Sugar Crystals

Properties keywords:  solid, crystalline, granular

Analogue keywords: phenocrysts in silicate melt, crystals, granular solid, basalt lava

Common names: glucose crystals, sucrose, table sugar, granulated sugar

General Information:  Sugar crystals are readily available, cheap and non-toxic.  They are ubiquitous in food preparation as a sweetener.

Properties

Sugar crystals are a granular material that come in irregular shapes and sizes, however they may be sieved in order to achieve a uniform grain size.  Grains are roughly cubic in shape and densities are approximately 1500 kg m-3.  Sugar crystals do not technically have a melting point but they decompose at temperatures between 160 and 180°C.

Sugar crystals are not often used as an analogue material as they are not particularly reproducible in size or shape. Sugar can be used as a suspended solid phase in certain sugar syrups as it will not dissolve in an already saturated solution. Mixtures of granulated sugar and flour show variable cohesion depending on the ratio of the materials (Mastin and Pollard, 1988).

Fudge is formed by rapid nucleation of crystals and is therefore a viscous fluid composed of a large number of small sugar crystals. Its rheology is dependent on crystal content, temperature, water content of the liquid phase and strain-rate, exactly the same as basaltic lava flows (Rust et al., 2008). It also has a yield strength, forms a solid skin on cooling and will break instead of flow at high strain-rates as a result of sugar crystal interactions.

Applications

Sugar crystals have been used in analogue experiments by Castruccio et al. (2010) to simulate suspended phenocrysts in a silicate melt.  They used sugar syrup as the analogue basaltic melt and, by varying the amount and size of sugar crystals, found that lava flow dynamics depend on crystal content and size distributions.  Sugar crystals were considered appropriate as they did not dissolve on the timescale of the experiments.

Buisson and Merle (2002) used sugar crystals, not as a volcanological analogue, but as marker particles of internal strain on experiments showing the velocity gradients of spreading in silicone ‘lava dome’ formation.

Due to their granular nature they have also been used dry, to simulate a volcanic cone resting on a ductile golden syrup layer, in a study of volcano spreading and sagging by van Wyk de Vries and Matela (1998).  Mastin and Pollard (1998) combined them with flour to replicate a weak granular solid in order to model dyke emplacement.  However, the only scaling consideration was that the mixture produced similar fracture patterns to that observed in nature.

Fudge has been shown to be a good analogue for basaltic lava flows, as it replicates the grain size and crystal content dependence of pahoehoe and ‘a’a flows and shows similar flow behaviour.  It has been used to show the importance of crystallization and strain-rate on the style of lava flow produced (Rust et al., 2008).

Limitations and tips for use

If using sugar crystals as a suspended solid phase in an analogue melt, ensure that the crystals and the liquid are of similar densities to avoid particle settling or rise.  Particle segregation may occur when liquid viscosity is low and crystals are large (Castruccio et al., 2010).

When using fudge as a basaltic lava analogue it should be noted that, whilst it does form a skin on cooling, it forms much slower than a lava flow crust due to a much smaller temperature change (Rust et al., 2008).

References

Buisson C and Merle O (2002) Experiments on internal strain in lava dome cross sections. Bulletin of Volcanology 64: 363-371

Castruccio A, Rust CA, and Sparks RJS (2010) Rheology and flow of crystal-bearing  lavas: Insights from analogue gravity currents. Earth and Planetary Science Letters 297: 471-480

Mastin LG, Pollard DD (1988) Surface deformation and shallow dike intrusion processes at Inyo craters, Long Valley, California. Journal of Geophysical Research 93 (B11): 13221–13235

Rust AC, Cashman K, and Wright HM (2008) Fudge factors in lessons on crystallization, rheology and morphology of basalt lava flows. Journal of Geoscience Education 56: 1, 73-80

van Wyk de Vries B and Matelar R (1998) Styles of volcano-induced deformation: numerical models of substratum flexure, spreading and extrusion. Journal of Volcanology and Geothermal Research 81: 1–18