Abstract: As a noted member of seal stones, red-brown larderites fascinate people by their unique colours. A series of modern test technologies have been employed to study the cause of colour of Gaoshan stone and Duchengkeng stone, which are varieties of red-brown larderites.
Petrographic microscopy shows that dark red minerals in the Gaoshan stone samples exist in two forms.Intact crystallized minerals are surrounded by minerals arranged in cluster,which jointly formed unique red dots in the surface of the stone.While the vivid red dots on Duchengkeng stones are made up by the fine and disarrayed dark red mineral crystals.
X-ray powder diffraction spectrometer results indicate that both Gaoshan stone and Duchengkeng stone samples are in major composed of dickite with a modicum of kaolinite and quartz.Raman spectra show that red minerals in both crystallization forms in Gaoshan stone samples are hematite.And in Duchengkeng stone samples,the well-crystallized dark redminerals are rutile while the small and cluttered red-brown minerals are hematite.
Energy dispersive X-ray fluorescence analysis and SEM indicate that dickite is the ground mass of Gaoshan stone and Duchengkeng stone and both species contain the elements Al,Si,O,with Fe as the cause of colour.
It can be deduced that red-brown colour of Gaoshan stone is originated from hematite while the red-brown colour of Duchengkeng stone is caused by both hematite and rutile.