Study on Gemmological Characteristics of a Kind of Imitated Larderite

Some samples claimed as larderite were received at Yanshan University Gemstone Identification and Research Center recently. The refractive index, density and hardness of the samples were studied by using the conventional gemmological methods. The results show that the refractive index was 1.53, the density was 2.31 and the hardness was 2.5. Although the samples were very similar to larderite in many aspects, such as in colour, refractive index, hardness and so on, its density was slightly lower. X-ray diffraction (XRD) and Fourier transform infrared spectrum (FTIR) were also used to identify the samples in order to decide whether they were larderite or not. The results of XRD and FTIR show that the samples were a kind of imitation since they were composed of gypsum and calcined gypsum with uncertain water content, and that the minerals such as dickite, nacrite, pyrophyllite and ilite were not found in the samples. The imitation was very similar to larderite both in appearance and in parameters except that the density of the imitation was a lttle lower. The density of larderite should be tested in the routine check at the inspection stations. If the density of the stone is lower than the ordinary level, other methods should be used to make further identification.