Stable carbon storage gemstones
There is a particular focus on using highly abundant elements
to tie the carbon down nigh permanently
preventing it to ever get back into the atmosphere.
Excellent highly stable carbides
These are slack forming and fire self quenching.
These are very hard and resistant against erosion and bio-degradation.
Gemstone grade silicon carbide SiC aka moissanite
- Silicon as element is most abundant right after oxygen and highly accessible right after nitrogen
- SiC is extremely thermally stable, much more so than diamond.
- SiC can't be ignited in air, neither by natural means nor artificially
- SiC does not naturally erode or biodegrade (which can be seen as a problem too)
- SiC is quite chemically stable, but it can be dissolved as everything can given aggressive enough unnatural(!) liquid chemistry.
Titanium carbide TiC aka khamrabaevite
- Titanium is most abundant transition metal element after iron which makes much less stable carbides.
- TiC is extremely thermally
- TiC likely does not naturally erode or biodegrade (which can be seen as a problem too)
- TiC is quite chemically stable. Since it is electrically conductive electrochemical degradation is likely possible
https://en.wikipedia.org/wiki/Titanium_carbide
Boron carbides (B4C and others)
- Boron is less abundant and accessible as element
- Boron carbides are extremely thermally
- Boron carbides likely does not naturally erode or biodegrade (which can be seen as a problem too)
- Chemical stability likely high?
- https://en.wikipedia.org/wiki/Boron_carbide B4C – only one fifth of atoms is actually carbon here
- https://en.wikipedia.org/wiki/Boron_carbides – similar fro others
Diamond – normal cubic form and exotic hexagonal form aka lonsdaleite
- more limited heat resistance than SiC
- can be ignited in extreme enough conditions (as metals can) or when in filligree form
- does not naturally erode or biodegrade (which can be seen as a problem too)
- is quite chemically stable, but it can be dissolved as everything can given aggressive enough unnatural(!) liquid chemistry.
Monocrystalline graphite
- similar to diamond
- slightly more heat resistance than diamond
- maybe slightly less fire resistant than diamond
- possibly less stable against erosion than diamond (flakes coming off)
Carbonic acid salts
Earth alkali carbonates
- CaCO3 calcite aragonite and other forms
- MgCO3 magnesite and other forms
- Both are slack forming and already oxided so completely fire immune.
- Due to being softer and slightly water soluble these do naturally erode (& biodegrade?) (which can be seen as an advantage in some cases)
Both Ca & Mg can be sourced from metal rich ultramafic silicate rock.
APM/APT may help making that energy efficient enough.
This is actually attempted by current day (2025) technologies
pumping CO2 in ultramafic rock to be permanently chemically bound
or bringing out crushed ultramafic rock on fields.
Alkali carbonates (and aluminum carbonates) are much more water soluble and volatile
thus of less interest for permanent carbon storage.
Sodium can be sourced from seawater but what to do with the chlorine counter-ions then.
Iron carbonate
FeCO3 siderite – similar story as with earth alkali carbonates
Less stable carbides
Iron carbide Fe3C aka cementite or iron-cohenite – decent choice?
- Iron is by far the most common transition metal element thus self suggesting to use.
- Cementite is known from iron processing where small crystallites can inprove quuality of carbon steels.
- Mid erosion: The material as a single crystal on it's own has a decent mid hardness of Mohs 5.5 to 6.
Much less than SiC and TiC - Unclear biodegradability.
Aluminum carbide (Al4C3) – bad choice!
- Aluminium is the most abundant metal and nigh as abundant as silicon thus self suggesting
- Big issue: The "gemstone" readily reacts with water to aluminum hydroxide and methane
Methane is a really bad greenhouse gass and dissolved aluminum hydroxide makes for aggressive lye
that even neutralized as salt may pose health hazards. - Quite thermally stable (1400°C in air, 2100°C melting)
- https://en.wikipedia.org/wiki/Aluminium_carbide
Acetylides
These react with water – bad choice.
- https://en.wikipedia.org/wiki/Calcium_carbide CaC2 – known from carbide lamps historically used in mining