The textural and compositional characteristics of a spinifex rock (MS-1) from Munro township, Ontario have been investigated in some detail. When projected into Mg–CaO–Al2O3 and CaO–MgO–FeO space, MS-1 appears late in the fractionation trends for peridotites, associated spinifex rocks and high Mg basalts. The latter trend extrapolates almost linearly to olivine tholeiite compositions.There are two types of blade megacrysts in the rock, both apparently representing preexisting olivine: (1) an earlier phase of composite, skeletal platy crystals of clinochlore, about 10 × 10 × 0.5 mm in size, which have a preferred orientation in the rock and show pronounced parallel growth development in the plane of the blades. The clinochlore is crystallographically related to the host olivine with c* and [100] parallel to a and b axis of olivine, respectively; (2) sheafs of oriented skeletal plates about 0.1 mm in shortest dimension which are completely altered to chlorite, calcite, magnetite and amphibole. The appearance of the composite blade megacrysts varies markedly with thin section orientation and this seems to account for the various types of blade (or platey) crystals reported in the literature. The interblade areas are largely of acicular and spherulitic pyroxene and interstitial groundmass. The acicular pyroxene is augite. Its composition varies considerably, especially with the area analysed, the greatest variations being in Cr, Ti and Na. The augite is characterized by high Al contents—preferentially accommodated in tetrahedral sites—and this leads to a distinctive field in Ti, Al plots compared to those for lunar and other terrestrial pyroxene. The rock also contains a few relict rounded olivine phenocrysts, amygdules, accessory chromite octahedra with magnetite rims, pentlandite and pyrrhotite.The Mg–Fe ratios for the clinochlore, augite and interstitial groundmass do not vary greatly from that of the whole rock value in contrast to the situation in a quenched plateau basalt.

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