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

Abstract

A B S T R A C T. A range of authigenic sedimentary chloritesfrom sandstones has been studied by analytical transmission electron microscopy. Selected area (single crystal) electron diffraction patterns are of the Ib (fl = 90 ~ polytype confirming the earlier observations of Hayes (1970).TEM analyses show all samples to be relatively rich in both A1 and Fe. In the general formula (Mg,Fe,A1), [Sis_xAlxO20](OH)~6, 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 AI 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 Ib (fl = 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.

Microstructures, Mixed Layering, and Polymorphism of Chlorite and Retrograde Berthierine in the Kidd Creek Massive Sulfide Deposit, Ontario

“…However, in a strict sense, polymorphs must have identical compositions. Overlapping of compositional fields of chlorite and berthierine has been noted by Curtis et al (1985), Hughes (1989), Jahren and Aagaard (1989), and Velde (1989), but emphasis has focused on chemical comparisons rather than on polymorphic relations. This, in part, is because most available analyses of berthierine have been obtained by electron microprobe on rock samples or clay separates, for which contamination by other phases is likely.…”

“…The most obvious example is 1Md and 2M~ illite, which may have different compositional ranges that have not been well defined. In the same sense, berthierine or poorly-defined 7-• phases in low grade rocks are commonly viewed as a precursor polymorph of chlorite, analogous to the dioctahedral mica sequence (Velde et al, 1974;Iijima and Matsumoto, 1982;Lee and Peacor, 1983;Ahn and Peacor, 1985;Curtis et al, 1985;Jahren and Aagaard, 1989). However, in a strict sense, polymorphs must have identical compositions.…”

“…If vacancies do indeed occur in berthierine but not in associated chlorite, it is possible that normalization of berthierine formulae on an incorrect basis might imply a fictitious similarity in composition between berthierine and chlorite, and therefore in polymorphism. As noted above, such vacancies are generally assumed to exist if normalization is based on charge balance relative to an ideal number of anions, resulting in an excess of octahedral trivalent cations relative to tetrahedral trivalent cations, as is common for chlorite (e.g., Curtis et al, 1985;Hillier and Velde, 1991). There is no apparent chemical discrimination between berthierine and Fe-rich chlorite in terms of the number ofoctahedral vacancies if they both are normalized in the same manner.…”

“…Powder X-ray diffraction and chemical analytical data have led to the identification of berthierine in only a small number of shallow marine sediments and unmetamorphosed clastic sedimentary rocks (Velde et al, 1974;Frey, 1970Frey, , 1978Iijima and Matsumoto, 1982;Curtis et al, 1985;Taylor, 1990;Walker and Thompson, 1990). However, transmission electron microscopic (TEM) observations of clastic sediments have shown that berthierine typically is admixed with diagenetic chlorite at very fine scales (e.g., Lee and Peacor, 1983;Ahn and Peacor, 1985;Amouric et al, 1988;Jahren and Aagaard, 1989;Jiang et al, 1990).…”

“…Ahn and Peacor (1985) reported apparent overlap in the compositions of intergrown berthierine and chlorite in Gulf Coast sediments; however, the composi-tions of coexisting phases were only determined indirectly. Based on literature surveys, Curtis et al (1985), Hughes (1989), and Jahren and Aagaard (1989) also made comparisons of the compositions of berthierine and chlorite, but not in the context of polymorphic relations. The principal difficulty in documenting such a relation lies in access to well-characterized, pure berthierine.…”