TERRIUM — A mineralogy cabinet

Jarosita

Jarosita is a mineral with formula KFe₃(SO₄)₂(OH)₆, in the Sulfatos group. This specimen comes from Túnel del Arteal, Cuevas del Almanzora, Almería, Andalucía, España and joined the Terrium collection in 2025.

Jarosita from Túnel del Arteal, Cuevas del Almanzora, Almería, Andalucía, España — Terrium

Description

Toenail-size jarosite with intergrown, lenticular-pseudohexagonal crystals of high quality, with satin lustre, free-growing in a small vug near the top of the specimen. The cavity recalls an open leather pouch.

History of this specimen

First piece I bought at the La Unión Fair 2025. It marks my return to mineral collecting after many years of paying it little real attention beyond a few isolated pieces, mainly fossils. It is true that before this jarosite I had bought one or two others from Meli, Amalia Molina, but none of this quality. I took up the hobby again after walking 5,534 km across the Iberian Peninsula and, when writing about the adventure, realising that in the Villaricos area what made sense to tell was the encounter with the "mineral of Mars". I wanted to have one… and from there everything started again.

About Jarosita

Jarosite is the mineral that confirmed the ancient presence of water on Mars, and the chemical epilogue of the silver rush at Cuevas del Almanzora. A potassium iron(III) sulphate hydroxide, first described in 1852 at Barranco del Jaroso (Almería). It is a secondary, supergene mineral, a product of alteration and neoformation, linked to open workings, water, and abandonment. It represents what remains when the metallic system oxidises: water enters, iron transforms, and mining stops (or the other way round). On Earth it marks closure. On Mars it is another matter. In January 2004 the rover Opportunity detected jarosite in Martian soil, confirming a humid and geochemically active past. Water also flowed there. The mineral commonly occurs as crusts or small crystals; vugs with well-developed crystals, like this specimen, are uncommon.

About the locality

A mining district historically focused on silver, mainly as argentiferous galena, with export of crude ore abroad during British mining-industrial colonialism from the start of the 19th-century silver cycle. Around 1840 the State prohibited the export of untreated ores. Although the regulation was disliked by local entrepreneurs and those of other mining areas, it favoured local industrial development: ore-washing plants, furnaces and smelters sprang up along the coast. Today these remain as industrial heritage in varying states of conservation.

Technical data

Catalogue No.
0001
Composition
KFe₃(SO₄)₂(OH)₆
Name
Jarosita
Group
Sulfatos
Category
Cavella
Mine
Túnel del Arteal
District / Municipality
Cuevas del Almanzora
Province
Almería
Region
Andalucía
Country
España
Size (cm)
3.5 x 3 x 1.5
Weight
25.3 g
Acquired
2025
Ex-collection
Eduardo Ruiz Contreras (Geoterraminerales)
Etymology
Named after Barranco del Jaroso (Almería), where it was discovered.
Quality
Top
Value trend
Al alza

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