Summary: Epidote and clinozoisite are widely distributed in the andesitic lavas and tuffs of the Borrowdale Volcanic Series. Four main types of occurrence may be distinguished, namely quartz-epidote vein fillings; ‘Shap type’ veins, bordered by pink reaction zones rich in SiO₂, Na₂O, and K₂O; autometasomatized lavas and tuffs; and tuffs subjected to alkali metasomatism. Epidote alone formed in the quartz-epidote veins; clinozoisite is found with epidote in the other types of occurrence.

Epidote and clinozoisite formed in vesicles at pressures of 10–280 kg/cm², at temperatures of 300–550° C; epidote associated with pre-Bala mineral veins formed in the range 130–550° C at 1000 kg/cm²; epidote around the Caledonian granites probably formed up to 600° C at about 2000 kg/cm². Clinozoisite crystallized together with epidote in the vesicles, demonstrating a gap in the clinozoisite-epidote solid solution series. Early formed epidote in vesicles is more strongly coloured than late epidote, although the compositions of the earliest and latest fractions are identical.

Analyses and optical determinations gave a range of epidote compositions from 20·8 to 29·6% pistacite. There is no evidence for any effect of rock composition, or of pressure and temperature of formation on the composition of the epidotes studied. Varying oxygen partial pressure controls the activity of ferric iron, and thus has an effect on composition. Epidotes of small grain size tend to show high FeO contents.

The principal epidote-forming reactions are those involving the conversion of anorthite and orthopyroxene to epidote plus chlorite, the devitrification of volcanic glass, and the breakdown of anorthite during alkali metasomatism. Reactions involving ortho- and clino-amphiboles, clinopyroxene, and garnet were less important. Reaction zones rich in Na₂O plus K₂O probably form at 260–330° C, whilst silicification is the dominant process outside this temperature range.