"Rock of the Month # 28, posted for October 2003" --- Gabbro boulder.

The boulder in the photograph is a handsome mass of a coarse-grained igneous rock, an example of a plutonic rock named gabbro of probably-local origin (see, e.g., Davies and Whitehead, 2001). The rock is massive (lacking bands of distinct composition or orientation) and coarsely crystalline, composed of crystals roughly 4 mm in size. The small broken area exposes tabular grains of shiny greyish plagioclase feldspar and dark greenish-black silicates rich in iron, calcium and magnesium (pyroxene, probably metamorphosed to amphibole, and possibly olivine, not identified) and traces of iron oxides. The rock was found submerged in a local swamp, which may account for its unusual surface texture, in which bladed feldspar crystals, etched to a white and frosted finish, stand proud over pits presumably developed where acid swamp waters dissolved the more reactive dark minerals.

Gabbro is a dark "cousin" of granite, formed like granite by slow cooling of molten magma, usually several km below the Earth's surface. The slow process of solidification of the melt permits crystals to grow large, resulting in a mass of coarse crystals visible to the naked eye. Gabbro is dark because it contains more iron and less silica than granite, and typical examples are rich in greenish-black pyroxene and grey plagioclase feldspar, lacking the clear to pale grey quartz, white to pinkish feldspars and shiny mica flakes which comprise the bulk of many granites.

What tells us that this is not, after all, a meteorite? Most meteorites are composed largely of silicate minerals. A minority of sometimes very-large examples are mostly metal ("iron meteorites"). The "stony meteorites", unless cut into slices or heavily weathered (very rusty), generally display a thin (roughly 0.5 to 1 mm) fusion crust of material that melted during the meteorite's passage through our atmosphere. This black, glassy crust may be rippled, or cracked or partially flaked away. The stone at Marmora displays a pale, roughened exterior, and a dark interior is visible where a fragment was broken off. This is consistent with a gabbro boulder. If this sample was a stony meteorite, the interior would display certain textures, depending on its type. Typical stony meteorites contain shards of silvery to steel-grey nickel-iron metal, and/or a yellowish or brownish iron sulphide known as troilite.

Reference

DAVIES,JF and WHITEHEAD,RE (2001) CO₂, alkalies and REE systematics in hydrothermally altered gabbro hosting the Cordova gold-bearing veins, Ontario. Explor.Min.Geol. 10, 321-328.