Serpentine stone (magnesium iron silicate hydroxide) has a striped pattern like a snake and is used as an admixture for mortar plastering by smashing it. There are three important mineral polymorphs of serpentine: antigorite (non-asbestos mineral), lizardite and chrysotile (asbestos mineral).
X-ray diffraction cannot discriminate among these polymorphs because their peaks appear at nearly the same position in the spectrum. Therefore, the amount of chrysotile in the admixture for mortar plastering is obtained from the difference of the dehydration temperature by means of differential thermo-gravimetric analysis (DTG)1. The DTG method requires only 10 to 20 mg of sample, so if the location of chrysotile is determined beforehand using X-ray diffraction, it is easy to sample the stone at the measurement position.
Measurement of a small region ( ≤ 1 mm diameter ), Part I:
For a highly sensitive analysis of a small (≤ 1 mm diameter) section of serpentine stone, Rigaku's Ultima IV multipurpose diffraction system was equipped with Cross Beam Optics (CBO), a 2 kW sealed tube X-ray source and a high-speed one dimensional detector.
The X-ray diffraction patterns shown in the figure below are for two small sections (one white, one black) of a serpentine stone. The sections are circled in red in the inset, above a millimeter ruler for scale.
The database reveals that the white portion consists primarily of calcite (calcium carbonate) and the black portion includes chrysotile, antigorite and lizardite.
1 As ordered by the Ministry of Health, Labor and Welfare of Japan in July, 2004.