Epidote in Cheviot igneous rocks
Epidote occurs as a secondary mineral in Cheviot rocks that have been affected by propylitic alteration and also in the andesites that lie within the Cheviot pluton metamorphic aureole that have been subjected to contact metamorphism at relatively low temperature and pressure. It can appear as a replacement mineral in plagioclase and ferro-magnesium minerals . Al-Hafdh also states that epidote-rich veins occur in areas of propylitic alteration.
Al-Hafdh made two electron microprobe analyses of epidote that he found in one sample of coarse-grained rock from the Threestone Burn area. His data revealed the relative proportions of iron (30%) and aluminium (70%) in the sample places its composition well with the analyses quoted in Deer, Howie and Zussman (1962) (vol.l, Table 33, Nos. 8 and 10).
Al-Hafdh also says that the epidote in Cheviot rocks is easy to identify due to its colourless to pale yellow pleochroism and first order birefringence colours; characteristics that accord with it’s low iron content that defines as a clinozoisite-type of epidote. Higher iron content is associated with a pale green adsorption colour and higher interference colours.
Identification
In hand specimens of altered andesite, as outcrops in the Carey Burn, small yellow crystals, in amongst the plagioclase phenocrysts, have been identified as epidote.
Iron rich epidote and low-iron clinozoisite form a solid solution with each end member exhibiting different optical properties.
In thin section, the generally small epidote grains appear with high relief, poor, parallel cleavage, a yellow-green adsorption colour and some pleochroism in green. Clinozoisite shares the has high relief and poor cleavage, but exhibits a weaker adsorption colour and is sometimes colourless. Its pleochroism is also weaker and may be absent.
With polarising filters crossed, epidote displays a range of interference colours that extend up to the third order. These can sometimes be very patchy. Clinozoisite’s interference colours range only in the first order and can be an anomalous blue or yellow-green.
Photomicrographs of Cheviot Epidote
Locality: Brough Law, NT994162
Note the high relief, glassy appearance, pseudohexagonal crystal form and the transparent yellow-green pleochroism when the polariser is rotated.
Locality: Brough Law, NT994162
The epidote crystals with the polariser rotated to reveal their pleochroism.
Locality: Brough Law, NT994162
Locality: Housey Crags, Northumberland.
The high relief and faint green adsorption colour of the epidote are visible in this patch that has formed within an altered plagioclase crystal.
Locality: Housey Crags, Northumberland.
The high order interference colours of the same patch of epidote contrast strongly with the first order mid-grey colour of the plagioclase.
Return to list of minerals in Cheviot rocks