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I am putting 100 g of boric acid, 400 g of water and trying to crystallize at low temperature and letting cool slowly, however it recrystallizes as needle like texture. I need soft large crystals like mica. Can someone help out?

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The ratio seems about right, though the solution could be a bit more saturated as the solubility of boric acid in $\pu{100 mL}$ of boiling water is $\pu{27.5 g}$, so for $\pu{400 mL}$ of water you can theoretically use $\pu{110 g}$ of boric acid.

From my experience it is crucial that initial nucleation occurs at the surface, then one gets flake-alike crystals. That's said, you need to start to cool down the solution very slowly, preferably using a warm water bath and a closed vessel with large cross-sectional area (wide beaker with a watch glass on top should do). Don't rush the cooling process, especially at the beginning – crystallization should begin on surface, not in the volume, otherwise you get a bunch of crystals of different morphologies.

P. S. When drying the crystals at the end after rinsing them with cold water on a filter make sure not to exceed $\pu{130 °C}$, otherwise boric acid will start to decompose and form metaboric acid $\ce{HBO2}$, then subsequently upon further heating, $\ce{H2B4O7}$, and, finally, $\ce{B2O3}$ [1, pp. 119–120].

Reference

  1. Patnaik, P. Handbook of inorganic chemicals; McGraw-Hill handbooks; New York, 2003, ISBN 978-0-07-049439-8.
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