Gibbsite - Hydrargillite and Psilomelane Composition & Structure

Gibbsite - Hydrargillite
Aluminum hydroxide, A1 (OH)₃. Monoclinic. Rarely in hexagonal-shaped tabular crystals. Stalactitic or botryoidal. Basal cleavage. H. = 2-3.5.  G. = 3.2-2.4. Luster pearly, vitreous or dull. Color white. Infusible. Insolublein hydrochloric acid. Water in C.T. A rare species of secondary origin resulting from the alteration of other aluminum bearing minerals, especially the silicates. Often associated with bauxite and a constituent of many laterites.

Psilomelane
Of uncertain composition, chiefly manganese oxides, MnO₂ with MnO and H₂O, also small amounts of barium oxide, cobalt oxide, etc. Noncrystalline. Massive, Botryoidal, stalactitic. H. = 5-6. G = 3.7-4.7. Submetallic luster. Black color. Brownish black streak. Infusible. A small amount of mineral fused in O.F. with sodium carbonate gives an opaque bluish green bead. Gives much water in C.T. Distinguished form the other manganese oxides by its greater hardness. An ore of manganese, occurring usually with pyrolusite, and having similar origin and associations as that mineral.