Spectroscopic and microscopic (particularly HRTEM) techniques were used to investigate
the origin of the colour of natural blue Zn-carbonate (smithsonite). Blue smithsonite is
rich in copper, but substitution of zinc cations by copper cations, as proposed in the past
for the origin of the colour, is questionable considering the absence of anhydrous divalent
copper carbonates in nature. In this work, optical microscopy, SEM-EDS, XRD and laser
micro-Raman could not resolve distinct phases either than Zn-carbonate, while NIR spectra
excluded known chromophore Cu-hydroxycarbonate minerals. HRTEM studies however
could clearly resolve nano-sized (3-7 nm) Cu-rich inclusions (specifically Si/Ca/Cu/As-rich
inclusions of at least one phase), which are organised in bands with no topotaxial relation
to bulk smithsonite. Electron-beam sensitivity of the samples, even at low electron current
densities, did not allow the exact identification of the inclusions. However, it can be safely
suggested, for the first time in the literature, that they are the cause of the blue colour in
smithsonite
