Andesite

Name: Andesite
Diagram No.: 1188
Type of diagram: TTT
Chemical composition in weight %: See the table
Group: Slags
Note: Time-Temperature-Transformation (TTT) diagram built from the kinetic variation of viscosity. Open circles correspond to pure melt phase (at t1), open and black diamonds correspond to the presence of crystals in some amount but not in equilibrium (oxide +- plagioclase at t2 and oxide + plagioclase + pyroxene at t3, respectively) and gray diamonds correspond to the presence of crystals in constant amount (at t3).
Using the rheological response of the material to undercooling over time, a TTT (Time-Temperature-Transformation) diagram, commonly used to describe the behavior of nucleation and crystallization, can be constructed (Figure 1). Each temperature and time interval is assigned to a petrographic description: ‘‘melt’’ defines the area where only liquid phase exists; ‘‘melt+crystals’’ defines the area where crystals are present in some amount but not in equilibrium, this area is delineated somewhere between t1 and t2 and until thermodynamic equilibrium is achieved, at t3, where the ‘‘melt+crystals’’ area starts, representing the area where crystallization content is fixed (thermodynamic equilibrium between crystal and melt) and viscosity becomes constant. We noted that for the lowest temperature experiments (1152 and 1144 C), although viscosity was observed to be constant through time, evidence exists (heterogeneous pyroxene distribution) that thermodynamic equilibrium may not have been achieved. We observe that the time needed for crystals to form decreases with increasing undercooling temperature, in agreement with classical nucleation theory and previously reported experimentally [e.g., Tsuchiyama, 1983; Kouchi et al., 1986] and that it may increase at lower temperature. This TTT diagram represents the crystallization nose for an andesite under isothermal stirring conditions and at ambient pressure. The diagram also suggests that the liquidus temperature must be above 1180 C. Estimating the temperature or the time required to form crystals and thus change of the rheology of the lava now becomes possible.
Reference: Not shown in this demo version.

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