#monoclinic

• Acanthite — A mineral belonging to the sulfide class, representing the low-temperature monoclinic crystalline form (stable below 173°C) of silver sulfide (Ag2S). This precious and heavy mineral is frequently found in hydrothermal veins or secondary enrichment zones of silver ore deposits, showcasing a lead-gray to black color with a dull metallic luster. It is dimorphous with argentite, the cubic mineral that pseudomorphically converts into acanthite upon natural cooling.
• 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).