A fossilized or petrified bacillus, or a mineral form resembling bacilli.
From bacillus + -ite (Greek -ites, a suffix for minerals, fossils, and rock formations). The -ite suffix is standard in geology and paleontology for naming rocks and fossils.
Ancient bacillites found in rocks billions of years old provide evidence that bacteria-like organisms have existed nearly as long as Earth itself—some of the oldest fossils we know.
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