Revision as of 09:51, 23 June 2021

Moissanite is the name for transparent silicon carbide (SiC) of gemstone quality.
It may be an especially interesting (if not the most interesting) base material for gemstone based metamaterials in gem-gum products because of its set of peculiar properties.

Basic properties:

• Hardness: Mohs 9.25
• Melting point: 2730 °C (decomposes)
• Does not form a macroscale surface oxidation layer at room temperature
• Is stable in macroscale bulk under oxygen atmosphere at high temperatures
• Is not water soluble
• Crystal structure: diamondoid cubic or hexagonal (analog to diamond and lonsdaleite) and all mechanosynthesizable neo polymorphs
• Density: ~3.22g/ccm (for comparison: diamond ~3.53g/ccm, silicon ~2.33g/ccm)
• Optically fully colorlessly transparent in the visible range
• Refractive index: n_ω=2.654 n_ε=2.967 (can be strongly birefringent)

• Heat conductivity: very high ...
• Electrical conductivity: highly isolating ...

Resistance against heat

Compared to diamond and its polymorphs such as lonsdaleite moissanite has a much better resistance against high temperatures.
Diamond is only metastable at room temperature and converts to the lower energy state of graphite if it's heated up far enough. That is not the case with moissanite due to silicon not wanting to form graphite like sheets.

Resistance against larger scale fire

Compared to diamond moissanite is more resistant against oxidation and fire. Because it contains the nonvolatile (slack forming) element silicon. Moissanite is not a fully oxidized gemstone material (like e.g. quartz or leukosapphire is) thus it is not immune to oxidation an indeed gets its surface nanosctructure destroyed when contacting an oxygen containing atmosphere at somewhat elevated temperatures. But …

• simple sealing against entry of atmosphere can prevent that
• there is no oxidation on a larger scale because a protective slack layer out of quartz-glass is formed that prevents further oxidation and runaway fires.

While diamond cracks splinters and burns under a strong flame moissanite just turns yellow and back to clear again when it cools again.

Neo polymorphic structure control

Natural moissanite and thermodynamically synthetic moissanite come with a rather random layer order (not ABAB hexagonal or ABCABC cubic but something in-between). When it's produced via mechanosynthesis instead this layer ordering can by precisely controlled. See neo-polymorph.

Natural moissanite is:

• neither cubic (layer order ABAB wurtzite structure)
• nor hexagonal (layer order ABCABC zincblende structure)

It's a more complex layer order.
Of course with mechanosynthesis this could be arbitrarily controlled.

Interplanetary applications (Venus)

Unlike diamond moissanite is a very good refractory material that has no problems with the harsh surface conditions of Venus (~500°C ~90bar). So it would be a useful base material to make ground mining equipment out of.

To make moissanite one needs silicon but that is not present in the atmosphere since it is a nonvolatile element. At best there is some silicate dust and that predominantly in the lower parts of the atmosphere. So to make moissanite silicon needs to be minded from the surface. Silicon is extremely common on Venus, second only to oxygen. Just as on earth. So to get it one basically can haul up almost any random rock that lies around loosely. No serious ground mining (with drills or so) is required. The rocks can then be chemically processed at ~50km height where machines (and especially humans) can easier operate. (Picking mafic basaltic rocks gives some common metals as a bonus.) In short retrieving silicon (and other common non-volatile elements) from the harsh ground shouldn't be too difficult.

Related

• Diamondoid – Diamond like compounds
• Diamond
• Lonsdaleite

External links

Wikipedia:

• Moissanite