1 1 1 1 1 Rating 0.00 (0 Votes)

Lemon Magnesite as Gaspeite

Lemon Magnesite

»Lemon Magnesite« with a small quartz vein

Foto: K. Sieber, www.makrogalerie.de

A special variation of nickelous Magnesite is traded under the deceptive name "Lemon Chrysoprase". The material comes from the Eastern Goldfields area of Western Australia. The rock is found as nodules and veins in a strongly weathered serpentinite in a belt of ultramafic rocks that stretch from  Norseman in the south to near Wiluna in the north. For this reason, the raw pieces often show an outer edge of heavily corroded, brownish serpentine that has weathered to laterite. Now and then narrow, green, partly translucent areas of chrysoprase or green opal are next to it. However, the largest part of this rock consists of a greenish-yellow, opaque magnesite, which is at best interspersed with translucent, colorless veins of quartz. Studies conducted by the Gemmological Institutes of America (GIA) showed that these veins of quartz are indeed colorless and can not be named "Chrysoprase".

Since 1996, when the first large lots had been offered on the Tucson Mineral Show, the demand has increased and the supply of high quality stones has fallen continuously. Since 2004 we have been observing a divergent trend. Initially, yellow-green nickelous magnesite (without Chrysoprase) was offered. After that, a beige, almost white silicified magnesite without nickel was offered under the same name. The material may come from the periphery of the mining areas or from other deposits like Lake Rebecca, the Bulong Goldfield near Lake Yindarlgooda, near the cities Kambalda and Wingellina or sporadically from an area near Yundamindera Station.

Confusion with Gaspeit

Gaspeite, Australia

Gaspeite, Australia

Foto: K. Sieber, www.makrogalerie.de

An iron-containing nickel-magnesite called gaspeit is very similar in appearance, but has a different chemical composition. The mineral was named after its discovery site on the Gaspe Peninsula in Quebec, Canada, where it was first discovered in 1977. The world's most important deposit is located in Western Australia, from where the first specimens came onto the mineral market in early 1992 (GRACANIN,1997).
Chemically it is an anhydrous carbonate, with admixtures of nickel, magnesium and iron with the formula (Ni,Mg,Fe2+)CO3. Gaspeite forms a solid solution serie with nickel-containing magnesite (MgCO3) and siderite (FeCO3). This siderite component is completely missing in "lemon magnesite". An elemental analysis by the laboratory of the German Gemmological Society (DSEF) on various "lemon chrysoprase" samples revealed only a trace of nickel, but no iron, in addition to magnesium, silicon and carbon. Because of the lack of iron, »Lemon Magnesite« cannot be called "gaspeite" (HENN et al.,1997).

Conclusion

In its best quality this bright green rock consists of partially silicified nickelous Magnesite that includes veins of quartz. The stones the EPI institute has examined have either been not silicified greenish magnesite or silicified beige to white magnesite, but in no case Chrysoprase. For this rock the name »Lemon Magnesite« is more adequate. Because of the lack of iron »Lemon Magnesite« is not consistent with gaspeite.

References:
BROWN, G., BRACEWELL. H. (1987): "Citron Chrysoprase", Austral. Gemmol., 16, 6, 231-233.
GRACANIN, L. (1997): Gaspeit - ein seltener Schmuckstein, Z. Dt. Gemmol. Ges., 46, 2, 107-108.
HENN, U., MILISENDA, C.C. (1997): "Zitronen-Chrysopras" aus Australien, Z. Dt. Gemmol. Ges., 46, 1, 45-47.
JOHNSON, M.L., KOIVULA, J.I. (1996): Gems News: Quartz-magnesite rock, so-called "lemon chrysoprase" from Australia, Gems & Gemology, 32, 3, 217.