"Rock of the Month # 103, posted January 2010" --- Sample GCW 2263, an example of pegmatitic amazonite feldspar from Saint-Ludger-de-Milot, in Lac-Saint-Jean-Est county, some 25 km northeast of the N.E. shore of Lac St. Jean, Quebec, Canada. It was donated by Jayanta Guha, professor at UQAC, the Université du Québec à Chicoutimi, and its Department of Geology (Sciences de la Terre à l'UQAC), located in a vibrant community in the nearby Saguenay valley.

The main sample is a 25x20x2-5-cm slab of pale bluish-green amazonite pegmatite, dominated by microcline K-feldspar of delicate green hue. The amazonite feldspar is a popular lapidary material or semi-precious stone (Sinkankas, 1976), a colourful counterpoint to the more familiar pink perthitic feldspar in many pegmatites. The piece is not appreciably magnetic. Grey quartz is also present in individual grains and patches of oriented "graphic granite", as are traces of muscovite and biotite micas and rusted (?) pyrite. The area is part of the Allochthonous monocyclic belt of the Grenville province (Hocq, 1993).

The close-up at the right (the one-cent coin is 19 mm in diameter) of a smaller, more richly coloured sample shows the perthitic texture very clearly. This piece (GCW 1699) is from another Grenville locality, a small quarry in the Magnetawan district, near Georgian Bay, south-central Ontario.

Amazonite, occasionally dubbed amazonstone, is a green variant of microcline K-feldspar. It occurs at a number of localities across southern Quebec, such as Lac Godin and south of Lac Blue Sea in the Maniwaki-Baskatong area (Lapointe and Gauthier, 1992); at Lac Sairs in Villedieu township, 35 km east of Kipawa (Sabina, 1974; Remick and Bellemare, 1992); the Berger occurrence (Sabina, 1971); and in pegmatite at the Leduc mine (Rimsaite, 1968). As in Ontario to the west, the Grenville province in Quebec is home to many mineral showings with a wealth of mineral species, such as tourmaline, pyrite, apatite, graphite, scapolite, feldspar species such as amazonite and peristerite, zircon and phlogopite (Bellemare et al., 1992). The Lac St. Jean region hosts numerous interesting intrusions of Grenville ages, most notably the Lac St. Jean anorthosite suite, dated at roughly 1157 to 1140 Ma. Amazonite in the area is found in pegmatite dykes (Ider et al., 1997; Higgins et al., 2002).

The variable yet characteristic colour of amazonite appears to result from the quantity and valence state of lead impurity (Julg, 1998). Microcline is the typical feldspar of granitic pegmatites, and pegmatites may form relatively late in the tectonic and metamorphic history of the host terrane. Thus Pb remobilized in metamorphism may be concentrated in the feldspar (Stevenson and Martin, 1986). Amazonite has long been noted as a typical if unusual pegmatite mineral, with other species of feldspar, mica, quartz, beryl, Mn-enriched garnet, etc (Fontaine, 1883). At one gemstone mine in Vietnam, amazonite is considered an indicator mineral for more valuable tourmaline (Blauwet, 2007). The classic locality is Pikes Peak, Colorado (Foord and Martin, 1979; Muntyan and Muntyan, 1985).

Amazonite occurrences are numerous, with examples in Russia, Mongolia, Vietnam, Norway, Egypt, the Sidamo region of Ethiopia, Madagascar, India and Sri Lanka, and the Broken Hill area of New South Wales, Australia. North American localities are scattered from Colorado and Nevada to Virginia and North Carolina, as well as Quebec, Ontario and the Northwest Territories.

References

Bellemare,Y, LePage,M and Delisle,S (1992) Liste des Sites des Minéraux de Collection au Québec, tels que Répertoriés dans la Littérature. MERQ MB-91-17, 143pp. (in Fr.).

Blauwet,D (2007) The Minh Tien tourmaline mine, Luc Yen mining district, Yenbai province, Vietnam. Mineral.Record 38, 443-452.

Fontaine,WF (1883) Notes on the occurrence of certain minerals in Amelia County, Virginia. Amer.J.Sci. 125, 330-339.

Foord,EE and Martin,RF (1979) Amazonite from the Pikes Peak batholith. Mineral.Record 10 no.6, 373-384.

Higgins,MD, Ider,M and van Breemen,O (2002) U-Pb ages of plutonism, wollastonite formation, and deformation in the central part of the Lac-Saint-Jean anorthosite suite. CJES 39, 1093-1105.

Hocq,M (1994) La province de Grenville. In "Géologie du Québec" (Dubé,C editor), Gouvernement du Québec, 154pp., 75-94 (in Fr.).

Ider,M, Higgins,MD and van Breemen,O (1997) U-Pb ages of the Canton-St-Onge wollastonite deposit and associated plutons in the central part of the Lac-St-Jean anorthosite complex, Grenville province, Canada. GAC/MAC Abs. 22, 72, Ottawa.

Julg,A (1998) A theoretical study of the absorption spectra of Pb⁺ and Pb³⁺ in site K⁺ of microcline: application to the color of amazonite. Physics and Chemistry of Minerals 25, 229-233.

Lapointe,S and Gauthier,M (1992) Typologie et chronologie des minéralisations de molybdène, d'uranium et de thorium de la région de Maniwaki-Baskatong: implications pour l'exploration minière. In `Séminaire d'information 1992', MERQ DV 92-03, 89pp., 23-24 (in Fr.).

Muntyan,BL and Muntyan,JR (1985) Minerals of the Pikes Peak granite. Mineral.Record 16 no.3, 213-230.

Remick,J and Bellemare,Y (1992) Minéraux du Québec. MERQ GT 92-02, poster (in Fr.).

Rimsaite,J (1968) Micas and associated minerals from two Li-pegmatites. Can.Mineral. 9, 580.

Sabina,AP (1971) Rocks and Minerals for the Collector: Ottawa to North Bay, Ontario: Hull to Waltham, Quebec. GSC Pap. 70-50, 130pp.

Sabina,AP (1974) Rocks and Minerals for the Collector: Cobalt-Belleterre-Timmins: Ontario and Quebec. GSC Pap. 73-13, 199pp.

Sinkankas,J (1976) Gemstones of North America, Vol. II. Van Nostrand Reinhold, 494pp.

Stevenson,RK and Martin,RF (1986) Implications of the presence of amazonite in the Broken Hill and Geco metamorphosed sulfide deposits. Can.Mineral. 24 Pt.4 (Ferguson Volume), 729-745.