Study On The Gemological And Mineralogical Characteristic Of Strawberry-red Grossular Garnet In Mexico

The strawberry-red grossular garnet,discovered in 1974 in the Sierra de Cruces in Coahuila,Mexico,has been very popular in the international collection market because of its rare color and distinctive characteristics.Mexico is the only known origin in the world so far.In this paper,the gemological properties,chemical composition,crystal structure and spectral characteristics of Mexican strawberry-red grossular garnet are systematically analyzed and discussed by means of conventional gemological testing,electron probe,X-ray powder diffraction,backscatter electron image testing,X-ray fluorescence spectroscopy,infrared spectroscopy,Raman spectroscopy and ultraviolet-visible spectroscopy.The chromogenic mechanism and geological growth environment are also discussed.Mexican strawberry-red grossular garnet is pink to rosy red.Some of the samples are black in the central area,with ring and band uneven distribution,accompanied by white masses,mostly translucent to opaque,euhedral,rhombic dodecahedron,internal cracks developed,less inclusions.The refractive index?1.741¹.755?and relative density?3.5874³.6987?are both within the theoretical range of grossular garnet,and the hardness of the black core region is slightly higher than that of the red edge.The cell parameters a₀ value of black-core strawberry-red grossular garnet is slightly larger,which lies between the theoretical range of grossular garnet and andradite garnet.It is caused by Al³⁺-Fe³⁺isomorphic substitution in[AlO₆]octahedral lattice.Electron probe analysis shows that the purity of strawberry-red grossular garnet without black core is very high,mostly over 90%,and the highest content of Mn is1.543%.It is the chromogenic element.Al³⁺can be replaced by Fe³⁺and Ti⁴⁺to form andradite-grossular garnet in the black core region.The highest content of Fe₂O₃ and TiO2 can reach 9.175%and 5.052%respectively.The infrared absorption peaks and Raman characteristic peaks of black-core strawberry-red grossular garnet shift to the low wavenumber,which is caused by Al³⁺-Fe³⁺isomorphic substitution in[AlO₆]octahedral lattice.Three absorption bands of 277,327 and 507 nm in UV-VIS spectra indicate that strawberry-red is colored by Mn²⁺.Mexican strawberry-red grossular garnet is associated with quartz,calcite,pyroxene,wollastonite and other minerals.It belongs to the classical zoning model of Skarn Deposits——proximal garnet and distal pyroxene.According to its composition characteristics,it can be inferred that strawberry-red grossular garnet was formed in the calcareous skarn formed by contact metasomatism under the geological conditions of medium temperature?450?⁷00??and low pressure?500×100⁵Pa⁸00×100⁵Pa?.In the early stage of iron-rich strong oxidation environment,the isomorphic substitution of Fe³⁺-Al³⁺in[AlO₆]octahedral lattice forms the andradite garnet component,it makes the center of strawberry-red grossular garnet appears black core.With the environment gradually stabilized down,the content of Fe decreased,and Mn ions enriched to a certain extent in the lattice,the outer edge of the crystal appears to be strawberry-red.