A metallographic and microprobe examination of a metallic nodule from the Bondoc Peninsula meteorite

Extract

The Bondoc Peninsula meteorite appears to be unstable and is disintegrating as the result of terrestrial corrosion. The meteorite is complex with occasional 1 to 3 cm dia. ‘Nodules’ of ‘iron-class’ material, fig. 1, within which are numerous non-metallic inclusions. These range in size from 0·5–1·5 mm and vary in shape from the extremes of angular to globular, shown in figs. 2 and 3. The metallic groundmass of fig. 1 is a polycrystalline array of kamacite grains that are equant in shape and about 3 to 4 mm dia. The boundaries between these kamacite grains are heavily invaded by corrosion product. The films of cracked schreibersite and strips of compositionally zoned taenite that are present at the kamacite boundaries each contribute about 2 % by volume and their average Ni contents are about 45 wt% and 40 wt% respectively. When these figures are combined with the average 6·2 wt% Ni, 0·75 wt% Co, and 0·02 wt% P of the kamacite the resulting bulk composition of the metal is ∼ 7·5 wt% Ni, ∼ 0·7 wt% Co, ∼ 0·3 wt% P. Nital etching reveals partially annealed Neumann bands in the kamacite, indicating a late stage reheating below about 500°C. However, the M profile method of Wood (1967) when applied to the zoned taenite yields a cooling rate of 0·1–0·5°C/Myr before this reheating event.

The angular inclusions are relatively coarsely crystalline and commonly have massive chromite and tridymite crystals, with pyroxene, anorthite, and whitlockite usually present. Most angular inclusions also contain small particles of metal (10–40 µm dia.) interstitial to the mineral phases in the depth of the inclusion. Small quantities of sulphide and phosphide are similarly located. The metal in the angular inclusions is not detectably zoned and both kamacite and taenite show a range of Ni contents that vary with particle size in the manner previously observed by Powell (1969) for a range of conventional mesosiderites. We have in addition measured the Co and P contents of this metal and find P < 0·1 wt% and the Ni and Co analyses are consistent with equilibration in the ternary Fe-Ni-Co equilibrium diagram at ∼ 450 °C.