ti, V, Zr) 2 [SiO4] 3;
In addition, there are blythite(Mn3Mn2 [SiO4] 3), calderite (Mn3Fe2 [SiO4] 3), skiagite, (Fe3Fe2 [SiO4] 3), khoharite, (Mg3Fe2 [SiO4] 3), knorpringite (Mg3Cr2 [SiO4] 3), yamatoite (Mn3V2 [SiO4] 3), etc.
The Andradite belongs to the Andradite series, which is named in memory of the Brazilian mineralogist J.B.d’Andradae Silva. The color of Andradite is mainly yellow, green, brown and black, and those black Andradite with titanium are called melanite; Yellow is called topazolite and Green is called demantoid.
Basic properties of Andradite
[Chemical composition]
Ca3Fe2 (SiO4) 3 is often mixed with a certain amount of grossular molecular isomorphic mixed crystals, while spessartite molecular content is small.
Andradite garnet also often contains Ti. If the content of TiO2 is 1~5%, it is called Melanite garnet. If the content is higher, it is called titanium garnet, and the maximum content can reach 20%.
There are two opinions about the position of Ti in the structure; one is to replace Si, and the other is to replace Fe3+. The latter one is better. However, when the content is high, it may also partially replace Si in addition to Fe3+.
[Crystal parameters and structure]
The crystal system, symmetry type, and space group are the same as almandine; only the unit cell is slightly larger, ao=12.048, which is the largest mineral in this group.
Due to the large cell size, the structure can contain Ti to replace Fe and even a small amount of Ti to replace Si, forming a much larger [TiO2] tetrahedron without affecting the entire crystal structure.
Therefore, the garnet with high titanium content is mainly composed of Andradite garnet. The crystal structure is the same as that of grossularite.
[Form]
Andradite garnet is the most common type of calcium series garnet, which has the same morphology as almandine, but mainly consists of rhombic dihedron.
[Physical Properties]
The general property is the same as the almandine, but the difference is that the specific gravity is 3.859. The color of Andradite garnet varies greatly, ranging from dark to light. Those with high Mn and Ti content are dark in colors, such as Melanite garnet and titanium garnet, which are dark, brown-black and black respectively.
Those with less Mn and Ti are light colors, which can be red, brown, yellow, and green. Garnet group minerals, including Andradite garnet, can appear in various colors, but there is no blue one. In addition, Andradite garnet also often shows ring structures with different colors.
[Optical properties]
The color of the thin slice of Andradite is darker than that of other garnets, showing yellow, brown, and red, while Melanite garnet is dark brown or dark reddish brown, with poor transparency, regular polygon outline, and sometimes various irregular shapes. It often has twin crystals.
The Melanite garnet usually has a regular crystal shape and an obvious banded structure. The refractive index of the Melanite garnet is higher than that of the ordinary Andradite garnet, N=1.94-2.01.
In the orthogonally polarized light, Andradite garnet is one of the garnets with the most common optical anomalies. The interference color can reach first-grade gray and often has a ring structure.
[Under polarizer]
It is light pink or light brown, and the individual is thick brown or dark reddish brown, with homogeneity. The Andradite garnet series shows obvious heterogeneity.
The refractive index of Andradite is affected by composition and structure.
Generally, with the increase of a0, the content of Fe, Mg, and Al decreases, while the content of Ca, Fe, and Zr increases, and the refractive index increases.
The stability in water is good. Those containing OH – are easily soluble in acid.
The Structural water is removed from Hydrougrandite garnet At 705 ℃, and there are two exothermic peaks at 440 ℃ and 900 ℃- Fe2+→ Fe3+ at 440 ℃; Phase transition occurs at 900 ℃. Therefore, the thermal stability of garnet is not high when it contains iron and other variable valence elements.
[Change]
It can be changed into epidote, chlorite, serpentine, limonite, calcite, and feldspar.
[Occurrence]
It occurs in the contact metamorphic zone between igneous rock and limestone and is one of the main minerals of skarn, often associated with diopside and hedenbergite. In the complex skarn, it can be replaced by later epidote and actinolite. The Melanite garnet mainly occurs in alkaline rocks.
[Identification characteristics]
Andradite is different from other garnets according to the characteristics of darker color, larger refractive index, and Occurrence.
In terms of genesis and Occurrence, the Melanite garnet and titangarnet are relatively unique. They are products of alkaline rocks, such as nepheline syenite, Ijolite nepheline, and their corresponding volcanic rocks.
There are four kinds of Andradite garnet minerals; their morphology, physical properties, and crystal structure are very similar.
Pyrope
It can be found in ultrabasic rocks, such as peridotite, kimberlite, and basic volcanic block. The red garnet in the eclogite also contains many pyrope garnet molecules.
Spessartite
Spessartite is a rare garnet, which can be found in contact metamorphic belts of manganese ore mines or regional metamorphic belts of manganese-rich sedimentary rock stratum. In addition, it also occurs in granite pegmatite.
Grossularite
Grossularite is contact metamorphic and regional metamorphic mineral. Through contact metamorphism, impure limestone or calcareous shale can form almandine.
However, although almandine is common in contact metamorphic zones, it is still inferior to Andradite. After regional metamorphism, the impure limestone will also generate almandine, but it is not as common as contact metamorphism.
Uranovite
Uranovite is the one of rarest garnets . It is only found in ultrabasic rocks rich in chromite.
Andradite garnet deposit
According to the different compositions of Andradite garnet, it can be divided into Melanite garnet, Demantoid garnet, and Topazolite garnet. In addition, there are varieties of light black red and light black brown, which is generally transparent, translucent, or opaque.
Melanite garnet is composed of a large amount of titanium, and the content of titanium can reach 5%. Green to yellow is due to Fe3+, Green is due to Cr3+, and brown is due to Ti3+and Ti4+.
There are two different genetic types of Andradite garnet: skarn type and serpentinite type.
Andradites from China’s Xinjiang, Italy, Switzerland, Russia’s Urals, Korea, the United States, Pakistan, Iran, etc., belong to the serpentinite type, which occurs in serpentinized ultramafic rocks.
Andradites from Madagascar and Namibia belong to the skarn type. As a component mineral of the skarn, Andradite is a green garnet, which was first discovered in 1864 at the foot of the Ural Mountain, 115km northwest of Yekaterinburg, Russia.
Because of its strong refractive index and dispersion, it was mistaken for emerald.
Later, it was determined that its composition was chromic-containing Andradite, called demantoid.
The famous Andradite garnet (demantoid) is produced in the Sanjaowal Malenko deposit in Italy, the Elongo Mountain in Namibia, and the Suohan area in Koman Province, Iran, San Benito County, California, USA and Stanley Bates, Arizona, North Korea, the Democratic Republic of the Congo, Armenia, Pakistan and Canada.
Conclusion:
The color of Andradite garnet varies with its composition. According to the crystal shape, color, grease luster and high hardness of demantoid garnet, we can simply identify it.
If you need to accurately identify mineral species, it is necessary to accurately determine the composition, density, and refractive index or conduct X-ray diffraction analysis.
Andradite garnet is mainly used as abrasive, water filter, decorative, and gem raw material.