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The calorimeter sees all: Testing methods to determine crystallinity of partially amorphous samples using Differential Scanning Calorimetry (DSC)

Presentation #220.12 in the session Laboratory Investigations (Poster + Lightning Talk)

Published onOct 23, 2023
The calorimeter sees all: Testing methods to determine crystallinity of partially amorphous samples using Differential Scanning Calorimetry (DSC)

Rocks produced by diverse planetary processes contain both crystals and amorphous material. For example, chondrules range from fully crystalline to almost completely glassy. The crystal population of volcanic rocks and pyroclasts exerts strong controls on lava rheology, which in turn feeds back to ascent rate and eruption style. Partial or complete melts may be produced by bolide impacts, in addition to igneous processes. Regolith on airless bodies, where micro-impacts are common, produces semi-crystalline agglutinates. In all cases, since glass is not an equilibrium phase, crystallinity provides information on the thermal history of the sample.

Crystal characteristics are typically explored using microscopy, using polarized light or backscattered electrons. However, many samples present visibly ambiguous textures such as intimate intergrowth of crystal phases, crystal sizes extending down to the nanometer spatial scale, and crystals that are indistinguishable from glass on the basis of average atomic number. Crystallinity often cannot be accurately determined in such samples via microscopic inspection alone. Here we apply calorimetric methods involving heat capacity and enthalpy to assess the crystallinity of a series of volcanic samples that have also been evaluated using microscopy.

We tested two different calorimetric approaches, using a Netzsch DSC404 Differential Scanning Calorimeter, and 30-40 mg aliquots of powdered basalt. The first approach involves determining the magnitude of the increase in heat capacity (CP) at the glass transition (Tg). Pure glass undergoes a significant CP increase at Tg, which corresponds to the configurational heat capacity of the liquid. A mixture of glass and crystal undergoes a proportionally smaller increase, with a purely crystalline sample exhibiting no change at Tg. Crystallinity can be estimated to ±10% or better using this method. The second approach is based on the enthalpy of fusion, although this is a fairly complex procedure and results are typically no better than for the heat capacity method. Preliminary results for lavas erupted from the 2018 Kilauea Lower East Rift Zone eruption indicate that crystallinity based on calorimetric data can be tens of percent higher than that identified using microscopy and petrographic analysis.

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