Tag: Rare Earth Elements

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.

Abukumalite: The Yttrium Star of the Japanese Highlands

While the previous minerals we’ve discussed have been neon-yellow or deep-sea blue, Abukumalite brings us into the realm of the “Earth Tones”—a mineral that looks like the soil but holds the secrets of rare-earth elements.

If you look for “Abukumalite” in a modern textbook, you might find it listed under a different name: Britholite-(Y). However, the story of its discovery in the mountains of Japan remains a classic piece of mineralogical history.

What is Abukumalite?

Abukumalite is a rare-earth silicate mineral belonging to the Apatite supergroup. It was first discovered in 1938 in the Suishoyama pegmatite, located in the Abukuma Range of the Fukushima Prefecture, Japan.

In 1966, mineralogists decided to standardize the names of rare-earth minerals. Because Abukumalite was chemically similar to Britholite but dominated by the element Yttrium, it was officially renamed Britholite-(Y). Despite the name change, seasoned collectors still use the old name to honor its Japanese heritage.

Quick Facts:

  • Chemical Formula: $(Y, Ca)_5(SiO_4, PO_4)_3(OH, F)$

  • Color: Reddish-brown, dark brown, or black.

  • Hardness: 5 on the Mohs scale (similar to a pocket knife blade).

  • Luster: Resinous to greasy (it can look a bit like hardened tree sap).

The “Metamict” State: A Mineral in Chaos

One of the most scientifically fascinating things about Abukumalite is that it is often metamict.

Because Abukumalite frequently contains trace amounts of radioactive elements like Thorium or Uranium, it undergoes a process called “self-irradiation.” Over millions of years, the radiation emitted from within the crystal actually breaks down its own internal structure.

The result? The mineral looks like a crystal on the outside, but on the inside, the atoms are in a state of chaotic disorder, more like glass than a structured crystal.

Where Does it Come From?

Abukumalite is typically found in granite pegmatites—massive, slow-cooled volcanic rocks that allow rare and “exotic” elements to concentrate into large crystals.

  1. Japan (Type Locality): The Abukuma massif remains the most famous source, specifically the Suishoyama pegmatite.

  2. Norway: Found in the high-alkaline rocks of the Drag area.

  3. Russia: Notable specimens have been found in the Kola Peninsula, a “Mecca” for rare-earth mineral hunters.

Why Is It Important?

Abukumalite isn’t just a curiosity for the shelf. It is a vital source of Yttrium and other Rare Earth Elements (REEs).

Yttrium is a “tech-metal” used in:

  • LEDs and Phosphors: Creating the red color in older television screens and modern LED bulbs.

  • Superconductors: Vital for high-tech energy and transport research.

  • Camera Lenses: Adding yttrium oxide to glass makes it heat-resistant and shock-resistant.

Safety and Identification

Like many rare-earth minerals from pegmatites, Abukumalite can be mildly radioactive. While it’s not as “hot” as pure uranium minerals, it’s best to keep it in a display case rather than carrying it in your pocket.

Visually, it can be hard to distinguish from other brown minerals like Allanite. The key is often its association—if you find a resinous brown mineral in a pegmatite known for yttrium, you might just be looking at Abukumalite.

The “A-Group” Mineral Summary

Mineral Color Key Element Claim to Fame
Abernathyite Yellow Uranium Neon fluorescence
Abriachanite Blue Iron The “Blue Earth” of Loch Ness
Abukumalite Brown Yttrium The tech-metal workhorse