Tutton's Salts

A2B(SO4)2.6H2O where A is univalent, K or larger, normally NH4, and B is divalent. B must exist as hexaaquo ion. So, there is in fact no water of crystallization. These compounds are usually highly stable due to crosslinking hydrogen bonds, being resistant to oxidation and erosion in air even if B is Cr, Fe or V.

Simply mix the sulfates in solution, and they quickly reacts to form double salt crystals. Even solid sulfates are OK, as a saturated solution of Tutton's salt is not saturated for the simple salts.

Monoclinic. If you use no fishing string like me, you may get two different shapes due to different directions(see below), and you can manually turn the diamond one to the brick one by simply turning the seed upside down.

It seems that +3 cation strongly impacts transparency.

Known examples:

• Mg: TODO, maybe just similar to the Zn one.
• V: TODO, purple, extremely reducing in solution. This is far harder than I thought, and crystals shown here are accidentally dissolved. Paraffin liquid is not enough to stop oxygen.
  
  
  
• Cr: TODO, extremely reducing in solution and Cr2+ is Jahn-Teller distorted. May be very similar to the Cu one. Haven't succeeded.
• Mn: Pink. Highly soluble(close to 2mol/L), maybe due to Mn2+'s high radius. Somewhat reducing in solution, produces MnO2, but does not matter. Shape seems to be different from others.
  
• Fe: TODO, reducing in solution.
• Co: Dark red, I've never seen such a deep color in a +2 aquo ion! CoSO4 can be prepared with CuSO4 and Co. Acid and Co is quite slow.
  

  
  
  
• Ni: TODO
• Cu: Light blue. Pure compound has extremely irregular shape and is very opaque, probably due to Jahn-Teller effect of Cu2+. However, mixing it with the Zn salt at Zn:Cu=4:1 produces good crystals, even better than pure Zn one.
  

  
• Zn: Colorless. Nothing to say, but mold may appear. I added some potassium sorbate.
  
• Ru: TODO, but I must know how to deal with damn RuO4 and get enough money first! Very reducing in solution.
• Cd: TODO, but this element is way too dangerous by any mean...

Good luck!