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.

The curved crystal faces diagnostic of Araçuaí siderite — the so-called saddle-shaped rhombohedra — form when growth rates vary across the face during rapid crystallisation from cooling pegmatitic fluids. This gives each crystal a slightly rounded, undulating surface that catches light differently from the flat faces of calcite or dolomite. Siderite from Araçuaí often occurs as loose crystals or small drusy groups free of gangue, making them easy to display. The warm honey to deep amber colour is most prized; very deep brown material with strong pearlescent lustre represents the finest collector grade.