This and the following five papers were presented at a joint meeting of the Metamorphic Studies and Clay Minerals Groups of the Mineralogical Society under the auspices of IGCP 294 entitled "Phyllosilicates as indicators of very low-grade metamorphism and diagenesis" held from 4-6 July 1990 at the University of Manchester. Nevertheless, the correlation of composition with temperature and similarities to the temperaturerelated evolution of synthetic chlorites, suggest that diagenetic chlorites are compositionally distinct from, but metastable with respect to, fully trioctahedral metamorphic chlorites. Temperature-related trends are modified by bulk composition, complicating their potential use for low-temperature geothermometry.

A B S T R A C T : Pore-lining chlorites are often responsible for the preservation of porosity in deeply buried sandstones because they inhibit the formation of quartz overgrowths, but little is understood about when and how they form. Chemical analyses and XRD data indicate that there are at least two common types: Fe-rich, and Mg-rich. The Fe-rich examples occur as individual euhedral crystals, are invariably interstratified with 7 ~ layers (probably berthierine) and form as the Ib g = 90 ~ potytype at low temperatures. With increasing temperature, their chemistry changes, 7 ~ layers are lost, crystal size increases, and eventually they transform to the high temperature llb f3 = 97 ~ polytype. The Mg-rich examples occur as boxwork arrangements of crystals, are not interstratified with 7 fi, layers and are exclusively the lib g = 97 ~ polytype.The Fe-rich examples occur most frequently in sandstones that were deposited at the transition between marine and non-marine environments and the presence of Fe-rich oolites in many samples suggests a link to the ironstone facies. They probably formed originally at surface or near-surface conditions as a 7 ~ mineral, such as berthierine or even odinite, in a fresh water/marine water mixing zone in tropical regions. The Mgrich varieties tend to be found in aeolian or sabkha sandstones in close association with evaporites. They are probably replacements of Mg-rich smectites via the intermediate mineral corrensite. Precursor Mg-rich smectites formed originally from evaporite brines at near-surface conditions; chlorite itself was not formed until temperatures were high enough to crystallize the lib 13 = 97 ~ polytype.

Abstract--Chlorite and corrensite are common clay minerals in lacustrine mudrocks from the Devonian Orcadian Basin, Scotland. The relationship of their occurrence to vitrinite reflectance data demonstrate that they are authigenic minerals, formed during burial diagenesis/metamorphism at temperatures of -> 120"C. Whole rock mineralogical and chemical analyses show that chlorite authigenesis occurred by reactions between the detrital dioctahedral clay mineral assemblage and dolomite that was formed under early evaporitic conditions in the lacustrine environment.XRD and electron microprobe analyses indicate that phases intermediate between corrensite and chlorite are probably mixed-layer chlorite/corrensite with a tendency towards segregation of layer types. Chemically, the conversion of corrensite to chlorite involves an increase in A1 for Si substitution in tetrahedral sites, but there is no change in the Fe/Mg ratio of octahedral cations. There is also no relationship of mixed-layer proportions to paleotemperature; only a general paleotemperature interval of approximately 120 ~ to 260~ in which a range of phases between corrensite and chlorite occurs. Chlorite polytypes are exclusively IIb, indicating the formation of this polytype at diagenetic temperatures.The occurrence of corrensite and Mg-rich chlorite in evaporite and carbonate successions is probably a reliable indicator ofdiagenetic alteration at temperatures of >-100~ Burial diagenetic reactions between dioctahedral clay minerals and Mg-rich carbonates may possibly explain many occurrences of corrensite and Mg-rich chlorite in such rocks.

The biogeochemical activities of free-living and symbiotic fungi must be acknowledged in attempts to understand uranium cycling and dispersal in the environment. Although the near-surface geochemistry of uranium is very complex and a wide variety of mineral phases is known, uranium trioxide (UO3) and triuranium octaoxide (U(3)O(8)) can be used as well characterized models in the study of biotransformations. We have used a complex methodological approach involving advanced solid state speciation and scanning electron microscopy to study the ability of saprotrophic, ericoid and ectomycorrhizal fungi to transform these model oxides. This study has revealed that fungi exhibit a high uranium oxide tolerance, and possess the ability to solubilize UO3 and U(3)O(8) and to accumulate uranium within the mycelium to over 80 mg (g dry weight)(-1) biomass. X-ray absorption spectroscopy of uranium speciation within the biomass showed that in most of the fungi the uranyl ion was coordinated to phosphate ligands, but in ectomycorrhizal fungi mixed phosphate/carboxylate coordination was observed. Abundant uranium precipitates associated with phosphorus were found in the mycelium and encrusted the hyphae. Some of the fungi caused the biomineralization of well-crystallized uranyl phosphate minerals of the meta-autunite group. This is the first experimental evidence for fungal transformations of uranium solids and the production of secondary mycogenic uranium minerals.