"Rock of the Month #128, posted for February 2012" ---

A lustrous pebble of (?) chiastolite-muscovite schist, a metasediment containing coarse crystals of silicate minerals. The fine-grained matrix is evidently micaceous, probably due to the silvery mica, muscovite. The crudely rectangular prisms that are the most striking feature of the rock are thought to be another aluminous silicate. They may be chiastolite (see below) or another species of aluminosilicate.

This little pebble, collected by Mrs Laura Murray in August 2008, was found at Notre Dame Bay on the northeastern, Atlantic coast of Newfoundland, near the Exploits River. The flattened pebble, with a smooth surface and discoid shape, appears to be a waterworn sample of a metamorphosed sedimentary rock, a schist with internal fabric defined by alignment of flattened flakes of silvery muscovite mica. The stubby purplish prisms are thought to be a form of andalusite, a locally common aluminosilicate mineral, which shares with its polymorphs kyanite and sillimanite the basic composition Al₂SiO₅. In favourable orientations, these prisms may display cross-like inclusions, and this form is known as chiastolite.

Chiastolite is referred to in numerous textbooks of metamorphic petrology, old and (relatively) new (Harker, 1932; Battey, 1981; Barker, 1990; Yardley et al., 1990; Blatt and Tracy, 1996). The development of so-called sector zoning in metamorphic minerals such as andalusite is commonly related to the presence of carbonaceous matter, including its metamorphic equivalent, graphite (Rice and Mitchell, 1991). Chiastolite is known from localities in England (such as the Lake District and Dartmoor), from Sardinia, Brittany, northeast India, parts of Australia (e.g., Simpson, 1990) and Canada, and elsewhere. Relatively coarse material has been confirmed in the Northwest Territories (Cerny et al., 1989), and closer to the current example, in the Meguma terrane of Nova Scotia (Raeside and Jamieson, 1992).

The region of Notre Dame Bay is part of the northeastern Appalachian orogen of North America, noted for its fossil sea-floor rock assemblages known as ophiolites (Cyprus in the Mediterranean is often cited as a prime example in more recent geological time). The volcanic and associated sedimentary rocks of Paleozoic age in Newfoundland and Nova Scotia include strata favourable to production of andalusite and other minerals in relatively low-pressure regional metamorphism. This would have occurred during the mountain- building episode that produced high peaks, now represented by their rugged remnants, the Appalachians.

The Cambrian and Ordovician volcanic rocks host base-metal deposits enriched in copper and in trace metals such as gold (Sillitoe, 1973). The volcanism is part of the geological history of an extinct waterway, the Iapetus Ocean, and sedimentary strata and fossils (such as graptolites and conodonts) of this ancient domain are preserved across the younger Atlantic ocean, in Scotland and elsewhere in Europe (see, e.g., Colman-Sadd et al., 1992; Lafrance and Williams, 1992; Spicer et al., 2010).

The Exploits River area was prime country for the Beothuk native people, the principal or only inhabitants of the region for more than 1,000 years. Falling foul of European explorers and settlers, the Beothuk were eventually driven inland from their preferred coastal habitat, the last member of the tribe dying in 1829.

So, there it is --- a whole page about a pebble --- and without proper analysis we may never know whether those curious crystals actually are chiastolite, or not. Onwards...

References

Barker,AJ (1990) Introduction to Metamorphic Textures and Microstructures. Blackie, Glasgow, 162pp.

Battey,MH (1981) Mineralogy for Students. Longman Inc., New York, 2nd edition, 355pp.

Blatt,H and Tracy,RJ (1996) Petrology: Igneous, Sedimentary, and Metamorphic. W.H. Freeman and Company, 2nd edition, 529pp.

Cerny,P, Wise,MA and Tomascak,P (1989) Reconnaissance of M1, M2 and Qulik 1 claims, Chantrey Inlet, NWT. EGS 1989-3, NWT Geology Division, INAC, Yellowknife, 16pp.

Colman-Sadd,SP, Stone,P, Swinden,HS and Barnes,RP (1992) Parallel geological development in the Dunnage Zone of Newfoundland and the Lower Paleozoic terranes of southern Scotland: an assessment. Trans.Roy.Soc.Edinburgh: Earth Sciences 83, 571-594.

Harker,A (1932) Metamorphism, a Study of the Transformations of Rock-Masses. Methuen & Co. Ltd., London, 1st edition, 360pp.

Lafrance,B and Williams,PF (1992) Silurian deformation in eastern Notre Dame Bay, Newfoundland. Can.J.Earth Sci. 29, 1899-1914.

Raeside,RP and Jamieson,RA (1992) Low-pressure metamorphism of the Meguma Terrane, Nova Scotia. GAC/MAC Field Excursion Guidebook C-5, 25pp., Wolfville, Nova Scotia.

Rice,AHN and Mitchell,JI (1991) Porphyroblast textural sector-zoning and matrix displacement. Mineral.Mag. 55, 379-396.

Sillitoe,RH (1973) Environments of formation of volcanogenic massive sulfide deposits. Econ.Geol. 68, 1321-1325.

Simpson,PG (1990) Tom's Gully gold deposit. In `Geology of the Mineral Deposits of Australia and Papua New Guinea' (Hughes,FE editor), Australasian Institute of Mining and Metallurgy Monograph 14, 1828pp., 743-746.

Spicer,B, Morris,B, Ugalde,H, Slavinski,H and Skulski,T (2010) Structure of the Betts Cove ophiolite complex beneath the western margin of Notre Dame Bay, Newfoundland. Can.J.Earth Sci. 47, 181-198.

Yardley,BWD, MacKenzie,WS and Guilford,C (1990) Atlas of Metamorphic Rocks and their Textures. Longman Group UK Ltd, 120pp.