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.