Oxfordshire minerals

Oxfordshire minerals

 

Oxfordshire’s mineral diversity is surprising. The Jurassic and Cretaceous rocks that underlie the county are formed from crystals for example, calcite is the major constituent mineral of our famous limestones and the Chalk. In the Banbury area rocks composed partly of chamosite and goethite give the characteristic orange brown to the signature building stones. Other minerals are found in veins and nodules, and were deposited in existing rocks by water rich in dissolved chemicals. Water filtering down through the rock has deposited calcite, gypsum, baryte and celestine in cracks, vugs and fissures.

 

The Oxford Geology Group lexicon of Oxfordshire Minerals:

 

Aragonite is the second most common polymorph of naturally occurring calcium carbonate, the most common being calcite. Aragonite owes its relative scarcity to the fact that as a metastable form of calcium carbonate it reverts to calcite as a result in changes in environment. Shelly fossils are very common in Oxfordshire’s suite of rocks and are sometimes composed of aragonite.

 

Small, white, crested aggregates of baryte crystals have been found in septarian nodules found in the Oxford Clay at Cassington and Yarnton, near Oxford.

 

The limestones of Oxfordshire are composed mainly of calcite and therefore it is a very common mineral in the county. Calcium carbonate rich rocks are our famous limestones and the Chalk, either of Jurassic or Cretaceous age. Clay deposits in the county may contain substantial amounts of fine-grained calcite, and assemblages of shelly fossils found throughout most of county’s strata are usually composed of calcite. Fine dog tooth spar can be found lining shells vugs, and shrinkage cracks in septarian nodules. Speleothem can be found in natural cavities and mines.

 

Celestine was first discovered in Oxfordshire rocks in 1979 during the construction of the Cassington Water Treatment Facility, and has subsequently been found elsewhere in the county. It occurs as colourless to blue transparent crystalline masses in the shrinkage cracks in concretions, in association with calcite, pyrite and baryte.

 

 

Chamosite is an iron-rich chlorite mineral (end member) that is found in the Lower Jurassic Marlstone ironstones of the Banbury area to the north of the county. It occurs as ooliths cemented by siderite, and gives the stone a distinctly green colour. Much of the iron oxide content of the Banbury ironstone has been weathered to goethite.

 

 

Goethite is an iron bearing oxide mineral found in soil and other low-temperature environments and occurs as a brown coating in limestones and clays, but individual crystals are rarely seen in Oxfordshire. The pigment 'yellow ochre' is a mixture of goethite and clay minerals, and in the past it was quarried at Shotover. Weathering of the Marlstone ironstones found in the Banbury area converts chamosite and siderite to goethite. The building stone, coloured by has been used widely in this part of the county.

 
 

 

Gypsum is the most ubiquitous of all the Oxfordshire minerals. Museum standard crystals from the former quarries of Shotover are held in public collections across the world. Gypsum is found almost anywhere in the county where beds of clay are exposed. Crystals that have been found in the county can be transparent and colourless, but most are grey due to clay inclusions. Satin spar, the fibrous form of gypsum is rare in the county.

 

 

This sulfate mineral, Natrojarosite, occurs as yellow powdery masses in the Oxford Clay, usually associated with crystals of gypsum. It is derived from the weathering of pyrite. Natrojarosite has been found in excavations in the Oxford Clay at Cassington & Bicester.

 

 

Pyrite is a common mineral in the Oxfordshire clays and in other rocks of the county. It can occur as well-formed cubic or pyritohedral crystals, but is often found as thin coatings lining fossil shells. It may be tarnished, giving an iridescent appearance.

 

 

Although one of the most common of minerals worldwide, quartz crystals are rather rare in Oxfordshire, where they occasionally line small cavities in limestone. Quartz is found most extensively in the county in the form of sandstone and gravel river deposits.

 

 

Siderite occurs with chamosite in the green oolitic Marlstone ironstone found in the Banbury area of north Oxfordshire. Both chamosite and siderite are commonly altered by weathering processes to orange-brown goethite.

 

 

Sphalerite is very rare in Oxfordshire. Spahlerite is the main ore of zinc, consisting largely of zinc sulfide in crystalline form but usually contains variable iron. Flattened brown crystals were discovered in calcareous nodules during the construction of the M40 motorway just inside the county border south west of Kings Sutton.

 

 

Vivianite occurs as oxidised deep blue nodules and powdery masses and concretions in clays throughout the county. It forms as a result of weathering of fossil bone (composed of calcium phosphate) and pyrite (iron sulphide), both of which are locally abundant in Oxfordshire clay deposits.