Figs. 1-2: Here are two images of samples of apparently pure spodumene, with traces of what seem to be thin films of dark Mn and Fe oxides. The mineral forms elongate columnar prisms, as much as 10 or 11 metres in maximum length at this site! Samples are 9x5x4 cm and 17x9x(3-4.5) cm, respectively. Spodumene is found in highly evolved siliceous granitic pegmatites, and the largest examples are candidates for the largest crystals on Earth. A 13-metre crystal was unearthed at the Etta mine in South Dakota. No microscope required!
The crystals are striated parallel to length. Some edges display the typical pyroxene cleavage at 87°. Spodumene is brittle but also hard and tough. When struck with sufficient force, the resultant breakage generates sharp splinters. It is difficult to get a clean break across a prism, given the perfect pyroxene-type cleavage parallel to prism length, and numerous sharp chips and shards result. A rock saw would be a better bet.
Samples collected at Plumbago North on 11 September 2022. Weights (Figs 1-3) are 442, 1460 and 1971 grams, respectively. Measurements on five pieces from this deposit, 74 to 718 grams in size, yield a tight estimate of specific gravity (SG), 3.16±0.02 (2 s.d.), range 3.151 to 3.174. within the reported range of 3.03-3.23. In terms of other physical characteristics: a pale pinkish to orangey wash was observed in SW/LW ultraviolet light, and the sample in Fig. 2 contains a few 1-mm angular inclusions with electric-blue SW fluorescence (tiny crystals of calcite?). The fluorescence, though rather weak in two low-powered UV units, is enough to differentiate the spodumene from associated quartz and feldspar. The gemstone variant kunzite (typically lilac in colour) may fluoresce pink or violet in SW UV light (Schneider, 2006, pp.130,145). A silicate such as spodumene would be expected to have very low magnetic susceptibility, and indeed, observed values are <0.001x10-3 SI units.
"Rock of the Month # 257, posted for November 2022" ---
Spodumene is a monoclinic lithium aluminosilicate, the type locality being the famous Varutrask pegmatite in Sweden. The formula is LiAlSi2O6. It is an inosilicate (a silicate with a structure based on chains of SiO4 tetrahedra) related to more familar pyroxenes such as diopside, CaMgSi2O6. Spodumene is usually close to the ideal formula, often with a slight excess of Si, and some replacement of Li by Na. In the gemmy variants, the green colour of hiddenite is attributed to traces of Cr, the lilac kunzite is coloured by elevated Mn and a low Fe/Mn ratio (Deer et al., 1966, p.136). There is also a colourless to pale yellow variant, triphane. Spodumene, then, should have a composition near 8 wt.% Li2O (lithia), 27 wt.% Al2O3 and 65 wt.% SiO2. However, the substitution of Na for Li can be substantial (e.g., Ahlfeld and Angelelli, 1948, pp.251-252).
The samples from the Plumbago North pegmatite are snow white to yellowish-white with a vitreous lustre, the external faces variably sprinkled with black and orangey-brown spots that appear to be 2-dimensional films of secondary oxides of manganese and iron, which most probably have a negligible effect on bulk Mn + Fe contents and the SG.
The Plumbago North pegmatite is a lithium deposit in evolved leucogranitic rocks, located north of the villages of Bethel and Newry in western Maine. There is a tremendous concentration of pegmatites in southern Maine (Maine Geological Survey, 1957). The Plumbago Mountain area in Newry township has long been noted for beryl and spodumene, amongst the many unusual and/or gemmy mineral species to be found in the state. This “Rock of the Month” reflects the recent discovery of a new spodumene pegmatite occurrence on the north side of Plumbago Mountain, some 1.5 km northwest of the famed Dunton gem tourmaline pegmatite. This area of Oxford county is known as Spodumene Brook. The new find (Simmons et al., 2020) is an albite- quartz- spodumene pegmatite with huge crystals of spodumene and montebrasite. The upper parts of the pegmatite contain up to 50% spodumene, thus this is a potentially significant Li deposit. Some spodumene laths exceed 11 m in length (and are thus some of the biggest crystals of any mineral, found anywhere). An initial consideration of geometry and grade suggests the metric resource is some 10 MT grading an average 4.68% Li2O, which is a very high grade of lithium. Plumbago North is hosted by Devonian-Silurian biotite schist. Pegmatites in Oxford county have been dated to roughly 270-250 Ma: these may be anatectic pegmatites formed by decompressional melting, associated with the early stages of rifting of Pangea (see Simmons et al., 2022, pp. 238-254 for a detailed discussion of this). The most abundant minerals are quartz, albite, muscovite and spodumene, with sparse microcline K-feldspar and local pods of the Li phosphates montebrasite and triphylite, plus other minerals, including secondary phosphates.
Figs. 3-4: Another hand specimen, 16x8x6 cm, and (right) part of the quarry face showing metre-scale laths of pale spodumene.
Lithium is becoming more and more important as an economic commodity, as the global move toward electrical vehicles gathers speed, and demand for lithium batteries rises. Lithium is used also in some small domestic batteries of AA and AAA size. Though more expensive, the Li cells have the twin advantages over alkaline cells of their longer life, and of being relatively impervious to low temperatures. As such, they are very useful for critical uses in outdoor equipment such as thermometers / weather stations and camping / mountaineering gear. While battery concerns dominate the market today, Li compounds have long been used in a range of other applications, such as ceramics, speciality alloys with magnesium and other metals, lubricants (as LiOH in grease formulations) and pharmaceuticals (such as anti-depression drugs).
Lithium is a highly reactive metal, and so does not occur naturally as a native element. It is also the lightest (least dense) metal. It is the lightest (smallest) alkali metal, its chemical "cousins" being sodium, potassium, rubidium and caesium. Lithium was discovered by Arfvedson in Stockholm in 1818, while examining the Li silicate petalite (LiAlSi4O10 from another Swedish locality) at the laboratory of Berzelius (Newton Friend, 1961, pp.145-146).
The lithium market operates on two basic forms (snapshot of prices as of 31 October 2022 --- late in a banner year!):
The major sources of lithium are brines and pegmatites (the latter dominating “hard rock” sources). The largest producer is currently Australia, largely from pegmatites. To enlarge a little on this...
In recent years it has become possible to analyse for Li in the field using a handheld device utilizing LIBS. Laser ionization breakdown specroscopy works by firing a tightly collimated laser beam at a small (mm scale) area of a rock sample. The immediate target material is ionized to some degree, and the photon emission from the resultant plasma is analysed in an optical spectrometer. Thanks to a helpful staff member of the SciAps booth at the annual Prospectors and Developers Association meeting in Toronto, 07 March 2023, two samples from the Oxford pegmatite field were analysed with a Z-901 unit, yielding values on pairs of 2x2 mm areas (a 100-micron-wide laser beam is rastered over a wider area to obtain a more representative result). A small piece of the spodumene averaged 0.939% Li (2.01 wt.% Li2O) while the graphic granite from the Havey quarry ** returned 115 ppm Li. Note that the latter, a first possible value for a granitic background level in that region, is still several times the clarke (mean concentration in the continental crust) which is 16 ppm lithium. The patchy nature of UV fluorescence in the analysed spodumene suggests there may be intergrown quartz and/or feldspar in the sample, which may explain the Li value which is lower than for theoretically pure spodumene. Given calibration with samples analysed in a conventional lab, this technology has great potential to characterize suites of rocks prospective for lithium in a given area.
Acknowledgements: The opportunity to visit this and other localities, and to learn about the geology and history of pegmatites and pegmatite research, arose through my participation in the Pegmatite Workshop held on 8-11 September 2022 at the recently-opened Maine Mineral & Gem Museum in Bethel, Maine. The course was organized by Skip Simmons, Karen Webber, Alexander Falster, Encar Roda-Robles and Donald Dallaire, the authors of the manual "Pegmatology" (2nd edition released in 2022: Visit Rubellite HERE). Thanks to them, Larry Stifler and Mary McFadden and the staff of MMGM, Mary and Gary Freeman of Freeman Resources, LLC, and especially to Richard Bedell, who suggested and facilitated my participation, at a time when I otherwise might have stayed at home!
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** Graphic granite of the Havey quarry, Maine