But similar shapes can be produced in many non-biological ways, including from laboratory processes such as cleaning and glueing and coating samples. It is not absolutely clear that the shapes are not contaminants. Again, McKay et al. made a good faith attempt to demonstrate this but the evidence is not unequivocal. Microscopic Mineral Grains McKay et al. show that these grains are clearly associated with the carbonate globules which clearly are of Martian origin. Thus, terrestrial contamination is not an issue.
Magnetite is made by some magnetotactic bacteria on Earth.
Pyrrhotite and greigite are produced as waste products by some bacteria. Iron oxide and iron sulfide are not usually found in the same biological environments, but some bacteria can produce both. All these mineral grains do look similar to the sizes and shapes of such grains produced by terrestrial bacteria. But non-biological processes can also produce all these minerals of similar shapes and sizes. McKay et al. suggest that the grouping of these minerals is highly unusual in non-biogenic environments, but this is not certain.