AbstractNeedle‐fibre calcite occurs in association with root structures in a Lower Carboniferous calcrete crust. The origin of this form of calcite is discussed and the scattered literature from soil science, microbiology and geology is reviewed. It is concluded that while needle‐fibre calcite is not diagnostic of any environment its origin is related to the activities of various micro‐organisms, especially fungi. The use of needle‐fibre calcite as a climatic indicator is reviewed and it is used, with other criteria, as evidence of fluctuating climates in the Lower Carbon