Sulfur

Sulfur is not one of the extremely abundant elements, but it often occurs in concentrated form making it decently accessible.

Sulfur is located under oxygen in the periodic table. Like oxygen sulfur too tends to form directed covalent bonds, which is very desirable in the context of APM. Due to its notably bigger size this character is not quite as strong though.

In nature sulfur tends to form minerals with heavy/toxic/rare elements leaning to the right side of the periodic table. In contrast to oxides many sulfide compounds are rather soft.

Common natural sulfides (occurring as minerals)

• Iron sulfides [1] – pyrite marcasite (both FeS₂), troilite (FeS) and many more
• Zinc sulfide [2] – Minearls sphalerite [3] (ZnS)
• Copper sulfides [4] – Many minerals including very soft Covellite (CuS) and soft Chalcocite (Cu₂) (analog to mid hard transparent cuprite?)
• Lead sulfides: [5] galena (PbS)

Rare but notable:

• Typical sulfides: Cinnabar HgS; Arsenic sulfide minerals [6]
• Despite titaniums abundance titanium sulfide minerals are very rare. – Wassonite (TiS) & [7] (TiS₂)
• Millerite NiS – (nickel is rare on earth but common in metallic asteroids)

Rare exotic synthetic compounds

Continuously going right in the periodic table pairing more and more electronegative elements with the already electronegative element sulfur can lead to uncommon oxidation numbers and more and more reactive species less and less suitable as structural material.

Boron group:

• Boron sulfide B₂S₃ [8] reacts with water (bad combo with boron)
• Aluminium sulfide Al₂S₃ [9] reacts with water (bad combo with aluminium) (Isostructural sulfur analog to sapphire Al₂O₃? If so pseudo polymorphs might be stable)

Carbon group:

• Carbon disulfide [10] – the "thermodynamic polymorph" is a a highly toxic volatile liquid
• Silicon disulfide [11] (chains)
• Germanium disulfide [12] – thermodynamic production gives glassy amorphous 3D polymers indicating strongly covalent behaviour (desirable) – But germanium is a rather rare element not suitable as structural material.
• Tin disulfide [13] (mineral berndtite) (cadmium iodide structure)
• Lead disulfide [14] (not to confuse with the as mineral occurring lead monosulfide: galena) (cadmium iodide structure)

Pnictogen group:

• Sulfur nitrides: [15] especially terasulfur tetranitride [16] (explosive)
• Phosphorus sulfides: [17] e.g. P₄S₁₀ P₄S₃ (all very unhealthy)

Chalcogenide group (sulfurs own group):

• Sulfur oxides: [18] – precursor to: sulfuric acid, sulfurous acid, ...

Halogenide group (counting hydrogen to the halogenides):

• (Di)Hydrogen sulfide [19] – (infamous)
• Sulfur fluorides: [20] – all but one are toxic (as expectable) with the odd exception of sulfur hexafluoride [21]
• Sulfur chlorides: [22] – beside SCl₂ there are S₂Cl₂ and SCl₄

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Earth alkali sulfides:
Calcium sulfide [23] and magnesium sulfide [24] occur as mineral despite hygroscopic it calcium-oldhamite & magnesium-oldhamite

Sulfur in the solar system

Sulfur is a borderline volatile ("atmosphere seeking") element.

• On Earth the bulk of all the sulfur was drawn from the atmosphere by life (just as the carbon).
• On Venus a huge amount of sulfur has accumulated in the atmosphere in the form of sulfur trioxide SO₃ (alongside the carbon dioxide and nitrogen)
• On Titan all the sulfur is certainly frozen out due to the cold temperatures (maybe even sunk down to the silicate core - we don't know yet).
• On Mars the lack of sulfur in the atmosphere is probably because a combination of low temperatures and limited volcanic activity

Extreme amounts of sulfur can be found on Jupiter's giant moon Io.
Io is the nearest giant moon of Jupiter (one of four). So near that the tidal forces heat it up enough to convert it into a giant volcano moon. The heat seem to have evaporated off the lightest volatiles like water. Lacking a wealth of heavy elements too (known from low density) all thats left are mid mass elements like mostly silicon, (oxygen) and a lot of sulfur. Beside lava flows there are gargantuan geyser like vents where material re-sublimes to "sulfuric snow". All this makes Io it very colorful and pretty (poisonous pretty). Since Io has no atmosphere ejected "snow" falls back to the surface in a throwing parabola. (Also since Io has no atmosphere if water comes up it boils even at O°C).

We have no ground based images of Io's surface yet. But there is a place on earth that might look similar (Ethiopia – Danakil Depression – Dallol) "Dallol" means dissolution / colorful area / ? (to check). Very fitting for all the concentrated sulfuric acid puddles there.

All the color images we have of Io likely do not show the colors as our human eyes would perceive them. This is because scientific interest had higher priority thus the color fiters in both visiting spaceprobes ( did not match the RGB of our screens or the "RGB" of our eyes. In contrast newest mars images are excellent in this regard, even taking the dynamic color temperature adaption of the human eyes into account.

Related

Sulfur as drop in replacement for oxygen (directly above in the periodic table and thus chemically similar):

• TiO₂ (rutile sructure) => TiS₂ [25] (insoluble in water) thermodynamic production leads to but a layered structure very different to the rutile structure. (TODO: investigate mechanosynthesizability and degree of metastability stability of rutile structure TiS₂) (infos about mechanical properties overshadowed by electronic properties)

• Al₂O₃ (sapphire) or other other phases, including the cubic γ and η phases, the monoclinic θ phase, the hexagonal χ phase, the orthorhombic κ phase and the δ phase that can be tetragonal or orthorhombic.
• Al₂S₃ (sapphire structure) still hard but reacts with water -- more than six other crystalline forms are known to be thermodynamically accessible (TODO: How can Al₂S₃ feature wurtzite structure when stoichiometry is not 1:1)

Sulfur for surface passivation:
Unlike silicon dioxide (SiO₂) silicon disulfide (SiS₂) forms polymer chains. This might point to sulfur being a good candidate for passivating silicon rich surfaces that shall contact each other and slide on each other.

Due to:

• its larger size than oxygen
• its still quite covalent behaviour and
• its bond order o two

Sulfur seems especially useful for:

• superlubricating diamonoid strained shell brearings
• single atom intermeshing diamondoid strained shell gears

Sulfur with lots of oxygen as nearest neighbours (highly oxidized sulfur):
Oxygen likes to (double)bond to sulfur: (may be more of a reactive functional broup than a passive capping - depends on environment)

• Sulfoxides Sulfurous_acid
• (Disulfuric_acid)
• Sulfones Sulfuric_acid (anhydride SO₃)
• Sulfates (salts of sulfuric acid) are ternary compounds adding oxygen to the "mix". Check out the page: "Salts of oxoacids"

External links

Wikipedia

• Dimethyl_sulfide C₂H₆S -- a small organic sulfur carrying molecule with low toxicity -- possibly useful as elemental storage medium
• Sulfide_minerals
• Dallol, Ethiopia (Tipp: do a picture / video search - drone footage)

(wiki-TODO: maybe add illustrative images -- Dallol, Io (real color trouble), ...?)

• Disulfur (unlike O₂ weak unstable S=S double bond - but also a dirarical)