Abstract: Larderite is one of the most famous carving stones in China, which is widely rewarded and respected by all walks of life for its gorgeous colour and fine texture.With the ever-increasing demand of the larderite, extensive researches on its gemmological and mineralogical properties were carried out.
This paper investigated two specific kinds of red larderite called Yuewei stone and Laoling stone on their gemmological and mineralogical properties, chemical compositions, mineral components and origin of colour by using conventional gemmological instruments, XRD, Fourier transform infrared spectroscopy(FTIR), laser Raman spectroscopy and environmental scanning electron microscope(ESEM) with energy dispersive spectrometer(EDS).
Differences exsit between Yuewei stone and Laoling stone in visual inspection.Compared with Laoling stone, Yuewei stone is lighter in colour, milder in luster and clearer in transparency.Bright red spots and dark red veins can be easily seen with naked eyes in Yuewei stone while only red veins in Laoling stone.Flash of cleavage plane can be seen in Yuewei stone which is then identified as diaspore in further study.While light yellow and yellowish white minor minerals can be seen in Laoling stone, which is later identified as rutile in further study.
The major ingredient of both stones is pyrophyllite.Observed by microscope, it is colourless and the crystal is very tiny, with no apparent boundary, and its interference colour is secondary blue and green.Characteristic peaks at 9.132, 4.579, 2.295, 3.056 Å of pyrophyllite can be seen in XRD results.Detailed research shows that pyrophyllite in Yuewei stone is 2M₁ type, while in Laoling stone a mix of 2M₁ type and 1Tc type.
IR spectra show that the spectra of both stones are similar, with 3 675cm^-1 Al-(OH) stretching vibration, 1 122cm^-1 Si-OH stretching vibration, 1 069cm^-1 Si-O-Si stretching vibration, 949cm^-1 Al-(OH) in-plane swing, 853, 835, 813cm^-1 Al-(OH) out-ofplane swing and 540, 483, 418cm^-1 Si-O swing.The result indicates that the major ingredient of both stones is pyrophyllite.Raman spectra show characteristic peaks at 195, 215, 262, 706, 814cm^-1 indicating pyrophyllite.From the results of ESEM, major minerals are in imbricate arrangement, plate-type structure or fragmentary structure.The EDS test shows that the elementary composition is w(Al)=11%, w(Si)=24%, w(O)=65%, which is in line with Al₂Si₄O₁₀(OH)₂.
Apart from pyrophyllite, those diaspore and hematite were found in Yuewei stone, while hematite, rutile, zircon, muscovite and chlorite, in Laoling stone.In Yuewei stone, diaspore can be divided into two types, well-crystallized one and poorly crystallized one, additionally, hematite appears as spot or vein, quite small and poor crystallized.
In Laoling stone, hematites are light brown, and some appear as spot or vein unevenly in pyrophyllite, some centre on rutile core, and some are well-crystallized as single crystal.Rutile is dark brown, darker than hematite, and appears as spot or needle.Around the spot ones, there are always hematite veins, and the needle ones are always arranged.Besides, some rutiles are colored dark green.Zircon always distributes at the edge of hematite and rutile.