The effect of lithium on the viscosity of pegmatite forming liquids

•The influence of Li2O on the viscosity of pegmatite forming liquids is determined.•The effect of Li2O is comparable to that of K2O.•The effects of alkalis are smaller compared to OH− groups at low water content.•The effects of alkalis become equal to OH− groups with increasing water content.•The low viscosity of pegmatite forming melts is related to H2O, F and alkalis.

Viscosity experiments were conducted to investigate the effect of Li2O on the viscosity of pegmatite forming melts and the possible difference to K2O. The individual effects of Li2O and K2O on melt viscosity were investigated by adding the same molar proportion of both components to a starting composition representing natural pegmatite forming melts. The viscosity experiments on hydrous melts (~ 1 to ~ 6 wt.% H2O) were carried out in the low temperature range at ambient pressure using the micropenetration technique and in the high temperature range at 200–300 MPa pressure using the falling sphere method.The addition of Li2O and K2O results in a decrease of melt viscosity and, on a molar basis, the effect of Li2O is comparable to that of K2O. Viscosity-related properties such as activation energy of viscous flow, melt fragility and glass transition temperature are affected in a similar way by both alkali oxides. The observed effects of alkalis on the viscosity of melt are smaller when compared to an equal molar amount of OH− groups at low water content but become equal with increasing water content. Finally, it is shown that the low viscosity of pegmatite forming melts can only be explained by the combined effects of H2O, F and of non-charge balancing alkalis.