Clinoeulite (magnesian clinoferrosilite) in a eulysite of a metamorphosed iron formation in the vredefort structure,South Africa

A unique clinopyroxene (En₁₉Fs₇₈Wo₃), clinoeulite, space group P2₁/c, $${\text{(Fe}}_{{\text{1}}{\text{.48}}} {\text{Mg}}_{{\text{0}}{\text{.37}}} {\text{Mn}}_{{\text{0}}{\text{.08}}}^{{\text{2 + }}} {\text{Ca}}_{{\text{0}}{\text{.05}}} {\text{Al}}_{{\text{0}}{\text{.01}}} {\text{)}}_{{\text{1}}{\text{.99}}} {\text{ Si}}_{{\text{2}}{\text{.01}}} {\text{O6],}}$$ contains sharp exsolution lamellae of ferroaugite (En₁₇Fs₄₃Wo₄₀) from which the former presence of a ferropigeonite near En₁₇Fs₇₀Wo₁₃ can be calculated. This two-pyroxene intergrowth is the main component of a eulysite containing also magnetite, olivine (Fo₉Fa₈₆Te₅), quartz, oligoclase-K feldspar inter-growth, and retrograde cummingtonite with about 76 % grunerite end member. The occurrence of this most unusual rock type in the center of the Vredefort structure is attributed to a period of high-temperature metamorphism (at least 800 °–850 °C) which was followed by hot deformation of the rock during the Vredefort event thus probably preventing the common formation of orthopyroxene through pigeonite exsolution and inversion upon cooling. After this tectonic deformation, the rock recrystallized within the low-temperature stability range of clinoeulite to yield fine annealing textures. Late-stage equilibria at temperatures well below 500 °C include the complete unmixing of a former high-temperature anorthoclase, a Mg/Fe redistribution in the clinoeulite and olivine and, with the introduction of water, the partial formation of cummingtonite through reaction of clinoeulite, olivine, and quartz. During weathering the olivine was transformed to a nearly opaque, anhydrous ferrisilicate which, except for the change of Fe²⁺ to Fe³⁺ and the oxygen introduction, largely retained its original chemistry.