"Rock of the Month #202, posted for April 2018" ---

Elbaite

is a frequently spectacular, colourful end-member of the borosilicate (cyclosilicate) tourmaline family (and tourmaline is by far the most common borosilicate mineral: currently with 1,904 records in the MINLIB bibliography, cf. 117 for axinite, 86 for dumortierite and 80 for datolite). The pink variety of elbaite seen here is called rubellite. Other varietal names are blue indicolite, green verdelite and clear achroite. Elbaite is often zoned from end to end, a reflection of the piezoelectric nature of tourmaline, and classic specimens exhibit "watermelon" hues (green to pink), rarely with deep blue terminations. As a tourmaline, elbaite is very hard (Mohs 7.5) and the prisms are typically striated. The mineral formula is Na(Li_1.5Al_1.5)Al₆(Si₆O₁₈)(BO₃)₃(OH)₄.Many of the finest specimens have been won from pegmatites in Afghanistan, Brazil, and southern California. The colour zoning, size and clarity of the striated prisms of tourmaline make this mineral, and especially elbaite, a darling of the mineral collecting world, especially in terms of the high-end specimens. In recent years, I'll guess that there have probably been more coffee-table books and collector-magazine articles on elbaite than on any other gemstone (see, e.g., Falster et al., 2002; Mauthner, 2015).

Locality

The labelling on the specimen, at the time of my last visit to the Miller Museum in 2017, does not state the locality. However, the combination of pinkish-red tourmaline and pale lilac mica-rich aplite host is very distinctive. I can think of just two likely possibilities, but since I have not visited either locality I cannot be personally sure of the provenance (I studied 1 or 2 thin sections from Meldon during my analyses of Cornish tourmalines, for my long-ago thesis, but do not recall seeing the hand specimen). The two ideas for locality are:

San Diego county, southern California, U.S.A.

The gem pegmatites of southern California include the Tourmaline Queen mine, on the east flank of Pala Mountain (aka Tourmaline Queen Mountain), in northern San Diego county (Swoboda, 2002; see also Barlow et al., 1996; Wilson, 2014), and the Himalaya mine (Mauthner, 2015). More fine elbaite has been won from the Stewart mine (Stewart Lithia mine, later re-opened as a gemstone mine, also on Tourmaline Queen Mountain) and the Tourmaline King mine, both in the Mesa Grande district (Wilson, 2007). The pegmatite dykes at the Stewart mine are of Cretaceous age. Mineralogically-banded "line rock" occurs in some evolved granitoids, such as the aplite portion of the George Ashley Block in the Pala pegmatite district. In this 8-m-thick composite pegmatite- aplite dyke, there is rhythmic layering, with garnet-rich bands alternating with albite- quartz- muscovite- rich bands. Cumulus (settling) textures are notably absent from the layered rock and the texture may form by an in-situ diffusion-controlled process of oscillatory nucleation and crystallization (Webber et al., 1997).

Meldon aplite quarries, near Okehampton, Devon, England

The two Meldon quarries have long been noted for their interesting associations of minerals. Although lithium-mica granites are common in Cornwall, other Li minerals found at Meldon, such as petalite and spodumene (McLintock, 1923; Drysdale, 1985), are not at all common. Meldon is known to host both rubellite and lepidolite (Dearman, 1962). An ion microprobe study of Li in tourmaline and quartz across Cornwall showed generally lower levels of the element, typically with 140 to 1700 ppm Li (up to 0.37 wt.% Li₂O) in tourmaline and 26-210 ppm in quartz (Wilson, 1980). Note that these are the common schorl and dravite (Fe and Mg-rich) compositions of tourmaline, black to brown in colour, not elbaite with its essential Li in the formula. Micas contain far more Li than most tourmaline (Henderson et al., 1989), as of course do the discrete Li minerals observed at Meldon. Located on the northwestern edge of the Dartmoor massif, the quarries display the interaction of the Hercynian granite batholith of southwest England with predominantly sedimentary host rocks such as shale, quartzite and chert (Anon, 2007).

The evidence favours the Stewart Lithia mine, even though other pegmatites around the world contain both rubellite and lepidolite, e.g., Varutrask in Sweden (Quensel and Gabrielson, 1939) and Urubu in northern Minas Gerais, Brazil (Cassedanne and Cassedanne, 1981). With all due thanks to colleagues I have consulted, their experience is conclusive: the Stewart mine in the Pala district, San Diego county, California, is the source of this sample. Such material is widespread in collections: I noted a similar piece (sample 4814) at the Yifu Museum, China University of Geoscience-Beijing, in 2016. According to pegmatite expert Mark I. Jacobson, such specimens were extracted from outcrop at the mine in 1892, and sold into collections around the world. The specimens were an immediate commercial success (Fig. 3) and multiple adverts appeared to sell the goods. Tonnes of specimens were shipped out, hence their wide distribution to this day (Figs. 4-5). The interesting early days of the gemstone mining in southern California are recounted in some detail by Jacobson (2017).