Iridium is a rare chemical element found in the earth’s crust. It has a silver-white appearance and is one of the densest elements known. In its raw form, unprocessed iridium has a distinctive shiny, silvery look. Let’s explore what raw iridium looks like and learn more about this unique metal.
The Appearance of Raw Iridium
In its pure form, iridium is a bright, whitish silver metal. It has a reflective, mirror-like surface when polished. Some key facts about the visual characteristics of raw iridium include:
- Color: Silvery white, similar to platinum or tin
- Luster: Metallic and highly reflective
- Surface: Smooth and shiny when polished
- Form: Brittle, hard solid at room temperature
Iridium’s silvery color is slightly warmer and less gray than nickel or chromium. When examined closely, it may have a faint yellowish cast. In its raw state, iridium can be found in small brittle granules or irregularly shaped grains. It has a high resistance to corrosion and oxidation, so its shiny metallic surface stays relatively unchanged in air.
Comparison to Platinum
Iridium is commonly compared to its sister metal platinum. Although they belong to the same group of elements, called the platinum metals, there are some key differences between raw iridium and platinum:
| Metal | Color | Surface | Density |
|---|---|---|---|
| Iridium | Whitish silver | Highly reflective | 22.56 g/cm3 |
| Platinum | Grayish white | Less reflective | 21.45 g/cm3 |
As seen above, iridium has a lighter, more silvery color while platinum is darker with a steel-gray tone. Iridium also has a higher density than platinum, making it over 5% denser. These subtle visual differences help distinguish the two precious metals.
Iridium’s Crystalline Structure
On an atomic level, iridium demonstrates a face-centered cubic (FCC) crystalline structure in solid form. This means its atoms are arranged in a cubic unit cell formation with atoms at each corner and in the center of each cube face.
Iridium’s FCC configuration contributes to its typical granular physical form and metallic sheen. The tightly packed crystalline lattice also makes iridium very hard and brittle, causing it to fracture when bent or worked.
Manipulating Iridium’s Structure
Heating, alloying, and processing can alter iridium’s natural FCC structure. For example:
- At over 2,443°C, solid iridium transitions from FCC to a body-centered cubic (BCC) structure.
- Melted iridium takes on a liquid disordered atomic arrangement.
- Mixing iridium with other metals like platinum forms substitutional solid solutions with a modified lattice.
- Mechanical working like pounding or drawing can introduce grain dislocations and defects.
These changes to the crystalline structure modify iridium’s properties and can improve its workability for various applications. But in its raw form, the undisturbed FCC configuration gives iridium its classic bright, white appearance.
Characteristics of Iridium Powder
One method of preparing high purity iridium is through metallothermic reduction to produce a fine powder. This powder has a distinctive dark gray color and is composed of very small irregular or spherical grains of iridium.
Some properties of iridium powder include:
- Composition: >99.9% Ir
- Color: Dark gray
- Particle size: Less than 25 microns
- Morphology: Irregular or spherical particles
- Purity: High, with low oxygen and carbon content
The dark gray hue differs from polished iridium’s usual light appearance. This is because the tiny powder particles scatter and absorb light differently than a smooth metal surface. However, the distinct powder is still over 99% pure iridium.
Consolidating Iridium Powder
Iridium powder can be further processed to make commercial iridium products. Techniques like vacuum sintering apply heat and pressure to consolidate the powder into solid forms. The resulting materials have the characteristic bright finish of bulk iridium again.
Natural Occurrences
In nature, iridium is almost always found finely dispersed in solid solution minerals. This is because it rarely occurs in sufficiently high enough concentrations to form its own distinct minerals.
The most common natural iridium-bearing mineral is osmiridium. This is an alloy of iridium and osmium, typically containing about 30% Ir. Osmiridium has a pale grayish color and appears within placer deposits of fine gravels. Iridium ratios are highest in osmiridium from Russia’s Ural Mountains region.
Iridium is also commonly associated with nickel-rich laterite ores. In these deposits, iridium occurs in extremely low levels measured in parts per billion. Nonetheless, the mining of laterite ores accounts for much of the world’s iridium production.
Refining Raw Iridium
In raw natural forms like osmiridium alloys, iridium needs to be separated and purified. This typically involves complex pyrometallurgical and hydrometallurgical processes.
Some steps include:
- Dissolution: Dissolving osmiridium in aqua regia acid to put iridium into solution
- Precipitation: Precipitating out an ammonium hexachloroiridate compound using ammonia
- Reduction: Reducing the chloroiridate to pure metallic iridium using hydrogen or sodium formate
- Consolidation: Melting and consolidating the refined Ir powder into ingots or sponge
This produces high purity iridium with the characteristic bright silver-white color and mirror finish. The refined metal may then be fabricated into various products and applications.
Typical Forms of Refined Iridium
Iridium is traded globally in various refined forms including:
- Powder
- Sponge
- Ingots
- Granules
- Wire
Powders have the dark gray appearance as described earlier. Sponge and ingots are consolidated iridium with the classic silvery-white look and metallic shine.
These forms are then used to manufacture finished iridium products like crucibles, fountain pen nibs, high performance alloys, and more.
Key Characteristics
To summarize, here are the key identifying characteristics of raw iridium in different forms:
| Form | Color | Properties |
|---|---|---|
| Natural osmiridium | Pale gray metallic | Alloy with osmium; brittle; found in placer deposits |
| Refined powder | Dark gray | Fine grain powder; 99%+ purity |
| Ingots/sponge | Silvery white metallic | Solid consolidated form; highly reflective |
Conclusion
Raw iridium displays unique visual properties that distinguish it from other metals. In natural osmiridium alloys it has a grayish look. When refined, iridium powder has a dark gray appearance while consolidated sponge and ingots are brightly reflective and whitish-silver.
Understanding what unprocessed iridium looks like provides insight into this rare and valuable element. Its distinctive coloring and shine make iridium an easily identifiable precious metal.