Learning more... Oxfordshire minerals Oxfordshire minerals Finding minerals in Oxfordshire This is an article about some of the There are few natural exposures of rock on minerals found in the county of public land. Minerals are usually found in Oxfordshire. working quarries and in temporary exposures made during road building and The sedimentary rocks of Oxfordshire contain construction work. It is essential to obtain a surprisingly large number of different permission from the quarry owner or minerals. Some make up the rocks manager, or the landowner, before themselves, for example, the ironstone that attempting to look for minerals on private underlies the Banbury area is composed partly land. of chamosite and goethite; it gets its green and orange-brown colours from these minerals, Some of Oxfordshire’s minerals can be and many buildings in and around Banbury seen in the Museum’s display British are constructed of this stone. Minerals: a heritage revealed. Other minerals are found in veins and nodules, and were deposited in existing rocks by water rich in dissolved chemicals. ‘Septarian nodules’ were formed by bacterial Banbury action at the bottom of the sea in Jurassic times. When they were buried by more sediments below the sea bed, they started to Chipping Norton dry out, developing shrinkage cracks. Water filtering down through the rock has deposited Bicester calcite, gypsum, baryte and celestine in the Kidlington cracks, sometimes as well-formed crystals. Witney Eynsham Oxford Septarian nodules are found in the Oxford Carterton Cumnor Clay, a Jurassic rock which stretches across Thame the county from west to east, underlying the Kingston Bagpuize Chinnor Abingdon Thames valley and the city of Oxford. Faringdon Drayton What is ‘Learning more’? Grove Wantage Didcot Wallingford ‘Learning more’ presents a series of articles about the Museum and its collections. It is Henley designed for older students, teachers, researchers, and anyone who wants to find Map of Oxfordshire out more about particular aspects of the Showing major towns. Many of the localities listed in this Museum’s work and its history. article, like Cowley, Cumnor Hill, Headington and Yarnton are found within, or close to, Oxford city itself. This article introduces some of the minerals found around the county of Oxfordshire. It was originally designed and Table of contents illustrated by a student from Oxford The minerals featured in this article are Community School during a week’s listed alphabetically. placement at the Museum, funded by the Introduction.......................................................1 British Gas Lattice Foundation. Finding minerals in Oxfordshire.......................1 ‘Learning more’ articles are free, and Aragonite-Goethite.............................................2 available to all for educational, non-profit purposes. Unless otherwise stated, the Gypsum-Quartz...................................................3 Museum retains copyright of all material Siderite-Vivianite...............................................4 used in this leaflet. Minerals in the Museum.....................................4 © Oxford University Museum of Natural History Page 1 Learning more... Oxfordshire minerals Minerals from around Celestine (strontium sulphate) Oxfordshire Celestine was first discovered in Oxfordshire rocks in 1979 during the Aragonite construction of the Cassington Sewage Shelly fossils are very common in Oxfordshire Works, and has subsequently been found rocks and are sometimes composed of elsewhere in the county. It occurs as aragonite. colourless to blue transparent crystalline masses in the cracks in septarian Baryte (barium sulphate) concretions, along with calcite, pyrite and Small, white, crested aggregates of baryte rare baryte. Crystals of intermediate crystals have been found in septarian compositions between baryte and celestine nodules (hard round concretions which are well documented. have shrinkage cracks lined with minerals) Celestine in the Oxford Clay at Yarnton. Similar and (strontium equally rare deposits of baryte have also sulphate) been found across the county border at Calvert, Buckinghamshire. Blue celestine with white and Calcite (calcium carbonate) brown calcite, from the ARC Calcite is a very common Oxfordshire mineral. quarry The limestones of Oxfordshire are composed between mainly of calcite. Some are of Jurassic age, but Cassington the beds of chalk, a very pure form of and Yarnton. limestone, are of Cretaceous age. Clay deposits in the county may contain substantial amounts of fine-grained calcite, and the shelly Chamosite (iron magnesium fossils which are abundant in most of the aluminosilicate) geological strata of Oxfordshire are often Chamosite is an iron-rich chlorite mineral composed of calcite. that is found in the Lower Jurassic Marlstone ironstones of the Banbury area Calcite (calcium to the north of the county. It occurs as carbonate) ooliths (tiny rounded grains resembling ‘Dog-tooth’ crystals fish eggs) cemented by siderite, and gives of calcite from the stone a distinctly green colour. Much of Bunkers Hill quarry, the ironstone has subsequently been Kirtlington. weathered to goethite. Well crystallised examples of calcite, most Goethite (iron oxide hydroxide) commonly the acute rhombohedral ‘dog-tooth Goethite occurs widely as brown coatings in spar’ form, are found lining mollusc shells and limestones and clays, but individual crystals infilling fractures and cavities in the are rarely seen. The pigment ‘yellow ochre’ is limestones. Calcite crystals also line shrinkage a mixture of goethite and clay minerals, and in cracks in septarian nodules - hard round the past it was obtained from Shotover, near concretions found in the Oxford Clay. Headington. Calcite occurs as stalactitic deposits in both Weathering of the Marlstone ironstones found natural cavities and human excavations such in the Banbury area converts chamosite and as mine adits in limestone. Examples come siderite to goethite. The resulting orange- from Stonesfield where the fissile limestone brown building stone has been used widely in has long been mined for roofing ‘slates’. this part of the county. © Oxford University Museum of Natural History Page 2 Learning more... Oxfordshire minerals Gypsum (hydrated calcium sulphate) Pyrite (iron sulphide) Gypsum is the most well-known of all the Pyrite is a common mineral in the Oxfordshire minerals, and fine crystals from Oxfordshire clays and in other rocks of the old quarries of Shotover, near Headington, the county. It can occur as well-formed are held in museum collections worldwide. cubic or pyritohedral crystals, but is often Gypsum is found almost anywhere in the found as thin microcrystalline coatings county where beds of clay are exposed. lining fossil shells. It may be tarnished, Some crystals are transparent and colourless giving an iridescent appearance. but most are clouded grey with inclusions of clay. Both single and twinned crystals are common, and rosette-shaped clusters of crystals are particularly attractive. The fibrous form of gypsum, ‘satin-spar’ is much rarer in Oxfordshire. Pyrite (iron sulphide) Gypsum (hydrated calcium sulphate) Crystals of pyrite in an ammonite, from the old Chawley A single crystal of gypsum (left) and twinned crystals (right), both brickworks, Cumnor Hill. from Shotover, near Headington. Quartz (silicon dioxide) Natrojarosite (hydrated sodium iron sulphate) Although one of the most common of minerals worldwide, quartz crystals are Natrojarosite occurs as yellow powdery rather rare in Oxfordshire, where they masses in the Oxford Clay, usually occasionally line small cavities in associated with crystals of gypsum. It is limestone. Quartz is found most derived from the weathering of pyrite. The extensively in the county in the form of first natrojarosite from Oxfordshire was sandstone and gravel, of which there are identified using X-ray diffraction analysis huge commercial deposits. on specimens collected by Museum staff in Cassington during the construction of the new sewage works in 1979. Large amounts were found in 1984 in a temporary pit in the Oxford Clay at the Army Ordnance Depot, Bicester. Natrojarosite (hydrated sodium iron sulphate) Natrojarosite from the Army Ordnance Depot at Quartz (silicon dioxide) Bicester. Crystals of quartz on calcite in limestone, collected in 1939. © Oxford University Museum of Natural History Page 3 Learning more... Oxfordshire minerals Siderite (iron carbonate) Minerals in the Museum Siderite occurs with chamosite in the green The rocks and minerals displays occupy oolitic Marlstone ironstone found in north the aisle south of the central aisle of the Oxfordshire. Both chamosite and siderite are main court. The displays on the north side commonly altered by weathering processes to of the aisle explain how minerals and rocks orange-brown goethite. form. They show how the rock cycle works, and explain the powerful forces of plate Sphalerite (zinc sulphide) tectonics. Sphalerite is very rare in Oxfordshire. Visitors are invited Flattened brown crystals were discovered to touch the large in calcareous nodules during the geological construction of the M40 motorway just specimens in the inside the Oxfordshire border south west of centre of the rocks Kings Sutton. and minerals aisle; they include a mass of golden pyrite from Peru, Sphalerite (zinc and a huge smoky sulphide) quartz crystal from Sphalerite crystals Russia. in a calcareous concretion discovered during On the south side of the rocks