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 scrystal symmetries and mechanical strength at the cots of a bit of thermal stability.

Low pressure magnesium endmember forsterite:

  • Mg2SiO4 Forsterite – orthorhombic dipyramidal – Mohs 7

High pressure modifications of fortserite:

  • Mg2SiO4 wadseylite (wikipedia) – sorosilicate – ortorhombic – dipyramidal – Mohs ?? – mid pressure crystal structure
  • Mg2SiO4 ringwoodite (wikipedia) – nesosilicate – cubic – Mohs ?? – 3.9g/ccm – high pressure crystal structure

Low pressure iron endmember fayalite:

  • Fe2SiO4 Fayalite – orthorhombic dipyramidal – Mohs 6.5-7.0 – 4.39g/ccm – (de)

High pressure modifications of fayalite:

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

Silicates of further rather common elements:

  • 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.

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

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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