Soft white mineral (6)
I believe the answer is:
gypsum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">soft white mineral</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for gypsum that I've seen before include "Chalky mineral from which plaster of Paris is made" , "Mineral used in making plaster of Paris" , "that provides material for casts" , "Mineral giving plaster of Paris" , "Alabaster" .)