Albite and siderite crystals. Albite (white) is a plagioclase feldspar mineral that occurs in pure white or, more rarely, colourless, tabular crystals. Feldspars are a very common group of silicate minerals that make up as much as 60% of the Earth's crust. Siderite (brown, iron carbonate) is a valuable source of iron and is commonly found in hydrothermal veins. Specimen found in Nadabula, Hungary.