While silver and uranium minerals often steal the spotlight with their luster and glow, the world of selenides holds its own quiet, metallic mystery. Achávalite (or Achavalite) is one of the rarest of the bunch—a mineral so elusive that for decades it was known from only a single mine system in the world.
Named after the Argentine engineer and professor Luis Achával, this mineral is a masterclass in “hidden” geology.
What is Achávalite?
Achávalite is an iron selenide mineral ($FeSe$). It belongs to the nickeline group, a family of metallic minerals that share a specific hexagonal crystal structure. First discovered in 1939, it was found tucked away in the selenide-rich veins of the Mendoza Province in Argentina.
In 2015, the International Mineralogical Association (IMA) officially updated the spelling from Achavalite to Achávalite to correctly reflect the accent in Luis Achával’s name.
Quick Facts:
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Chemical Formula: $FeSe$ (often with traces of copper)
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Color: Dark grey to iron-black.
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Luster: Metallic to sub-metallic.
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Hardness: 2.5 on the Mohs scale (soft and brittle).
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Crystal System: Hexagonal.
The “One-Mine” Wonder
For a long time, Achávalite was considered a “single-locality” mineral. Its home was the Cacheuta Mine in the Sierra de Cacheuta, Argentina.
In this remote location, Achávalite forms as tiny, dark grains or massive granular patches within calcite veins. It isn’t a mineral you find in giant, showy crystals; instead, it’s often microscopic, intergrown with other rare selenides like Berzelianite (copper selenide) and Clausthalite (lead selenide).
Today, only a handful of other sites—such as the Zapadno-Ozernoe deposit in Russia—have reported its presence, making it a true prize for specialized “rare species” collectors.
A Chemical Powerhouse
Though it might look like a simple grey stone, Achávalite is chemically significant. It is the selenium-based analogue of Troilite (iron sulfide).
Why Selenium?
Most iron in the Earth’s crust bonds with sulfur to create minerals like Pyrite (Fool’s Gold). However, in very specific geological environments where sulfur is low and selenium is high, the iron is forced to bond with selenium instead. This “chemical swap” creates Achávalite.
The Superconductor Connection
Interestingly, synthetic iron selenide ($FeSe$)—the man-made version of Achávalite—is a superstar in the world of physics. It is a superconductor, meaning it can conduct electricity with zero resistance at certain temperatures. While natural Achávalite isn’t used to build quantum computers, its existence helps geologists understand how these unique elements behave in nature.
Identification and Handling
If you happen to find a piece of Achávalite, don’t expect it to stand out. It looks remarkably like many other metallic grey minerals. Identification usually requires:
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Polished Section Analysis: Looking at the mineral under a reflected-light microscope.
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X-Ray Diffraction (XRD): Confirming the hexagonal structure.
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Chemical Testing: Proving the presence of selenium over sulfur.
Safety Tip: Like many selenides, Achávalite should be handled with care. While it isn’t highly toxic to touch, you should avoid inhaling dust from the mineral, as selenium can be harmful in high concentrations.
The “A-Group” Final Roundup
Now that we’ve covered five distinct “A” minerals, you can see how diverse the mineral kingdom really is:
| Mineral | Color | Key Element | Claim to Fame |
| Abernathyite | Yellow | Uranium | The glowing radioactive crystal. |
| Abriachanite | Blue | Iron | The “Loch Ness Blue” pigment. |
| Abukumalite | Brown | Yttrium | The tech-metal workhorse. |
| Acanthite | Black | Silver | The “soft” silver you can cut with a knife. |
| Achávalite | Grey/Black | Selenium | The ultra-rare Argentine native. |