Federico Pezzotta

Liddicoatite Tourmaline From Anjanabonoina, Madagascar

28 LIDDICOATITE FROM MADAGASCAR GEMS & GEMOLOGY SPRING 2002 three lithium tourmalines with the general formula (Ca,Na,K, )(Li,Al)3Al6Si6O18(BO3)3(OH)3(OH,F), which are defined on the basis of their X-site occupancy: Ca = liddicoatite, Na = elbaite, and a vacant ( ) X site = rossmanite. Elbaite is the most abundant gem tourmaline, whereas rossmanite has so far been identified from few localities (Johnson and Koivula, 1998b; Selway et al., 1998), and typically is not of gem quality. However, neither can be separated from liddicoatite without quantitative chemical analysis. Therefore, in this article we use the group name tourmaline to refer to material that has not been chemically analyzed. Although liddicoatite is well characterized mineralogically, little has been published about the history, sources, and gemology of this tourmaline species in particular. This article focuses on liddicoatite from Madagascar—which is the principal historic source— and in particular on the Anjanabonoina pegmatite, By Dona M. Dirlam, Brendan M. Laurs, Federico Pezzotta, and William B. (Skip) Simmons

Schiavinatoite, (Nb, Ta) BO4, the Nb analogue of behierite

The new mineral schiavinatoite (Nb, Ta) BO 4 , the niobium analogue of behierite, has been found in a boronrich pegmatite at Antsongombato, south of Betafo, in the Malagasy Republic, as portions of a greyish-pink bipyramidal crystal of Nb-rich behierite. Associated minerals are rhodizite, liddicoatite, spodumene, pollucite, danburite, apatite. The mineral is tetragonal, space group I 4 1 / amd , isostructural with behierite (zircon-type), with a = 6.219(5), c = 5.487(5) A, V = 212.2(5) A 3 , Z = 4, D calc = 6.548 g/cm 3 . The strongest lines in the X-ray powder diffraction pattern [d in A (I, hkl)] are: 4.115(100,101), 3.110(84,200), 2.328(49,112), 1.598(42,312), 2.481(36, 211), 1.939(29,301), 1.646(25,321). Mohs9s hardness is 8. Luster is vitreous, transparent in thin section, streak is white, the crystal is uniaxial (+) with average n = 2.30(5). The structure has been refined from single-crystal data, using 178 observed unique reflections to R = 0.011, R w = 0.014.

Yellow Scapolite from Ihosy, Madagascar

GEMS & GEMOLOGY WINTER 2010 (Ca4[Al6Si6O24]CO3). Intermediate compositions in the solid-solution series are named after the closest end member (Deer et al., 1992). Meionite (Me) possesses higher RI and SG values than marialite. The meionite end member has refractive indices of no = 1.600 and ne = 1.564, and an SG of 2.78; the marialite end member has RIs of no = 1.539 and ne = 1.531 and an SG of 2.50 (Deer et al., 1992). A third end member of the group, silvialite, is characterized by an SO4 anionic group (Ca4[Al6Si6O24]SO4; Teertstra et al., 1999); however, silvialite has not been considered in previous gemological studies of gem scapolite. It has not yet been systematically established whether the different colors of scapolite correspond to different compositions. Likewise, it is also unclear if properties such as RI and SG are related to specific colors. However, Couper (1991), reporting on a study of scapolite from Myanmar, noted that violet-topink samples plotted in the marialite field, and yellow and colorless samples fell in the meionite field. Because of the uncertainties concerning minerals of this group, a better knowledge is needed of the composition of gem-quality scapolites and the methods for their identification. This study characterizes yellow scapolite from the Ihosy deposit in Madagascar, and investigates the suitability of previously established methods for calculating scapolite composition so it can be properly identified.

Tsavorite and other Grossulars from Itrafo, Madagascar

GEMS & GEMOLOGY FALL 2012 G with the chemical formula Ca3Al2(SiO4)3, is a species of the garnet group that exhibits colors ranging from colorless to pink, brown, yellow, orange, and green. The latter is known by the varietal name tsavorite when the color is a saturated green (O’Donoghue, 2006), whereas less-saturated material is often referred to as green grossular or mint green grossular in the trade. Although tsavorite is not approved as a mineral name by the International Mineralogical Association (Nickel and Mandarino, 1987; O’Donoghue, 2006), we will use the term in this article for the sake of brevity and consistency with gemological convention. The most important deposits of gem-quality tsavorite occur in Tanzania and Kenya (Bridges, 1974). Other notable sources include Pakistan’s Swat Valley (Jackson, 1992) and the Gogogogo area in southwestern Madagascar (Mercier et al., 1997; Johnson et al., 1999). A new source of fine gem-quality grossular (figure 1), including some tsavorite, was discovered in 2002 at the village of Itrafo in central Madagascar. This article presents a detailed characterization of this material.