"Rock of the Month # 112, posted for October 2010" ---

The term ophicalcite is applied to a noted product of dedolomitization, in which the Ca-Mg carbonate dolomite is transformed to the (ideally) purely-Ca carbonate calcite, with Mg being liberated to form other minerals such as forsteritic (Mg-rich) olivine. The olivine subsequently hydrates to serpentine, generating a serpentinous marble (ophicalcite; Harker, 1954, p.261). Another phase found in many of these marbles is brucite, rhombohedral Mg(OH)₂. In sufficient volumes, this is a potentially economic industrial mineral, useful as a fire retardant and possible magnesium ore (Simandl et al., 2007). Ophicalcites have been documented elsewhere (Leake et al., 1975; Nelson, 1993; Wilson et al., 2001; Lavoie and Chi, 2010).

The sample is from a small quarry located some 3.8 km S.S.W. of the village of Broadford, 2.5 km E.N.E. of the summit of Beinn an Dubhaich, in the southeast quadrant of the island of Skye (Ordnance Survey, 1975). This is a classic area for igneous petrology, shaped by the magmatism of the Tertiary Volcanic Province, associated with the opening of the North Atlantic ocean some 65 Ma. Some of the finest landforms in the Scottish sector of the province lie just to the west, across Loch Slapin, in the rounded Red Cuillin summits, and especially in the jagged Black Cuillin beyond (see, e.g., Craig, 1965, pp.434-445; Brown, 1969; Bell and Harris, 1986). Further evidence of the northward-propagating Atlantic rifting event is afforded across the ocean, in east Greenland. The formation and unroofing of the igneous complex on Rhum, followed by further magmatism in Rhum, Canna and Skye, and the development of most or all of the Cuillins complex on Skye, all occurred in a single reversed polarity interval (26r) lasting some 2.8 million years, around 59 Ma (Mussett, 1984).

The pre-Tertiary bedrock of the area includes the Cambrian Durness limestone, a series of fine-grained, fossiliferous limestones and chert-bearing dolomites (Phemister, 1960, pp.47-51; Richey, 1961, pp.11,12,92). The marbles and skarns in this area were formed when Cambrian dolostones were invaded by granitic magma and associated out-streaming hydrothermal fluids. The metamorphic reactions and complex mineralogy were documented by Tilley (1948a,b, 1951) Some chert nodules in the original dolostone have survived, after a fashion, and added to the mineralogical diversity. The evident lack of garnet is consistent with this sample lying beyond the immediate contact skarns. The aureole beyond the skarn zones displays talc and tremolite, forsterite, diopside, periclase and rare wollastonite, spinel, idocrase (vesuvianite), grossular garnet, phlogopite mica, brucite, serpentinite, chlorite and hydromagnesite (Tilley, 1951). Metasomatism along dykes of dolerite (diabase) cutting the marbles is a further source of interesting mineral assemblages (Tilley, 1948b; Holness, 2000).

The transformation of dolostones to marbles and the development of garnet and other metamorphic minerals was documented in numerous papers and textbooks of the day (e.g., Harker, 1932, 1954). More recent workers have maintained an interest in the Beinn an Dubhaich granite and its metamorphic aureole (e.g., Kerrick, 1991; Holness, 1992, 1997; Holness and Fallick, 1997). Other recent metamorphic research includes Ferry (1995, 2001) and Ferry and Rumble (1996). The content of rare earth elements (REE), yttrium and uranium in the skarns is described by Smith et al. (2002, 2004). The oxygen isotope chemistry of the granites and hydrothermal alteration has been studied by Guthrie and Veblen (1991), Monani and Valley (2001) and others.

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Brown,GM (1969) The Tertiary igneous geology of the Isle of Skye. Geologists' Association Guide 13, 37pp.

Craig,GY (editor) (1965) The Geology of Scotland. Oliver & Boy, Edinburgh and London / Archon Books, Hamden, CT, 556pp. plus map.

Ferry,JM (1995) Three novel isograds in metamorphosed siliceous dolomites from the Ballachulish contact aureole, Scotland. GSA Abs.w.Progs. 27 no.6, 262, New Orleans.

Ferry,JM (2001) Calcite inclusions in forsterite. Amer.Mineral. 86, 773-779.

Ferry,JM and Rumble,D (1996) Formation of periclase by fluid flow in two contact aureoles. GSA Abs.w.Progs. 28 no.7, 131-132, Denver.

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Holness,MB (1997) Fluid flow paths and mechanisms of fluid infiltration in carbonates during contact metamorphism: the Beinn an Dubhaich aureole, Skye. J.Meta.Geol. 15, 59-70.

Holness,MB (2000) Metasomatism and self-organization of dolerite dyke-marble contacts: Beinn an Dubhaich, Skye. J.Meta.Geol. 18, 103-118.

Holness,MB and Fallick,AE (1997) Palaeohydrology of the calcsilicate aureole of the Beinn an Dubhaich granite, Skye, Scotland: a stable isotopic study. J.Meta.Geol. 15, 71-83.

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Nelson,JL (1993) The Sylvester allochthon: upper Paleozoic marginal-basin and island-arc terranes in northern British Columbia. Can.J.Earth Sci. 30, 631-643.

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Simandl,GJ, Paradis,S and Irvine,M (2007) Brucite - industrial mineral with a future. Geoscience Canada 34, 57-64.

Smith,MP, Henderson,P and Jeffries,T (2002) The formation and alteration of allanite in skarn from the Beinn an Dubhaich granite aureole, Skye. Eur.J.Mineral. 14, 471-486.

Smith,MP, Henderson,P, Jeffries,TER, Long,J and Williams,CT (2004) The rare earth elements and uranium in garnets from the Beinn an Dubhaich aureole, Skye, Scotland, UK: constraints on processes in a dynamic hydrothermal system. J.Petrol. 45, 457-484.

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Tilley,CE (1948b) Dolomite contact skarns of the Broadford area, Skye: a preliminary note. Geol.Mag. 85, 213-216.

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