TERRIUM — A mineralogy cabinet

Uranofana

Uranofana is a mineral with formula Ca(UO₂)₂(SiO₃OH)₂·5H₂O / Cu(UO₂)₂(PO₄)₂·8H₂O, in the Fosfatos group. This specimen comes from Cadalso de los Vidrios, Madrid, España and joined the Terrium collection in 1994.

Uranofana from Cadalso de los Vidrios, Madrid, España — Terrium

Description

Block of coarse-grained biotite with quartz, speckled with tiny lemon-green powdery crusts, practically invisible to the naked eye but showing an intense green fluorescence under UV, characteristic of uranyl phosphates. The crusts coat cleavage planes and fractures of the mica, conforming to its relief. Habit is pulverulent, with minute tetragonal laminae. The piece combines the platy texture of the biotite with the pinpoint glow of the fluorescence.

History of this specimen

More than thirty years ago, Óscar went out with Eloy and his parents to look for minerals. During that outing, Eloy found this piece, but as he already had a large mica in his collection, he passed it to Óscar. Much later, when we checked the pieces under UV light, we discovered it was covered with tiny lemon-green crusts, invisible to the naked eye but bursting with light under ultraviolet. Measured with the Geiger counter, it read 1.0 µSv/h at a few centimetres. In 2025, Carlos González Bargueño—co-author of the "Bocamina" No. 14 article (Cadalso de los Vidrios) and a regular contributor to the magazine—confirmed with me that this fluorescence is compatible with uranyl phosphates, pointing to autunite or uranophane as the most likely possibilities. He told me that during his outings they did not document autunite, although they did torbernite, but as it is not fluorescent, that supports the hypothesis of a different uranyl phosphate. In any case, the "uranium micas", as he calls the uraniferous laminar silicates, are almost impossible to distinguish without chemical or spectral analysis.

About Uranofana

Cadalso de los Vidrios hosts granitic pegmatites with micas, quartz and hyalite opal. In some spots, hydrothermal alteration produced powdery uraniferous crusts—uranophane, metatorbernite or autunite—difficult to differentiate without analysis. The intense green fluorescence of this specimen places it within that poorly documented but historically attested context.

About the locality

The locality has been little studied compared with classics such as Assunção or San Finx, but old catalogues and field forums cite finds of powdery uranyl phosphates at Cadalso, commonly associated with biotite and quartz.

Technical data

Catalogue No.
0174
Composition
Ca(UO₂)₂(SiO₃OH)₂·5H₂O / Cu(UO₂)₂(PO₄)₂·8H₂O
Name
Uranofana
Group
Fosfatos
Category
Núcleus Ardens
Matrix
Mica biotita y cuarzo
Associations
metatorbernita
District / Municipality
Cadalso de los Vidrios
Province
Madrid
Country
España
Size (cm)
9.5 x 6.2 x 5.8
Acquired
1994
Ex-collection
Oscar Herranz
Etymology
Uranophane from uranium and the Greek "phaíno" (to show), for its yellowish appearance; "torbernite" in honour of Torbern Bergman, Swedish chemist.
Quality
Muy buena
Value trend
Estable

Related specimens

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