SUMMARY Quarry sections at Welton-le-Wold, Lincolnshire, reveal up to 13 m of glacial tills overlying some 8 m of silts, sands and flint gravels. The gravels are the source of a sparse fauna of straight-tusked elephant, deer and horse, and of four Acheulean handaxes. The sub-till deposits, here designated the Welton Gravels, display two divisions. The upper was deposited by periglacial aeolian and niveo-fluvial processes, whilst the lower contains more water-bedded sands. Derived Lower Cretaceous materials are common. The youngest of the overlying tills is of Devensian age, while the Galcethorpe Till and the Welton Till (newly designated) are probably Wolstonian and form part of the Older Drift of the Lincolnshire Wolds. The faunal and artefact assemblages of the Welton Gravels have a Hoxnian affinity, and therefore the gravels were emplaced as valley-floor sediments during early Wolstonian or late Hoxnian times. They confirm the pre-Wolstonian existence of a valley draining eastwards through the Welton-le-Wold area from a head located in the eastern Wolds and possibly on Lower Cretaceous rocks.
THIS note offers the first record of an occurrence of brucite in the Mendip Hills. The locality is in Merehead Quarry (grid ref. O.S. I" map ST 698443), which is situated on the south flank of the Beacon Hill Pericline. The rocks quarried are well-bedded limestones that are referred to the Lower Carboniferous Clifton Down Limestone. A notable feature of this locality is the occurrence of a varied assemblage of rare lead and copper minerals. These have recently been described by Alabaster (2975: see references there to the no. 2 vein, at which brucite has now been discovered). The rare minerals are found within the pods of manganese oxide that occur, together with iron oxides, as replacement deposits set in faults and Trias-filled tissures developed in the Carboniferous Limestone. Doulting Stone (mid Jurassic: Upper Inferior Oolite) rests unconformably on the Carboniferous Limestone at Merehead and can be seen to truncate the mineralized fissures. The fissures occasionally contain inserted sedimentary materials ranging in age from Rhaetic to Upper Inferior Oolite.The pods of manganese oxide contain numerous small cavities, the ore from no. 2 vein being especially cavernous. The secondary minerals occur either as simple cavity fillings or else are contained in calcite nodules, which have themselves grown in, and may in some instances have enlarged, the original cavities. The brucite has been found occupying a small central cavity in two of these calcite nodules.Brucite occurs in each of these two cases as a compact mass of transparent, prismaticacicular crystals, arranged in radiating groups that are intergrown with one another. The crystals are elongate ![ [oooi] and grow out from the cavity walls. In both specimens there is some degree of alteration to white hydromagnesite and pseudomorphs after brucite are seen. A thin (up to o'5 mm) discontinuous light-brown rim occurs at the brucite/calcite interface. This light-brown material is translucent, has a vitreous-resinous lustre, and is appreciably harder than the brucite, which it invariably pseudomorphs. XRD analysis shows it to consist of a mixture of brucite and crystalline hydromagnesite. Both the brucite and the white hydromagnesite are seen to grade into this material.In one specimen (now in two parts deposited in the Geology Museum, University of Bristol, numbered B3624 and B3625: see the former in fig. I) brucite is associated with massive orange blixite, which shows partial alteration to hydrocerussite. Contacts between brucite and blixite are sharp. The gently undulating contact is uninfluenced by both cleavages and crystallographic orientation of the brucite crystal groups. There is no evidence to suggest that brucite is intergrown with the blixite/hydrocerussite. It seems to be the case that brucite formed before blixite in this nodule.The two brucites are of different colours, brownish green in the nodule that has blixite and pale green in the other. Refractive indices (determined using white light) too are different in
SummaryAnalcime, occurring both as small, pale-brown trapezohedral crystals and pale-brown slaggy masses, is reported growing into and replacing aragonitic shell debris in a heavily faulted, 15-km long, coastal exposure of L. Jurassic marine sediments. Associated minerals include authigenic baryte, sphalerite, pyrite (some framboidal), celestine, aragonite, calcite, dolomite, quartz, and solid hydrocarbon. No other zeolites have been found. Its modes of occurrence, crystal morphology, and relationships to these minerals are described. The analcime appears to be restricted in occurrence to the W. Somerset coast. Partial chemical analysis of one sample is given. The brown colouration is suggested to be due to organic compounds. XRD analysis of five samples reveals progressive decrease in silica content towards western extremity of the exposure. No evidence of volcanic ash contamination of the sediments is found. A hydrothermal origin is discounted; formation of the analcime by purely diagenetic processes is suggested.
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