Structural Variations in Chlorite and Illite in a Diagenetic Sequence from the Imperial Valley, California

Abstract

Abstract--Samples of cuttings from the Borehard A-2 well, Imperial Valley, California, were collected over a measured borehole temperature interval 135* to 275*(2. The < 0.5-/~m (e.s.d.) fraction was separated using high-gradient magnetic separation (HGMS) to create a nonmagnetic fraction rich in illite and a magnetic fraction rich in chlorite. Chlorite was less easily separated from illite in lower temperature samples (< 200"C), presumably due to the presence of polymineralic grains of chlorite and illite. Grains in higher temperature samples were more nearly monomineralic and more easily separated.The chlorite is the IIb polytype. The thickness of coherent scattering domains of chlorite increased until 220*(2 and then remained constant. The amount of 7-/~ interstratified material increased downhole until 195"C and then decreased. Over the same temperature interval, the illite polytypes varied systematically from 1Ma (135" to 175"C) to 1M + 2M~ (230* to 275"C) and coherent scattering domains in the mineral became thicker to about 200"C and then remained constant in thickness. The percentage of iUite in mixed-layer illite/smectite (US) increased from 40% at 135"C to 100% at temperature > 205*(2; ordering in the I/S changes from R0 to R1 between 135" and 155"C, and from R1 to R -> 3 at temperatures >155"C.The concurrent structural changes in chlorite and illite indicate a general improvement in the overall structural order of the clay minerals with increasing temperature. Differences between chlorite and illite suggest that the minerals may have reacted differently to changing conditions or that they may have formed by different mechanisms. The exclusive occurrence of IIb chlorite at temperatures as low as 135"C extends the limit of IIb chlorite stability to temperatures lower than previous estimates.

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

“…Such a sequence has been only rarely described in pelitic rocks (Frey 1978;Weaver et al 1984;Robinson and Bevins 1986), for which chlorite commonly has been inferred to form as a byproduct of the smectite-to-illite reaction or from alteration of kaolinite or berthierine (Hower et al 1976;Boles and Franks 1979;Ahn and Peacor 1985;Walker and Thompson 1990). Dioctahedral phyllosilicates such as illite or muscovite are usually present in amounts much greater than those of trioctahedral minerals in shales, commonly reflecting the presence of either dioctahedral smectite as an original detrital phase as in Gulf Coast sediments, or volcanic ash that was subsequently altered to form dioctahedral smectite.…”

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

“…Berthierine has been inferred to occur as a metastable prograde precursor to chlorite in rocks undergoing diagenesis (Velde et al, 1974;Iijima and Matsumoto, 1982;Jahren and Aagaard, 1989;Walker and Thompson, 1990). A small increase in diagenetic grade (or burial depth) results in replacement of berthierine by chlorite.…”

“…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).…”

“…Berthierine is generally considered a metastable precursor of chlorite in diagenetic environments (e.g., Kisch, 1983;Ahn and Peacor, 1985;Weaver, 1989;Walker and Thompson, 1990). On the other hand, berthierine has also been shown to be a retrograde alteration product of chloritoid or its retrograde successors (mainly chlorite) in metapelites of polymetamorphic origin (Banfield et al,, 1989).…”

“…On the other hand, berthierine has also been shown to be a retrograde alteration product of chloritoid or its retrograde successors (mainly chlorite) in metapelites of polymetamorphic origin (Banfield et al,, 1989). The onset of the transition from berthierine to chlorite during prograde metamorphism is believed to occur at temperatures of 70 ~ 200~ (Velde et al, 1974;Iijima and Matsumoto, 1982;Jahren and Aagaard, 1989;Walker and Thompson, 1990). Relations between berthierine and chlorite in a given occurrence therefore may have important implications for the thermal history of an area.…”