The Chlorite Series of Flagstaff Hill Area, California: A Preliminary Investigation

Abstract

Abstract--The results of X-ray diffraction, DTA, i.r. spectroscopy, and chemical tests are presented for some chlorites typical of the Flagstaff Hill area. The area is notable for the large variety of chlorite types occurring in considerable quantities. Chlorite, in this area, is found as veins, as pseudomorphs, and as individual crystals. Textures vary from massive, fine-grained aggregates to books which are more than 20 mm in width. Crystals more than 5 mm in size occur in parallel groupings at rock interfaces. Judging from 45 chlorite samples studied, sheridanite is most abundant; clinochlore and ripidolite are common. Penninite and its Cr-chlorite equivalent are less abundant. The parent rock is an irregularly shaped ultramafic body surrounded by low-grade schists and located very close to a granodiorite stock. The original ultramafic rocks have been highly altered by metamorphism and metosomatism into assemblages comprised mainly of serpentine, talc, hornblende, and chlorite with relics of olivine, pyroxene, and other less abundant original minerals. Much of the exposed rock is essentially monomineralic, mostly consisting of various polytypes of serpentine. Preliminary investigation indicates that the area merits much more study because of the opportunity for readily observing the various chlorite types and determining their genesis, alteration sequences and weathering characteristics. The area, being easily accessible, could also serve as an adequate source for samples to be used as reference standards.

Oxidation and Reduction of Structural Iron in Chlorite at 480°C

“…Using the criteria of Bailey (1975Bailey ( , 1980 and the monoclinic indices of Shirozu (1958), the chlorite is a monoclinic polytype IIb, in agreement with the results of Post and Plummer (1972). No changes in the diffractograms were found after heating the chlorite at 300~ in air or at 480~ in N2.…”

“…Chlorites from this area have been described by Post and Plummer (1972). The sample was hand ground and passed through a 0.25-mm sieve.…”

Thermal decomposition of Ferrian chamosite: an infrared emission spectroscopic study

“…Tables 2 and 3 show comparisons with ripidolite reported in the literature and with minerals containing building blocks comparable to the chlorite group minerals in order to assist with the assignment of the observed bands for ripidolite. There is a very good agreement with the spectrum for ripidolite from the same locality reported by Post and Plummer (1972). The infrared emission spectra at lower temperatures show some bands around 2900 cm A1 which are probably caused by impurities and therefore do not belong to the ripidolite spectrum.…”

“…This characteristic behaviour of the third band forms another argument for the correctness of the new assignment of these three hydroxyl-stretching modes. The disappearance of these bands around 650°C agrees well with the (Gadsden 1975) Assignment (Loh 1973;Farmer 1974;Borggaard et al 1982 strong endothermic dehydroxylation peak at approximately 610±635°C in the dierential thermal analysis (DTA) data (Post and Plummer 1972;Borggaard et al 1982) for samples from the same locality. In contrast to the bands associated with the hydrogen bonding between the tetrahedral sheets and the interlayer octahedral sheet, the hydrogen bonded inner OH band shows a much more moderate loss in intensity over the whole temperature range studied without completely disappearing.…”

“…Chlorites from this area have been described by Post and Plummer (1972). Table 1 gives the chemical composition reported by the CMS Source Clay Repository.…”