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Transition metals in silicate melts play an important role in the early stages of magma crystallisation, mineral fractionation and emplacement of ore bodies because of their influence on the rheologic behaviour of such melts and supercooled liquids. Here, particular interest is focused on the variation in glass transition temperatures (T_g), i. e. the plastic/elastic boundary, as a function of transition element enrichment. This is the critical control on rheological properties. For this study, dilatometric and calorimetric measurements have been carried out on Na-disilicate and anorthite-diopside (1 atm eutectic composition) glasses, containing up to 15 mol% NiO and up to 35 mol% CoO. Na-disilicate was chosen due to its rather simple glass structure, whereas anorthite-diopside represents a more realistic model for basaltic melts. The dataset, when complete, will be evaluated to yield molar expansivities (dV/dT) of the supercooled liquids up to 800°C, and thus help establish an equation of state for such liquids.

Preliminary results reveal that the glass transition temperatures of the Na-disilicate glasses (Fig. 3.6-7a) remain constant, within error, with increasing NiO content. For CoO there seems to be a minimum in T_g near 5 mol% and then the glass transition temperatures increase again with CoO content. At the highest CoO content studied (35 mol%), T_g is 12°C higher than for Na-disilicate. The anorthite-diopside glass transition temperatures (Fig. 3.6-7b) decrease by ca. 50°C with increasing CoO concentrations. The addition of 5 mol% NiO slightly increases T_g of a anorthite-diopside melt, whereas adding 15 mol% lowers T_g by ca. 15°C (Fig. 3.6-7). These trends may be explained in terms of the structural effects of the added transition metal cations: in the anorthite-diopside glasses, Co²⁺ acts as a network-modifier and increasing CoO contents allow an anorthite-diopside melt to supercool and exhibit a liquid-like behaviour at considerably lower temperatures than the undoped material. The maximum in T_g for NiO added to anorthite-diopside glasses could indicate a change in coordination from network-forming to network-modifying behaviour. An additional complication, however, is the existence of Ni²⁺ in five-fold coordination (cf. Galoisy and Calas 1993, Geochim. Cosmochim. Acta, 57: 3627-3633). Adding Co²⁺ to Na-disilicate melts seems to suggest a network-modifying role at low concentrations but at higher concentrations a network-forming character, indicating a coordination change of the Co²⁺ cation. Apperently no effect within error of NiO content on T_g is observed in Na-disilicate melts, which might be due a range in coordination numbers of Ni²⁺ compensating their effects on T_g.
 

Fig. 3.6-7: Variation of glass transition temperatures Tg of Na-disilicate (a) and anorthite-diopside (b) samples with various mol% of added transition element oxides (see text for details). Errors are within ± 3°C. Tg data of anorthite-diopside glass are from Knoche et al. (Geochim. Cosmochim. Acta, 56: 689-699, 1992).

Volume expansions of the samples were measured in a quartz push-rod dilatometer during heating of the samples following an initial heating and controlled cooling. Thermal expansivities (dV/dT) of the glasses can be regarded as true values up to the glass transition temperature, which in this study is defined as the peak temperature of the volume expansivity. Due to the viscous deformation of the supercooled liquids above the glass transition, the expansion of the glasses cannot be measured directly in the dilatometric run.

The equivalence of volume and enthalpy relaxation permits the calculation of molar expansivities of the supercooled liquids from specific heat capacity (c_p) data of the samples. To obtain the heat capacities, the samples were reheated after first cooling in a scanning calorimeter at cooling rates similar to those previously applied in the dilatometric run. By normalizing both the c_p and dV/dT data to the peak values we can then calculate the molar expansivities of the supercooled liquids.