Siderite

Description

Siderite (iron carbonate, FeCO₃) from the pegmatites of Araçuaí, Minas Gerais, forms rhombohedral crystals of warm yellow-brown to deep honey colour with curved or stepped faces and a characteristic pearlescent to vitreous lustre. The name derives from the Greek sideros (iron), reflecting its high iron content — up to 62 % FeO by weight.

Araçuaí siderite occurs in lithium pegmatites alongside tourmaline, quartz and feldspar and can develop into fine, well-formed rhombohedra prized by collectors for their colour saturation and crystal perfection. Siderite is also an important rock-forming carbonate mineral in sedimentary iron formations worldwide, but gem-quality crystals from Brazil are far less common.