
1) Carbonate mineral 2) Magnesium mineral
Found on
https://www.crosswordclues.com/clue/hydrotalcite

Hydrotalcite is a layered double hydroxide of general formula Mg6Al2CO3(OH)16·4(H2O), whose name is derived from its resemblance with talc and its high water content. The layers of the structure may stack in different ways, to produce a 3-layer rhombohedral structure (3R Polytype), or a 2-layer hexagonal structure (2H polytype) which was formerly...
Found on
http://en.wikipedia.org/wiki/Hydrotalcite

Hydrotalcite is a hexagonal, massive, foliated, fibrous white translucent mineral of pearly lustre and a greasy feel composed of aluminium, magnesia and water. It is found in serpentine.
Found on
http://www.probertencyclopaedia.com/browse/HH.HTM
No exact match found.