Salts of oxoacids

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 supralubricating bearings can be constructed from these types of diamondoid compounds]

(wikipedia:oxoacid)

Silicates (& Quartz)

Not exactly a salt but related ...

Phosphate minerals

(wikipedia)

• Especially interesting: Hydoxy- Fluor- & Clorappatite Ca₅(PO₄)₃(F,Cl,OH) - (Mohs 5) - a biomineral
• 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)
• YPO₄ Xenotime (wikipedia) - rare yttrium - high symmetry (tetragonal) - (Mohs 4.5)

Carbonate minerals

(wikipedia - minerals) (wikipedia - artificial)

• Calcite & Aragonite - calcium
• Magnesite - magnesium
• Siderite - iron
• Smithsonite - zinc
• Malachite & Azurite - copper
• Rhodochrosite - manganese (rare)
• ...

Sulfate minerals

(wikipedia)

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

Borate minerals

(wikipedia)

• Mg₃B₇O₁₃Cl Boracite (wikipedia) - (Mohs 7-7.5)
• 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 minerals

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