Gem News International Gems & Gemology, Summer 2017, Vol. 53, No. 2

Raspberry-Red Garnet with Black Core

Raspberry-red grossular garnet in matrix.
Figure 1. A very fine 4.5 × 3.1 × 2.1 cm specimen of raspberry-red grossular from skarn deposits, with crystals approaching 1 cm. The matrix consists of calcite, quartz, wollastonite, and scapolite. Photo by Kevin Schumacher.

Raspberry-red grossular garnet mineral specimens have been popular in the collector  market since their discovery in 1994 in the Sierra de Cruces range in the State of Coahuila, Mexico. V.W. Lueth and R. Jones (“Red grossular from the Sierra de Cruces, Coahuila, Mexico,” Mineralogical Record, Vol. 34, No. 6, 2003, p. 73) described the geological environment where the garnet was discovered and the petrological composition of the garnet and matrix (figure 1). An interesting internal characteristic of this type of grossular crystal is its raspberry-red rim and black core with sharp color transition at the core-rim boundary. Lueth and Jones reported that the raspberry-red rim is composed mainly of grossular and the black core contains up to 4.5 wt.% titanium that can be used to calculate additional morimotoite and schorlomite garnet end-member  species (table 1). No detailed chemical analysis of the black core was presented, however. To better understand the composition of this type of garnet, we prepared a thin section sliced from the middle of one garnet crystal (see the color-zoned image in the middle of figure 2) for detailed chemical analysis at GIA’s Carlsbad laboratory.

Table 1
TABLE 1. Cation site occupancies for garnet end members.
Garnet end-member percentage vs. position profile.
Figure 2. Garnet end-member percentage vs. position profile revealed that the black core of the garnet was rich in morimotoite and schorlomite. The pink rim of the garnet was composed almost entirely of grossular with minor andradite.

The section’s chemical composition was obtained with a Thermo Fisher iCAP Q ICP-MS coupled with a New Wave Research UP-213 laser ablation unit. 29Si was used as the internal standard. GSD-1G and GSE-1G were used as external standards. A line of 33 ablation spots was selected to cross the whole section from one outer rim to the opposite outer rim. The black core contained up to 27.91 mol.% morimotoite and up to 4.07 mol.% schorlomite (figure 2). The detailed composition of the black core of spot 7 to spot 15 (the light blue vertical zone in figure 9) is listed in table 2. The black core contained 4.10–5.04 wt.% TiO2, confirming the analytical results reported by Lueth and Jones, and 4.42–5.49 wt.% FeOtot. It also contained 0.26–0.84 wt.% ZrO2 that could be used to calculate a distinct kimzeyite garnet end-member species (tables 1 and 2; table 1 gives the composition and site occupancy of the end members). It is the first time a detailed chemical analysis of the black core of this type of raspberry-red garnet has been documented.

Table 2
TABLE 2. Chemical composition of grandite-morimotoite garnet spots by LA-ICP-MS.

Ziyin Sun is a staff gemologist, and Nathan Renfro is analytical manager of the gem identification department and microscopist of the inclusion research department, at GIA in Carlsbad, California.