Compositional variation within some sedimentary chlorites and some comments on their origin

A range of authigenic sedimentary chlorites from sandstones has been studied by analytical transmission electron microscopy. Selected area (single crystal) electron diffraction patterns are of the I b ( β = 90°) polytype confirming the earlier observations of Hayes (1970). TEM analyses show all samples to be relatively rich in both Al and Fe. In the general formula (Mg,Fe,Al) n [Si 8−x Al x O 20 ](OH) 16 , x varies between 1.5 and 2.6; Fe/(Fe + Mg) between 0.47 and 0.83 and n between 10.80 and 11.54. Octahedral Al is close to 3 in this formulation and Fe 2+ predominates over Fe 3+ . Swelling chlorites have significantly different compositions which are consistent with smectite/chlorite interstratifications. The I b ( β = 90°) polytype appears to be stable under conditions of moderate to deep burial. It replaces berthierine and swelling chlorites formed at lower temperatures. As commonly seen in grain coatings, however, it precipitates from porewater; solutes probably being contributed from several mineral decomposition reactions.