Identification and Spectroscopy Characteristics of Three Natural Minerals Similar to Turquoise

Turquoise is a natural mineral with a blue or bluish green colour，which has a long history in the jewelry market.Due to the contradiction of the high demands for the fantasy appearance and the small output，there are lots of treated products and artificial imitations.Yet some natural minerals with similar appearance of turquoise begin to emerge in the market as the exploitation of new mineral resources. Turquoise and its three similar minerals are observed and tested in order to determine the mineral components and find an efficient way to distinguish them. Basic gemmological parameters and spectroscopy characteristics, such as infrared spectra and laser Raman spectra, are obtained. The results of X-ray diffraction are analyzed. According to all the tests, it is found that the three similar minerals are gibbsite, variscite, and hemimorphite respectively. Based on the appearence and basic gemmological parameters, it can be seen that gibbsite has a loose structure and a low hardness so that it can be carved into powder easily. Variscite is brittle with the lowest refractive index. There may be brown surrounding rock and infection in the rough stone of variscite. Hemimorphite is a kind of mineral aggregate with an appearance of columnar．And there is a group of perfect cleavage in hemimorphite．Based on the former two features，hemimorphite call be identifie In all，by testing basic gemmological parameters，such as refractive index，hardness，specific gravity and fluo rescence，three samples can be distinguished preliminarily．Besides，spectroscopv of each mineral is typical．As a result，it can be accurate to distinguish turquoise from three similar minerals．In the infrared spectrum，the main peak of gibbsite is related to stretching vibra tion of Al—O—H(1 030 cm ^-1)．Variscite shows a characteristic infrared sDectrum caused by phosphate group with a main peak at 1 075 cm^-1 corresponding to asymmetrical stretching vibration of P—O．The infrared spectrum of hemimorphite is composed bv vibrations。f Si—O in silicate，and the main peak at 1 095 cm^-1 is caused by asymmetrical stretching vi bration of Si—O—Si．Turquoise has peaks related to phosphate and hydroxyl in the infrared spectrum．The Raman spectra of minerals are also distinctive．The Raman sDectrum of gibbsite is mainly caused by O—H flexural vibration mode．The peak at 1 018 cm^-1 is corresponding to δ(O—H)in plane bending vibration and the peaks at 892，506 cm^-1 to γ(O—H) out—plane bending vibration．And there is Al—O—H deformation vibrati。n and A1—O—A1 bending vibration(562，538 cm^-1)as well．Peak at 322 cm^-1 with a shoulder of 306 cm^-1 is arisen from δ(A1一O) flexural vibration．Variscite shows a characteristic Raman sDectrum caused by phosphate group，main peak at 1 020 cm^-1 corresponding to v_s(P—O)stretching vibration，1 075 cm^-1 to v_s(P—O)asymmetrical stretching vibration，and the peaks at 515， 450，420 cm^-1 to δ(P—O)．The Raman spectrum of hemimorphite reveals the existence of v(Si—O)(930 cm^-1)，v(Si-O-Si)(683 cm^-1)，v(Si-O—Si)(452，400 cm^-1) and v(Zn—O) (326 cm^-1)．Phosphate group leads to the Raman spectrum of turquoise(1 039，650—540， 500—410 cm^-1)，and there is also a weak band(330，230 cm^-1)，which may have a relationship with copper oxide．From the results of X—ray diffraction，it can be seen that gibbsite mainly consists of AI(OH)₃．But a little cordierite ferroan is mixed．Similarlv，variscite has a certain content of paravauxite which is also ferrous．Fe is found in gibbsite and variscite according to the mineral testing results of X-ray diffraction．It can be c。ncluded that the colour of gibbsite and variscite may be caused bv Fe．