Name that mineral

[DaveC426913]

I finally got around to cleaning out my wife's desktop pebble water fountain. It has been abandoned for a long time and the water has evaporated. I thought the reservoir was filled wih little bits of straw, but upon closer examination, see that they are all little crystals. The water obviously spend a lot of time trickling over the pebbles, and sitting in its reservoir of slate, so I imagine it's dissolved some minerals from there.

They're ~5mm long and .5 wide and of nearly uniform cross section end-to-end - few imperfections. I can't quite tell the angle of the faces, which is an important clue, right? They are opaque creamy white / very pale yellow. They don't look shiny, they look as if they'd feel like straw to the touch.

What mineral do you suppose these crystals are?

[chem_tr]

Hello,

As the first option, we may consider this material as a "crushed" pebble. So pebble is a calcium carbonate mineral (if I am not wrong); and water can remove some from it, revealing shiny crystals behind.

To sum up, I don't think it is different than the material used for manufacturing; it is plain pebble in my opinion, just etched by water a bit.

[DaveC426913]

Unless I misunderstand, it seems that you're suggesting these aren't crystals that grew in place, that they're left over from a dissolved pebble?

No, these are quite distinct crystals that have grown, physically separated from any pebbles, in the bottom of the water reservoir. The pebbles are not in the reservoir, they are on top, only water is (or rather, was) in the reservoir.

Whatever mineral they are has been grown from impurities that were in the water. Clearly, this mineral is most likely dissolved off the pebbles that the water had been running over all this time, before it evaporated. However, alternately, it could have been something in our tapwater.

[chem_tr]

Well, as a second option, we may think of this crystals' origin as the tap water just like you suggested. Tap water contains many minerals, and if their amount is sufficient, evaporation may cause some of the less dissolving materials behind as imperfect, somewhat "dirty" crystalline solids. They are probably sulfates, maybe calcium sulfate, which has less solubility in tap water than nitrates, chlorides, and so on.

Regards,
chem_tr