Amphibole Mineral Inclusions in Mozambique Ruby

Kazuko Saruwatari and Masumi Saito

Amphibole, a mineral supergroup with a diverse chemical composition of the general formula AB₂C₅T₈O₂₂W₂, where A =  (vacancy), Na⁺, K⁺, Ca²⁺, Pb²⁺, Li⁺; B = Na⁺, Ca²⁺, Mn²⁺, Fe²⁺, Mg²⁺, Li⁺; C = Mg²⁺, Fe²⁺, Mn²⁺, Al³⁺, Fe³⁺, Mn³⁺, Cr³⁺, Ti⁴⁺, Li⁺; T = Si⁴⁺, Al³⁺, Ti⁴⁺, Be²⁺; and W = OH⁻, O²⁻, F⁻, Cl⁻, is a collectors’ item as a loose stone; its members are known as constituent minerals of nephrite (X. Feng et al., “Characterization of Mg and Fe contents in nephrite using Raman spectroscopy,” Summer 2017 G&G, pp. 204–212).

In addition, amphibole appears as crystal inclusions in both ruby and sapphire reported from Mozambique and Kashmir (e.g., Winter 2018 Lab Notes, pp. 435–436). Recently, the GIA lab in Tokyo received a Mozambique ruby densely included with rounded anhedral crystals (figures 1 and 2).

The Raman pattern of the surface-reaching crystals most closely matched the RRUFF reference spectrum (no. 060072) of the calcic amphibole pargasite (figure 3). The Raman peak at 669 cm^–1 corresponded to the symmetric stretching vibration of T elements, which are tetrahedral ring structures composed mainly of the Si-O_b-Si linkage (O_b = bridging oxygen) and often used to fingerprint various amphibole species (N. Waeselmann et al., “Nondestructive determination of the amphibole crystal‐chemical formulae by Raman spectroscopy: One step closer,” Journal of Raman Spectroscopy, Vol. 51, No. 9, 2020, pp. 1530–1548).

The FTIR pattern showed some peaks between 3600 and 3800 cm^–1 in the principal OH-stretching region of octahedral metal components, which are C cations in the general formula of amphibole (figure 4). The FTIR pattern showed a peak at 3643 cm^–1 in addition to peaks at 3671 and 3704 cm^–1, suggesting the possibility that other elements such as Fe occupied those sites typical of pargasite in some portions (e.g., M.C. Day et al., “Gem amphiboles from Mogok, Myanmar: Crystal-structure refinement, infrared spectroscopy and short-range order–disorder in gem pargasite and fluoro-pargasite,” Mineralogical Magazine, Vol. 83, No. 3, 2019, pp. 361–371). The FTIR spectra of corundum between 2000 and 5000 cm^–1 were very useful for the identification of mineral inclusions in corundum. This is the first known report of an FTIR pattern identifying amphibole mineral inclusions in corundum.