In 1646 chemist Johann Glauber dropped a crystal of ferrous chloride (FeCl2) into a solution of potassium silicate (K2SiO3) and within minutes saw “philosophical trees,” the name he coined for the plant-like shapes that emerged during the reaction. More than three and a half centuries later, scientists still only qualitatively understand how the mineral scapes, now known as chemical gardens, grow into their rich variety of structures. Making them could hardly be simpler: Precipitates are produced when a metal salt dissolves in an alkaline solution of anions, such as silicate, phosphate, or carbonate. That simplicity makes chemical gardens a popular and dramatic demonstration among high schoolers. The science behind the drama, though, is not so simple.
As the salt crystal dissolves, it becomes enveloped in an insoluble semipermeable membrane of the solid metal silicate. Osmotic pressure pulls water from the outside to the inside...
