Soapsand--what is it, really?
Sariel's comments about posts are added in green.

Unfortunately, I lost most of the posts on this.  There were a lot of speculations, most of which I found silly, because the impression of soapsand I always got from the books agrees with what the DLG has to say: Soapsand is a naturally foaming sand found on Pern.
The one post I do still have does have a plausible suggestion:

Mike Mullen, 9/17/98

Would suggest borate, useful as a natural mineral detergent, and texture of sand.

Borate is actually a group of minerals which all contain the borate radical.  Borax is a specific borate mineral, which is commonly used as a soap on Earth, and does have the gritty texture of sand.  Borax forms as an evaporite, i.e. where a large body of water has evaporated, you might find a deposit of borax.  Outside of borax, I know of no minerals that naturally foam.   If borax is not equivalent to soapsand, I would suggest that soapsand might form because some plant growing in/nearby sand leaves a residue in it that foams.

Other people have suggested that soapsand is like lava soap--i.e. bar soap with grit such as pumice in it.  This is not correct, as bar soap is a rare luxury on Pern.  Bar soap is made from lye, which in turn is made from the ashes of burnt wood.  As wood has always been a precious commodity on Pern, which is rarely burnt because it can be put to better uses, it is not often that a holder can collect a large enough quantity of ashes to make soap.  The few bars that are made are usually given as gifts.  Now that trees are becoming more common, the new woodcraft hall is thinking of making soap as a sideline to their other products.