NTB12XXX: Group of lenticular crystals of Fe-bearing Siderite, light brown in colour, lustrous and well defined, forming a compact druse. The specimen displays a harmonious arrangement of the crystals, enhanced by the presence of white Calcite crystals that provide colour contrast.

Geologically, the Siderite formed from iron-rich hydrothermal fluids that circulated through fractures and cavities within the deposit, crystallizing under relatively low-temperature conditions. The Calcite corresponds to a later stage of the mineralization, having developed from calcium-rich fluids that precipitated over the previously formed crystals of Fe-bearing Siderite.
Santa María Mine (Cueva la Gitana), Las Carreras, Abanto y Ciérvana/Abanto Zierbena, Comarca Gran Bilbao, Vizcaya / Bizkaia, Basque Country (Euskadi)  Spain

Specimen size: 8.6 × 5.7 × 4.3 cm = 3.39” × 2.24” × 1.69”

Main crystal size: 1.1 × 0.9 cm = 0.43” × 0.35”