#monoclinic system
- Allanite — A mineral belonging to the epidote group, black, brown, or gray in color, with a resinous or submetallic luster, containing significant percentages of rare-earth elements (cerium, lanthanum, neodymium, yttrium), alongside traces of thorium or uranium, which give it a weakly radioactive nature. It crystallizes in the monoclinic system and occurs as an accessory mineral in igneous (granites, pegmatites) and metamorphic rocks.
- Anthreachite — A rare mineral belonging to the hydrated sulfate class, containing copper and aluminum, and crystallizing in the monoclinic system. Usually discovered as a secondary alteration product in the oxidation zones of hydrothermal copper deposits, this mineral occurs as microcrystalline aggregates or thin light blue-green to turquoise crusts.
- Armstrongite — A rare silicate mineral (hydrated calcium zirconium silicate, formula CaZrSi6O15·3H2O) characterized by monoclinic translucent crystals, light to reddish brown color, and vitreous luster. Discovered in the Khan Bogd massif in the Gobi Desert (Mongolia).