Formation and stability of viridine in microcline leptite from Ultevis District, Northern Sweden

Abstract

In the Ultevis District, Northern Sweden, viridine occurs in a potassium rich, microcline-bearing leptite together with quartz, plagioclase, phlogopite, opaques, and a retrogradely formed muscovite. The formation of viridine in this rock may be described by a dealkalization process of microcline, in which opaque minerals are involved. A diffusion of K⁺ and H₂O out of the centres of viridine formation and of H⁺ into these localities, which is a prerequisite of such type of reaction, may have been favoured by a contemporaneous migmatization of the rock, since the granite-like melts can be regarded as sinks for potassium and water. The following decomposition of viridine is initiated by the reverse reaction. Texturally this is indicated by small microcline seams along the contacts between viridine and quartz grains. The final alkalization of the viridine-quartz assemblage to form muscovite reflects increasing K⁺/H⁺-ratios and H₂O-contents of the vapor phase and/or decreasing temperatures. The potassium and water amounts necessary for the viridine decomposition reactions may have been generated by the cooling of the granitic liquids. An intergrowth of sillimanite and the muscovite can be described by a degenerated reaction. It can be interpreted as giving way to an equilibration of the alumosilicates und the actual metamorphic conditions, that is here P, T, $a_{{\text{H}}_{\text{2}} {\text{O}}}$, a _k ⁺, a _h ⁺ and a _Mn ³⁺.

The viridine investigated has the lowest Mn-contents recorded so far for andalusite-type minerals with viridine optics (1.3–2.2 mol-% Mn₂SiO₅-component). Since no andalusite with lower Mn-contents occurs in the rock and since no other Al₂SiO₅-phase formed at this metamorphic stage it may be assumed that the incorporation of manganese stabilizes the andalusite structure under the P-, T-, $a_{{\text{H}}_{\text{2}} {\text{O}}}$ ⁻, a _k ^+-, a _h ^+- and a _Mn ³⁺-conditions verified in this particular leptite.

The temperature of viridine formation is estimated to be about 600 ° C, the decomposition of viridine +quartz into the muscovite-sillimanite assemblage may have happened between 650–500 ° C at 6–2.5 kb.