Vanadium- and Chromium-Bearing Pink Pyrope Garnet: Characterization and Quantitative Colorimetric Analysis

Abstract

G arnets are a group of isometric nesosilicates with the general chemical formula X 3 Y 2 Z 3 O 12. X, Y, and Z represent dodecahedral, octahedral, and tetrahedral sites in the crystal structure, respectively. Natural rock-forming silicate garnets with the Z-site occupied by Si 4+ are commonly divided into the pyralspite and ugrandite groups. In pyralspite, Al 3+ occupies the Y-site and the X-site may contain Mg 2+ , Fe 2+ , or Mn 2+ ; these garnets are dominantly composed of the pyrope (Mg 3 Al 2 Si 3 O 12), almandine (Fe 2+ 3 Al 2 Si 3 O 12), and spessartine (Mn 3 Al 2 Si 3 O 12) end members. The ugrandite garnets have Ca 2+ on the X-site and Cr 3+ , Al 3+ , or Fe 3+ on the Y-site, giving uvarovite (Ca 3 Cr 2 Si 3 O 12), grossular (Ca 3 Al 2 Si 3 O 12), or andradite (Ca 3 Fe 2 3+ Si 3 O 12) end members. Stockton and Manson (1985) proposed a classification scheme for separating the pyralspite group into the gemological species of pyrope, almandine, spessartine, pyrope-almandine, pyrope-spessartine, and almandine-spessartine. Previously, two types of color-change garnets have been reported: pyrope with very high Cr 3+ (

Characterization of natural silicate garnets by means of non-destructive testing methods

“…Manganese is responsible for the colour in rhodolite and spessartite garnet. Lines attributed to it in the spectra of these stones are centered in the blue, violet and even in the ultraviolet [23][24][25][26][27].…”

The Effects of Munsell Neutral Grey Backgrounds on the Colour of Chrysoprase and the Application of AP Clustering to Chrysoprase Colour Grading

“…The colour of chrysoprase was first quantified by Sachanbi ński [3] based on the CIE1931 colour space system but the geometric distance between two colours in this system is inconsistent with human visual perception. In order to solve this problem, the CIE1976 L*a*b* uniform colour space was introduced and applied to the study of gem colours, particularly colour-change garnet [13][14][15][16], tourmaline [17,18], sapphire [19], alexandrite [20], peridot [21,22], cubic zirconia [23], blue amber [24], jadeite-jade [25][26][27][28][29], citrine [30] and amethyst [31]. Tang et al [21] analysed the colour appearance of peridot applying nine Munsell neutral grey backgrounds based on the CIE1976 L*a*b* uniform colour space system and found that the colours of peridot were much easier to distinguish in the high lightness background.…”

Genesis and Influencing Factors of the Colour of Chrysoprase: Colourimetric Analysis Based on the Cie 1976 L*a*b* Uniform Colour Space

“…The greenness of chrysoprase is in the form of a series of graded colours, which is of great value in the study of colourimetry. In recent years, colourimetry has played an important role in gemmology, involving mainly colour-change garnet [11][12][13][14] , tourmaline 15, 1 6 , sapphire 17 , alexandrite 18 , peridot 19, 2 0 , cubic zirconia 21 , blue amber 22 , jadeite-jade [23][24][25][26][27] , citrine 28 and amethyst 29 , among others. However, chrysoprase colour has been subjected to little evaluation or analysis of its influencing factors to date.…”