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I'm attempting to make sea salt pyramids like the one below.

Pyramid SAlt

I understand this needs to be evaporated at a reasonable speed to stop the crystals forming a cube, as the when formed quickly, the edges grow faster than the faces, creating the inverted pyramid shape.

When evaporating the saturated water at around 75 degrees celsius, I start with what look like great pyramids forming (like the below image). However, the crystals then begin to join together forming a large flat 'crust', with the individual small pyramids losing their unique shape.

I've attempted reducing the temperature of evaporation to a point where this mass of crust does not form, however at that point, the crystals which now form are cubic instead of pyramids.

Any help would be great!

Salt forming

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    $\begingroup$ When you say you get an unwanted crust, is this in the solid surface of the container you're heating the salt solution in, or is it on the surface of the liquid exposed to the atmosphere? If it is the latter, a completely 100% wild guess is that you may get something different by evaporating the water using a thin layer of, say, olive oil on top, to reduce excessively fast evaporation of the upper layer of the brine. $\endgroup$ – Nicolau Saker Neto Jan 6 '18 at 8:59
  • $\begingroup$ Hi Nicolau, thanks for your response. The salt is forming on the liquid surface which is exposed to the atmosphere. I've considered adding something like olive oil as an experiment. However, as the idea is to produce a food grade product, I'm not wanting to add anything with may leave residue on the salt. As shown in the video, youtube.com/watch?v=gCXTLRxqVmo to achieve the result I'm after, the salt should form and then drift to the bottom. However, once the individual crystals fuse into one layer, this no longer happens. $\endgroup$ – user57230 Jan 6 '18 at 21:03
  • $\begingroup$ Nice video, though unsurprisingly lacking details about the process. Adding olive oil certainly wouldn't affect the food grade status of the salt. Furthermore, it may be that a single drop of oil already has an effect, because over water oil will spread as much as it can, going all the way to a layer a single molecule thick. You may think such a thin layer would be inconsequential, but check this video out. Absolutely nothing useful may happen, but I figure it's simple enough to try in a small scale. $\endgroup$ – Nicolau Saker Neto Jan 6 '18 at 21:36
  • $\begingroup$ Wow, interesting video, I've read about this with sailing before but didn't understand how it would make a difference. Will give this a go and let you know the result. $\endgroup$ – user57230 Jan 6 '18 at 22:26
  • $\begingroup$ I gave this a go with the olive oil at first a couple of drops, then a bit more. At around 75 degrees celsius, it did seem to calm the movement of the water a bit and had the effect of isolating some crystals as the oil created a barrier. However, once the crystals met they would fuse and crust over like previously, though maybe a little slower. Appreciate the idea. $\endgroup$ – user57230 Jan 7 '18 at 8:05
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TL;DR: Use 10-20% urea solution for octahedral crystals and a water-ethanol mix for pyramidal crystals.


There is no universal method to predict and to control crystal habits. Very often the experimental results are explained with molecular dynamics; there are models (Wulff condition, Donnay–Harker correlation, Hartman–Perdok theory, Hartman–Bennema attachment energy) that can only be used to represent vapor-grown crystals. Crystal morphology is vastly dependent on many factors:

  • phase it's growing from (gas, liquid, melt);
  • growing rate (temperature);
  • concentration of the mother solution;
  • ionic strength of the solution;
  • presence of impurities (habit modifiers);
  • solvent etc.

Rock salt can take one of the three morphologies dictated by its FCC lattice with a strong tendency to prefer cubic crystals:

Morphologies of NaCl crystal denoted with Miller planes

Figure 1. Morphologies of $\ce{NaCl}$ crystal denoted with Miller planes: cube $\{100\}$; octahedron $\{111\}$; rhomb-dodecahedron $\{110\}$.

There are numerous literature sources providing experimental information on crystal growth. For sodium chloride there is a recent review by Aquilano et al. [1], not only providing a brief description of common habit modifiers, but also giving an insight on to why one crystal direction is preferred:

The morphology of natural halite is largely dominated by the cube $\{100\}$; the $\{110\}$ and $\{111\}$ forms are seldom present. When crystals grow by evaporation from aqueous solution the perfection of the cube faces is usually lost; owing to the high values of the supersaturation, hopper shaped $\{100\}$ faces appear along with dendritic branches developing in the $❬111❭$ directions. This is the main reason of the halite tendency to cake. Hence, the best way to face this problem is to search for additives able to favor the appearance of the $\{111\}$ form, thus avoiding the morphological instability such as the “hazardous” dendritic growth.

Here is a brief summary for the non-standard crystal shapes of rock salt and corresponding modifiers:

Octahedra. Formamide; urea; sodium tetraphosphate $\ce{Na6P4O13}$; cadmium chloride $\ce{CdCl2}$; polymer additive containing an amide functional group.

Dendrites and needles. Hexacyanoferrate(II) $\ce{Fe(CN)6^4-}$; polyvinyl alcohol.

Pyramides. Water-ethanol solution; growth from a supersaturated solution [2].

Presence of heavy metals ($\ce{Pb^2+}$, $\ce{Cd^2+}$) generally promotes formation of large single crystals instead of many small crystals. For more practical details, photos of crystals and manuals you can refer to Crystal Growing Wiki (originally in Russian, but now most articles are translated to English).

References

  1. Aquilano, D.; Otálora, F.; Pastero, L.; García-Ruiz, J. M. Three Study Cases of Growth Morphology in Minerals: Halite, Calcite and Gypsum. Progress in Crystal Growth and Characterization of Materials 2016, 62 (2), 227–251. https://doi.org/10.1016/j.pcrysgrow.2016.04.012.
  2. Desarnaud, J.; Derluyn, H.; Carmeliet, J.; Bonn, D.; Shahidzadeh, N. Hopper Growth of Salt Crystals. J Phys Chem Lett 2018, 9 (11), 2961–2966. https://doi.org/10.1021/acs.jpclett.8b01082.
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