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Cause of Color Modification in Tanzanite After Heat Treatment

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Cause of Color Modification in Tanzanite After Heat Treatment molecules Article Cause of Color Modification in Tanzanite after Heat Treatment Teerarat Pluthametwisute 1,*, Bhuwadol Wanthanachaisaeng 2, Chatree Saiyasombat 3 and Chakkaphan Sutthirat 1 1 Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand; [email protected] 2 Gem and Jewelry Program, College of Creative Industry, Srinakharinwirot University, Bangkok 10110, Thailand; [email protected] 3 Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima 30000, Thailand; [email protected] * Correspondence: [email protected] Academic Editor: Michael Bolshov Received: 11 July 2020; Accepted: 14 August 2020; Published: 16 August 2020 Abstract: Natural tanzanites usually show strongly trichroic coloration from violet to blue, and brown colors in different directions. However, this characteristic is easily changed to violet-blue dichroism after heat treatment. Moreover, the cause of color modification after heating is still controversial. A few researchers have previously suggested that trace amounts of either vanadium or titanium substituted in aluminum site should be the main determinant of color after the heat treatment. Alteration of either V3+ to V4+ or Ti3+ to Ti4+ may relate to light absorption around 450–460 nm, which is the main cause. UV/vis/NIR absorption spectroscopy and X-ray absorption spectroscopy (XAS), a utility of synchrotron radiation, were applied for this experiment. As a result, the violet-blue absorption band (centered around 450–460 nm) as well as green absorption band (centered around 520 nm) were obviously decreased along the c-axis after heating, and XAS analysis indicated the increasing of the oxidation state of vanadium. This result was well supported by the chemical composition of samples. Consequently, vanadium was strongly suggested as the significant coloring agent in tanzanite after heat treatment. Keywords: tanzanite; heat treatment; zoisite; color 1. Introduction Tanzanite, a violet-blue vanadium-bearing variety of zoisite (Ca2Al3[Si2O7][SiO4]O(OH)) [1], has been long known and has become a highly demanded precious stone in the world gem market owing to its fantastic color appearance. Natural tanzanites usually show strong trichroism between violet, blue, and brown (or green) colors (Figure1). Tanzanite was first discovered in 1967 [ 2] at Merelani area, north-eastern Tanzania (East Africa), by Jumanne Ngoma, a gypsum miner who was officially acknowledged by the Tanzanian government. The deposit only covers around 7 kilometers and its estimated life expectancy for mining is, unfortunately, less than 20 years [3]. Moreover, natural high-quality violet-blue colored tanzanites are rare; therefore, heat treatment has been applied to lower quality stones owing to the high demand in the market. Molecules 2020, 25, 3743; doi:10.3390/molecules25163743 www.mdpi.com/journal/molecules Molecules 2020, 25, 3743 2 of 15 Molecules 2020, 25, 3743 2 of 15 FigureFigure 1.1. TrichroismTrichroism ofof aa naturalnatural tanzanitetanzanite rotated in three directions directions showing showing violet, violet, blue, blue, and and green green pleochroicpleochroic colorscolors(photo (photo byby T.T. Sripoonjan,Sripoonjan, GIT). Zoisite,Zoisite, includingincluding tanzanitetanzanite variety, is an orthorhombic sorosilicate sorosilicate mineral mineral whose whose ideal ideal formula formula cancan bebe defineddefined as A A22MM3Si3Si3O3O12OH12OH [4], [ 4where], where A sites A sites are generally are generally substituted substituted by divalent by divalent cations cations (e.g., (e.g.,Ca, Mn, Ca, Sr, Mn, Pb), Sr, while Pb), whileM sites M are sites mainly are mainly occupied occupied by trivalent by trivalent cations cations(e.g., Al, (e.g., Fe, Mn, Al, Cr, Fe, and Mn, V) Cr, and[5–7]. V) Its [5 –structure7]. Its structure contains contains the base the of base edge of sharing edge sharing octahedral octahedral (M1,2) (M1,2)chain linked chain linkedby double by double Si2O7 (T1, T2) and single SiO4 (T3) tetrahedra. This edge sharing octahedral chain in zoisite also has Si2O7 (T1, T2) and single SiO 4 (T3) tetrahedra. This edge sharing octahedral chain in zoisite also has additionaladditional M3M3 octahedraoctahedra attachingattaching on one side of the chain [7] [7] (Figure (Figure 22).). The structure structure of of zoisite zoisite is is slightlyslightly di differentfferent from from clinozoisite clinozoisite (a member (a member of epidote of epidote supergroup), supergroup), with the with additional the additional M3 octahedra M3 attachingoctahedra on attaching both sides on both along sides the along b-axis the of b-axis the octahedral of the octahedral M1 chain M1 andchain the and octahedral the octahedral M2 chain M2 beingchain isolatedbeing isolated from the from M3-combined the M3-combined M1 chain M1 (see chain also Figure(see also2). AccordingFigure 2). toAccording this information, to this theinformation, Commission the on NewCommission Minerals, on Nomenclature, New Minerals, and ClassificationNomenclature, of the and International Classification Mineralogical of the AssociationInternational (IMA) Mineralogical has recently Association defined all (IMA) members has of recently epidote supergroupdefined all asmembers monoclinic of minerals,epidote andsupergroup thus tanzanite, as monoclinic a variety minerals, of orthorhombic and thus tanzanite, sorosilicate a variety zoisite, of orthorhombic no longer belongs sorosilicate to the zoisite, epidote no longer belongs to the epidote supergroup [8–10]. supergroup [8–10]. Molecules 2020, 25, 3743 3 of 15 Molecules 2020, 25, 3743 3 of 15 FigureFigure 2. 2. (a()a Zoisite) Zoisite octahedral octahedral chain chain (M1, (M1, 2) 2) combined combined with with additional additional octahedra octahedra (M3) (M3) on on only only one one sideside along along b-direction, b-direction, whereas whereas (b (b) )additional additional octahedra octahedra (M3) (M3) located located on on both both sides sides of of clinozoisite clinozoisite octahedraloctahedral chain chain (M1) (M1) with with (c ()c isolated) isolated octahedral octahedral chain chain (M2) (M2) (modified (modified after after Franz Franz and and Liebscher Liebscher [7]). [7]). TanzaniteTanzanite has has been been heated heated to to improve improve the the color color to to sapphire-like sapphire-like blue blue color, color, which which is is highly highly demandeddemanded in in the the market market [2,11]. [2,11]. It It is is believed believed that thethe violet-blueviolet-blue color color in in the the tanzanites tanzanites is is owing owing to to the thepresence presence of vanadium,of vanadium, which which replaces replac aluminumes aluminum in the octahedralin the octahedral site [3,5 ,site12–17 [3,5,12–17].]. Heating Heating tanzanite tanzaniteat approximately at approximately ~500 ◦C usually~500 °C producesusually produces the disappearance the disappearance of the yellow of the color yellow in thatcolor particular in that particulardirection direction in which thein which absorption the absorption around 450–460 around nm 450–460 is decreased, nm is anddecreased, alteration and of alteration pleochroism of pleochroismfrom trichroic from to dichroictrichroic violetto dichroic to blue violet color to [3 blue,13,14 color,17]. [3,13,14,17]. Heating tanzanite Heating at tanzanite too low or at too too high low a ortemperature too high a temperature may lead to may never lead reaching to never the reaching desirable the color desirable alteration; color noalteration; color alteration no color isalteration observed isbelow observed 250 below◦C[13 250], whereas °C [13], colorwhereas appears color toappears be faded to be at faded a high at temperature a high temperature [3]. In situ[3]. In heating situ heatingspectroscope spectroscope was used was to revealused to the reveal optimum the temperatureoptimum temperature (550 ◦C), which (550 causes °C), which the disappearance causes the disappearanceof the absorption of the around absorption 450–460 around nm [3 ].450–460 Color alterationnm [3]. Color behavior alteration after heat behavior treatment after can heat also treatmentbe observed can also in other be observed gemstones. in other For gemstones. example, blueFor example, intensification blue intensification and removal and ofgreen removal tint of in green tint in blue-colored variety (aquamarine) of beryls were performed by heating at 300–700 °C, leading to the reduction of Fe3+ to Fe2+. Consequently, Fe3+ absorption bands at 375 and 425 nm are Molecules 2020, 25, 3743 4 of 15 blue-colored variety (aquamarine) of beryls were performed by heating at 300–700 ◦C, leading to the reduction of Fe3+ to Fe2+. Consequently, Fe3+ absorption bands at 375 and 425 nm are decreased, while Fe2+ band at 810 nm is significantly intensified. However, heating aquamarines at higher temperature (e.g., 1100 ◦C) may lead to an opaque appearance, which relates to the disappearance of the 810 nm absorption band [18]. Another example is in diaspore (zultanite), where the chromophores involved in the variation in the color are owing to the presence of Ti-Fe and are associated with the intervalence-charge-transfer mechanism [19]. However, coloration in tanzanite after heat treatment has been still a questionable subject for decades. Previous researchers speculated that coloration of heated tanzanite is owing to oxidation changing of either vanadium or titanium. Vanadium may have been oxidized from V3+ to V4+ after heat treatment [5,14]; however, Olivier [3] argued
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