Platinide
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"Platinide" is a chemistry/engineering shorthand term for heavy Group 8A (see periodic table of elements) metals that share some of the physical and chemical properties of platinum. These properties include corrosion resitance (through the rapid formation of a thin, durable oxide layer), selective chemical reactivity, a relatively high melting point, good mecanical strength, and good electrical conductivity. All are very rare in the Earth's crust, though somewhat more common in extraterrestrial sources (for example, meteorites), suggesting a degree of sequestration in the planet's deeper layers due to high density and atomic weight.
The elements of this group, in order of ascending atomic number, are ruthenium, rhodium, palladium, osmium, iridium, and platinum.
Rhodium is prized as a plating metal for its durable, bright luster. Palladium is also used in plating, but much more widely used as a chemical catalyst. Osmium and iridium are extremely dense (iridium has the highest density, or specific gravity of any element), hard metals with specialized engineering applications. Platinum has been valued as a jewelry metal since the 1790s; far scarcer and even "heavier" than gold, it can be worked to a fine degree; under normal conditions it never tarnishes and is well-nigh indestructable. More recently it has found application as a catalyst material and in the construction of precision instraments, particularly where a low index of thermal expansion and high wear resistance are required.
The original international standards for metric weights and measures were constructed of a platinum/iridium alloy. Automotive catalytic converters typically employ a platinum and/or palladium coatings on a ceramic matrix.
"The Measure Of All Things" by Ken Alder (ISBN 0-7432-1675-x) discusses the considerations in choosing platinum alloy for the Metric Standard. It is a facinating look at the scientific politics of Nepolianic Europe.
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