Hauynite, Na2Ca(NaSO4Al)Al2(SiO4)3, is also a blue, cubic mineral, isomorphous with sodalite, with which it is often associated. Hauynite is of interest as being one of the very few silicates which contain sulphur.
Lapis-lazuli is a beautiful blue substance found in Persia, Afghanistan, Siberia, and Chili, and has been prized for ages as an ornamental stone. Density, 2.38 to 2.42; hardness, 5.5. It occurs embedded in limestone, from which it is extracted by heating the limestone and then breaking out the lapis-lazuli with a hammer. Lapis-lazuli was formerly supposed to be a simple mineral, but Brogger and Backstrom have shown it to be a rock, consisting of diopside, amphibole, muscovite, calcite, pyrite, and a blue, cubic mineral to which they gave the name lazurite. Lazurite is a sodium-aluminium silicate containing both sulphur and chlorine, and is regarded as being closely analogous and isomorphous with sodalite and haiiynite. Powdered lazurite constitutes the natural pigment ultramarine. |