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Is there any way to guess from these crystals what minerals formed them? They came from ordinary tap water.

I was cleaning out a room humidifier which hasn't been used since last year when I noticed lots of little crystals in the bottom of it. Most of them are square, which would suggest halite (NaCl) but some of them appear to have a pyramidal, or inverted pyramidal, shape. This picture was taken with a cell phone and the largest crystals in the picture are probably 1/8". Soaking them in warm water for an hour dissolved them. The color - a pale brownish-yellow in the picture is accurate.

enter image description here

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  • $\begingroup$ Sodium chloride is a possible answer, it forms crystals with such a shape in some conditions, I've seen it personally (the crystals are actually cubic, it's the way they grow that gives them this form). The color may come from some impurities. However, I have no idea how you can have so much solid residue in your tap water. $\endgroup$ – vapid Dec 14 '16 at 8:14
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    $\begingroup$ The key here is the source - a humidifier. This device is used to evaporate water, and is regularly refilled (and in the case of the one I own), and less regularly cleaned. This build up could be over many refills of the humidifier (and thus many liters of water). $\endgroup$ – Ben Norris Dec 14 '16 at 15:45
  • $\begingroup$ @Ben Norris - exactly - these were the result of last season's multiple refillings - so maybe 120-150 refillings without a cleaning. I'm unclear on the process oof the crystal formation since where they form is mostly underwater during the season, but because of the evaporation the water has a very high concentration of whatever this is so I assume it precipitates out - but why it doesn't dissolve again at the next refilling surprises me. $\endgroup$ – user316117 Dec 14 '16 at 17:09
  • $\begingroup$ You are right that multiple refill-evaporation cycles cause the building-up of the solid residue. But I would suspect a limescale-like residue, not something water soluble. I guess your tap water have an unusual mineral composition. $\endgroup$ – vapid Dec 15 '16 at 7:56

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