Salts of oxoacids

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The X-O-X bonds present in those compounds increase the space between the spacially linking X atoms. This lead to a lower density of bonds in cross sections inclusion of bigger voids thus higher porousity. Due to the porousness of these compounds it is harder to get the surfaces flat - figuratively like the surface of a pumice stone. It's impossible to get them as smooth as passivated diamond. [todo: investigate wheter superlubricating bearings can be constructed from these types of diamondoid compounds]

(wikipedia:oxoacid)

Silicates (& Quartz)

Silicates typically have pretty good mechanical properties.
Typically Mohs 5-6 sometimes up to almost ~8.

Of interest as base materials may be the pure end members of the mixing series of olivine (wikipedia) / peridot (wikipedia)
From Mg2SiO4 forsterite to Fe2SiO4 fayalite.
And especially the associated high pressure modifications.
High pressure modifications tend to have higher crystal symmetries and mechanical strength at the cots of a bit of thermal stability.

Olivin/Peridot end-members and their low pressure stable stable high pressure modifications

Low pressure magnesium endmember forsterite:

  • Mg2SiO4 Forsterite – orthorhombic dipyramidal – Mohs 7

High pressure modifications of fortserite:

Low pressure iron endmember fayalite:

High pressure modifications of fayalite:

  • γ-Fe2SiO4 ahrensite – cubic – 4.85g/ccm – high pressure crystal structure of fayalite – 3D structure (de)

Neo-polymorphic transitions

The low pressure modifications forsterite and fayalite are isostructural (atoms at the same places).
This allows for neo-polymorphs spanning the natural mixing series. Just checkerpatterned as deisred.
(wiki-TODO: Magnesium and iron are pretty far apart in the periodic table – find out why they behave so similar electronically in these minerals)

Q: Is there an iron analog to wadseylite?

Ringwoodite (Mg) and ahrensite (Fe), while both cubic, have quite different structure. No neo-polymorphs here.
But Maybe unnatural iron-ringwoodite and magnesium-ahrensite can be piezosynthesized.
At least a little bit of substitution should work if electronic similarity still holds in these high pressure modifications

Silicates of further rather common elements

Calcium:

  • γ-Ca2SiO4 Calcio-Olivine – Mohs 4.5 – orthorhombic
  • β-Ca2SiO4 Larnite – Mohs 6 – monoclinic (?)
  • Ni2SiO4 Liebenbergite (de) – (Mohs 6-6.5 or 4.5?) -- orthorhombic
  • Mn2SiO4 Tephroite (maybe less interesting since Mn is more scarce) – orthorhombic dipyramidal – Mohs 6
  • TiSiO4 Titanium Silicate (no natural mineral here?) [1] (broken)

Beyond that adding one more element there are an innumerable amount of natural silicates around.

Some semi random picks of other silicates

Misc, Not exactly a salt but related ...

  • PbCa3Zn4(SiO4)4 esperite [2] Mohs 5-5.5 (unabundant zinc | exceptionally hard lead mineral) Specific gravity: 4.28-4.42

Other pages listing silicates of interest

Context specific silicates are also listed on these pages:

Phosphate minerals

(wikipedia)

  • Especially interesting: Hydoxy- Fluor- & Clorapatite Ca5(PO4)3(F,Cl,OH) - (Mohs 5 defining mineral) - a biomineral [3]
  • Lazulite (wikipedia) & Scorzalite (wikipedia) magnesium iron aluminium phosphates - (Mohs 5.5-6)
  • Phosphophyllite (wikipedia) - zinc iron phosphate - rather soft (Mohs 3.5)
  • Pb5(PO4)3Cl pyromorphite (wikipedia) - relatively hard for a lead mineral (Mohs 3.5)
  • Y(PO4) Xenotime (wikipedia) - rare yttrium - high symmetry (tetragonal) - (Mohs 4.5)
  • Al2(PO4)(OH)3 augelite [4] Mohs 4-4.5
  • FePO4 heterosite Wikipedia:Heterosit(de) Mohs 4-4.5
  • MnPO4 purpurite [5] Mohs 4-5 (unabundant manganese)
  • (MgFe)2PO4F magnesium and iron wagnerite [6] Mohs (?)
  • FeAl2(PO4)2 anhydous iron scorzalite [7] Mohs 6
  • MgAl2(PO4)2 anhydous magnesium scorzalite Mohs 6
  • Zn2Fe(PO4)2•4H2O phosphophyllite [8] Mohs 3.5 (soft)

Carbonate minerals

(wikipedia - minerals) (wikipedia - artificial)

  • Calcite Mohs 3 (defining mineral) & Aragonite Mohs 3.5-4 - calcium
  • Magnesite Mohs 3.5-4.5 - magnesium
  • CaMg(CO3)2 Dolomite [9] – Mohs 3.5-4 – Trigonal Rhombohedral
  • Siderite - iron
  • Smithsonite - zinc
  • Malachite & Azurite - copper
  • Rhodochrosite - manganese (rare)
  • ...
  • Mg3Ca(CO3)4 Huntite [10] Mohs 1-2 (very soft!)
  • PbCO3 Cerussite [11] Mohs 3-3.5 (soft)

Sulfate minerals

(wikipedia)

Sulfate minerals are generally rather soft with few exceptions.
One of the harder ones is brochantite (wikipedia) - (Mohs 3.5-4)

  • CaSO4 anhydrite [12] (decomposes slowly to hydroxyde gypsum)

Borate minerals

(wikipedia)

  • Al6B5O15(F,OH)3 Jeremejevite (wikipedia) - (Mohs 6.5-7.5)
  • Mg7(BO3)3(OH)4Cl Karlite (wikipedia) - (Mohs 5½)
  • Ca2B5SiO9(OH)5 Howlite (wikipedia) - soft (Mohs 3.5)
  • MnSn(BO3)2 Tusionite (wikipedia) - (Mohs 5-6) - tin
  • CaZrAl9O15(BO3) Painite (wikipedia) - rare zirconium (Mohs 8)
  • ...

Nitrate and Aluminates

All rather water soluble. See: (wikipedia - minerals) (wikipedia - artificial)

Aluminates: (wikipedia)

Salts of metal oxoacids

TODO ...

Related

External links

Wikipedia:

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