Rubidium (Rb #37) is the next element on the periodic table for my APEX mineral collection.
Rubidium starts row 4 (Period 3) on the periodic table.
Rubidium Minerals - The very few Rubidium minerals which exist are ionic compounds (salts).
Pollucite ((Cs,Na)2Al2Si4O12•(H2O)) with 1% Rubidium is the only practical choice for my APEX mineral. It is only found at Bernic Lake, Manitoba, Canada. Lepidolite is mined for Rubidium traces but this mineral formula really doesn't contain Rubidium.
Rubidium is more common than Copper. It readily substitutes for Potassium in minerals, and is therefore fairly widespread. The problem is that there are no geologic processes which concentrate Rubidium. A lack of need and a lack of good ores has limited Rubidium production to 4 tons per year. Rubidium melts at a low temperature and reacts violently with water and air so pure element must be stored carefully.
Rubidium is not known to be necessary for any living organisms. However, Rubidium is easily substituted for Potassium in biologic functions. The only downside to this is that some Rubidium isotopes are radioactive which can pose a danger.