Autunite is a secondary uranium phosphate mineral with the chemical formula Ca(UO₂)₂(PO₄)₂·10–12H₂O, formed through the oxidation and alteration of primary uranium-bearing minerals in the near-surface environment. This specimen displays classic tabular to lamellar crystal growth, stacked in thin, platy sheets with sharp, stepped edges—a hallmark of well-crystallized autunite from Brazilian pegmatitic and hydrothermal systems. The vivid yellow to yellow-green coloration is caused by the presence of the uranyl ion (UO₂²⁺), which also gives the mineral its exceptional visual intensity.

Geologically, autunite forms in the oxidation zones of uranium-rich pegmatites and granitic host rocks, where circulating groundwater reacts with uranium minerals and phosphate sources. The São Pedro–Malacacheta region in Minas Gerais is known for complex granitic intrusions and pegmatites enriched in rare elements, providing ideal conditions for secondary uranium mineralization. The fragile, micaceous texture of this specimen reflects its low hardness (2–2.5 on the Mohs scale) and perfect cleavage parallel to the crystal plates, making intact examples particularly desirable.

Under longwave ultraviolet light, this specimen exhibits a strong neon green fluorescence, a diagnostic feature of autunite resulting from uranyl activation. The mineral is radioactive, though typically at low levels for small specimens, and is stable only in relatively dry conditions—over time, autunite can dehydrate and transition into meta-autunite. Overall, this piece represents a fine example of secondary uranium mineral formation, combining geological significance, regional provenance, and striking aesthetic appeal.

**Specimen from historic collection of Luiz Menezes, one of Brazil’s most respected mineral collectors from Brazil**

Approximate measurements: 14.42 x 8.12 x 7.13 mm

Weight: 0.93 g