Tag: Microcrystals

Achávalite: The Rare Selenium Sentinel of the Sierras

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:

  • Chemical Formula: $FeSe$ (often with traces of copper)

  • Color: Dark grey to iron-black.

  • Luster: Metallic to sub-metallic.

  • Hardness: 2.5 on the Mohs scale (soft and brittle).

  • 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:

  1. Polished Section Analysis: Looking at the mineral under a reflected-light microscope.

  2. X-Ray Diffraction (XRD): Confirming the hexagonal structure.

  3. 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.

Abernathyite: The Rare Yellow Treasure of the Uranium World

If you aren’t a dedicated mineral collector or a radiochemistry enthusiast, the name Abernathyite might sound like a fictional stone from a superhero movie. However, this rare secondary mineral is very real, strikingly beautiful, and carries a fascinating story that links 1950s geology with the complex world of uranium chemistry.

What is Abernathyite?

Abernathyite is a rare potassium uranyl arsenate hydrate mineral. It was first discovered in 1953 in the Fuemrol Mine in Emery County, Utah. Named after Jesse E. Abernathy, the mine operator who first found the specimens, it belongs to the autunite group—a family of minerals famous for their bright colors and radioactive properties.

Quick Facts:

  • Chemical Formula: $K(UO_2)(AsO_4) \cdot 3H_2O$

  • Color: Distinctive yellow to neon green.

  • Luster: Vitreous (glassy) to pearly.

  • Hardness: 2.5 on the Mohs scale (about the same as a fingernail).

  • Crystal System: Tetragonal.

Appearance and Characteristics

One of the most striking things about Abernathyite is its visual appeal. It typically forms in small, transparent, platy crystals. While it may look delicate, its chemical makeup is quite specific.

The Glow Factor

Like many uranium-bearing minerals, Abernathyite is fluorescent. Under ultraviolet (UV) light, it emits a moderate to strong yellow-green glow. This is a characteristic of the uranyl ion ($UO_2^{2+}$) present in its structure.

Radioactivity

Because it contains uranium, Abernathyite is radioactive. While a small thumbnail specimen in a plastic container isn’t a major health hazard, it requires careful handling. Collectors usually store it in “perky boxes” to prevent the inhalation of any radioactive dust or radon gas buildup.

Where is it Found?

Abernathyite isn’t something you’ll find in your backyard. It is a secondary mineral, meaning it forms when primary uranium ores (like uraninite) are weathered or oxidized by groundwater.

  • Utah, USA: The “type locality” remains the Fuemrol Mine.

  • France: Notable specimens have been found in the Lodève District.

  • Germany: Found in the Black Forest region.

  • South Africa: Small deposits have been identified in the Northern Cape.

Why Should You Care?

You might wonder why scientists and collectors get excited about a tiny yellow crystal. Abernathyite is a “chemical cousin” to more common minerals like Autunite (calcium-based) and Metazeunerite (copper-based).

By studying how Abernathyite forms, geologists can understand how uranium and arsenic move through the earth’s crust. This is vital for environmental science, particularly in managing old mining sites and protecting groundwater from contamination.

Handling and Safety

If you are lucky enough to see a specimen in person, remember the three rules of radioactive minerals:

  1. Time: Limit the time you spend holding it.

  2. Distance: Keep it away from your living spaces.

  3. Shielding: Store it in a dedicated mineral case.

  4. Hygiene: Always wash your hands after handling any unsealed specimen.

Abernathyite is a perfect example of the hidden beauty found in the world of radioactive mineralogy—a mix of bright, neon colors and complex chemical history.