Moissanite

In the future we will build with gemstones.

moissanite aka silicon carbide (SiC). A gemstone-like compound (and diamondoid) material. Without impurities it would be colorless and fully transparent. The silicon makes this material fireproof in bulk blocks. But it also inhibits the possibility to intentionally burn it up completely to gasses since (applying extreme heat) it it just partly turns into a glassy slag.

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)

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• 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.

Toughness – compared to diamond

Natural moissanite has higher toughness than natural diamond.
This is most likely because moissanite has a hard time choosing between
the cubic zincblende and the hexagonal wurzite structure and thus having more complex plane stacking orders
giving it some amorphous qualitues.

With materials synthetically made by piezochemical mechanosynthesis though:

• moisanite like stacking orders can be given to diamond/lonstaleite crossover materials – Dialondeite
• in small crystolecules, where there is not much space for plane stacking orders, there typically (on average) are no faults to begin with where cracks could start. So there should not be much difference due to cleaving planes.
  See: Superelasticity.

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 … ABCABC fcc (face centered cubic)
• Lonsdaleite … ABAB hcp (hexagoal close packed)
• Dialondeite … materials with mixes of the two stacking orders only accessible via piezomechanosynthesis

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• Base materials with high potential

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Ultrahard but not reliant on carbon, nitrogen or other not super extremely abundant elements:

• Leukosapphire (Al₂O₃) – Mohs 9 (defining mineral)
• Tistarite – Mohs ?? – might be notably harder than rutile as it shares the crystal structure with leukosapphire; soft hematite does too but iron generally does not like to form many good strong gemstone compounds
• Stishovite (dense SiO₂ polymorph) – Mohs 8.5-9.5 – metastable SiO₂ polymorph – rutile structure & very hard and dense – (Mechadensite maybe more heat resistant)
• Seifertite (dense SiO₂ polymorph) – Mohs ?? – metastable SiO₂ polymorph

External links

Wikipedia:

• Moissanite