Essexite (olivine monzogabbro)

Essexite, also known as nepheline monzogabbro is a holocrystalline igneous rock exhibiting a coarse grained texture with crystals easily being visible with the naked eye. The rock has a dark colouration, varying between black and grey and forms as intrusive plutonic bodies with a mafic chemistry. As such, this rock is often composed of nepheline and plagioclase feldspar which are visible as pale or white crystals alongside mafic minerals such as pyroxenes and minor amounts of olivine and magnetite. However, a striking and often eye catching feature of this rock is the presence of large, augite phenocrysts which exhibit well formed euhedral crystal forms. The rock is believed to have formed at some point between the Permian and Carboniferous period boundary, known as the Permo-Carboniferous age and intruded into the surrounded country rock of Ordovician age as a dyke. The rock was named after its type locality in Essex County, Massachusetts, USA, however, these particular specimens came from Lanarkshire in Scotland, UK and was a once sought after stone known locally as Crawfordjohns which was used to produce high quality curling stones. The samples are excellent for various level collectors due to the coarse textured crystals which are easily visible with the naked eye, especially the augite. They are also suitable for those interested in local history or curling as well as for educational use to help assist higher education students to identify phenocrysts in hand samples whilst working in the field. Specimens are available in three sizes and come supplied in a card tray with an information label.