An occurrence of brucite at Merehead Quarry, Cranmore, Somerset

Abstract

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