Calcite microcrystals in the pineal gland of the human brain: second harmonic generators and possible piezoelectric transducers

A new form of biomineralization has been studied in the pineal gland of the human brain. It consists of small crystals that are less than 20 μm in length and that are completely distinct from the often-observed mulberry-type hydroxyapatite concretions. Energy dispersive spectroscopy, selected-area electron diffraction and near infrared Raman spectroscopy established that the crystals were calcite. With the exception of the otoconia structure of the inner ear, this is the only known non-pathological occurrence of calcite in the human body. The calcite microcrystals are believed to be responsible for the previously observed second harmonic generation (SHG) in pineal tissue sections. There is a strong possibility that the complex twinned structure of the crystals may lower their symmetry and permit the existence of a piezoelectric effect.