AN UPDATE ON COLOR IN GEMS. PART 2: COLORS INVOLVING MULTIPLE ATOMS AND COLOR CENTERS By Emmunuel Fritsch and George R. Rossinun This is the second part in a three-part se- n the first part of this series (Fritsch and Rossman, 1987)) ries on the origin of color in gem male- I we reviewed how absorption of light by a dispersed metal rials. This article discusses colors pro- ion such as chromium (Cr3+)can produce color in a gem duced by (1) processes that involve multi- material such as ruby. However, color in gems is also ple atoms (e.g., blue sapphire, organic commonly produced by a variety of processes that involve products), and (2) a varieiy of defect the combined presence of two or more ions (e.g., charge- stnlctures that are generally created by irrtjdiation (ntjtr~mlor artificial), 1znow1 transfer processes), or by irradiation sometimes combined collective4y ps "color centers" (e.g., green with heating (the format'ion of a "color center"). In this diamond). ; article, we will describe and explain these mechanisms through a variety of examples, some of which are illus- trated in figure 1. The reader who desires more information about the origin of color in gem materials will find the reviews by Nassau (1975)) Loeffler and Burns (1976), Marfunin (1979),and Fritsch (1985)particularly helpful. It should be noted that in most cases, color-causing absorp- tion bands are so broad that they cannot be well resolved with a hand spectroscope. If visible, they usually appear as a broad smudge or as a gradual darkening toward one end of the visible range. PROCESSES INVOLVING MULTIPLE ATOMS Charge Transfer: When an Electron Jumps from One Atom ABOUT THE AUTHORS to Another. Color is caused by dispersed metal ions when Dr. Fritsch is research scientisl at the Gernologi- electrons undergo transitions between atomic orbitals cal Inslitute of America, Santa Monica, California. confined to a single ion (seepart 1 of this series, Fritsch and Dr. Rossrnan is professor of mineralogy a1 the California Institute of Technology, Pasadena, Cali- Rossman, 1987). During this transition, the electrons fornia. never leave the central atom. There are, however, a number Acknowledgments: E.F. wishes lo thank Professor of different ways that electrons can jump from one atom to Georges Calas, of the University of Paris VII, for another. When this happens, spectacular colorations may his help and encouragement in writing lhe original result. One of these processes is called charge transfer French version of th~sarticle. Special acknowl- edgment is given to Pat Gray for lyp~ngthe manu- because it can be described in simple terms as a mecha- scri,ot and improv~ngthe translation. Particular ap- nism that transfers a negative charge (i.e., an electron) from preciation is due to Laurel Barllelt and John Hum- one atom to another. me1 for their conslructive comments on this arti- cle. Jan Newel1 and Peter Johnston drew lhe line ~llustralions. When ~7nElectron Visits Its Nearest Neighbor: Oxygen + O 7988 Gemologica! Institute of America Metal Charge Transfer. Heliodor, the golden variety of Color in Gems, Part 2 GEMS & GEMOLOGY Spring 1988 3 Figrire I. Colors in gem materials can be caused by a number of different processes. This article concentrates on colors caused by processes in- volving multiple atoms and color centers. Exam- ples of such colorations are illustrated here (see tables 1 and 2 for the specific origin of color for each stone). Clockwise, from top right: pink fluo- rite, violet scapofite, greenis11 yellow tour- maline, andalusite, ame- thyst, crocoite, citrine, and yellow fluorite, with blue liyanite In the cen- ter. Stones are from the I -, GIA collection or coilr- , ..' , --- _ . 4, - Y.4. tesy of Pete Flusser, ..C - .- +-a- Overland Gems, Inc. .. h-,% ... .... - Photo 0 Tino Hammid. a-, .. .-- l - - beryl (seefigure 2),is colored by Fe3+ as it interacts in the beryl structure. The color results from light with other atoms in the beryl structure. The absorbed through the transfer of electrons from the transitions confined to the Fe"+ ion absorb in the oxygen ions to the iron ion. This color can also be blue and violet portion of the spectrum; they are induced artificially by the irradiation of blue beryl very weak and malze only a minor contribution to (see Fritsch and Rossman, 1987). the color. The deep yellow of heliodor is caused Usually oxygen + metal charge-transfer ab- mainly by an extremely strong absorption cen- sorptions are centered in the near ultraviolet and tered in the ultraviolet that extends into the blue are broad enough to extend into the blue end of the end of the visible spectrum and absorbs violet and visible spectrum, producing yellow to orange to blue (Loeffler and Burns, 1976). This ultraviolet brown colors. Their energy is fairly independent of absorption is due not to Fe3 + alone but rather to an the nature of the host mineral. The 02-+Fe3+ interaction between Fe3 + and its oxygen neighbors absorption described above for beryl is found at 4 Color in Gems, Part 2 GEMS & GEMOLOGY Spring 1988 transition is 100 to 1,000 times stronger than that resulting from an internal transition confined to the isolated ion (Mattson and Rossman, 1987a, 1987b). Consequently, charge-transfer colors are more intense than those caused by dispersed inetal ions. Of course, the intensity of the color also depends on the extent to which the charge-transfer absorption occurs in the visible portion of the spectrum. When an Electron Visits Its Next-Nearest Neigh- bor: Intervalence Charge Transfer. In some cases, an electron can travel further than one ion away, Figure 2. Some yellow sapl111ire.s (23.78 ct, left) such that two inetal ions separated by an oxygen and all golden beryls (heliodor, 9.94 ct, right) atom call actually exchange electrons, too. This owe their color to 0'-+Fe3' charge transfer. process can strongly influence the color of a gem. Photo by Shone McClure. When such a transition talzes place between two different oxidation states (e.g., Fe" and Fe3+),it is about the same energy- that is, the same region of called an intervalence charge-transfer (IVCT)tran- the spectrum-in corundum (Tippins, 1970). sition. Therefore, it contributes a similar strong yellow- Because of the natural abundance of iron, orange color to sapphires, despite the fact that the charge transfer is con~n~onlyobserved between details of the coordination environment are quite Fez+ and Fe" + A~populargemstone colored by this different,for the isolated ion in these two minerals type of lVCT is darlzer blue aquamarine, like the (in contrast, see Fritsch and Rossman, 1987, for the material from Coronel Murta (Brazil) and difference'in color produced by chromium [Cr3+] Tongafeno (Malagasy Republic). As we have seen in beryl [emerald]and corundum [ruby]). already, the presence of Fe"+ induces the absorp- The center of the oxygen 3 metal ion charge- tion of at least part of the violet, through 02-3 transfer absorption band moves from the ultravio- Fe3+ charge transfer. But when Fez+ is also present let toward the visible region as the positive charge in the adjacent site, the Fez+ +Fe3+ IVCT strongly of the central metal ion increases. 02-+Fez+ absorbs in the red end of the spectrum (Goldman et charge transfer is centered well into the ultraviolet al., 1978).As the violet is already suppressed, this and has minimal effect on color, whereas 02-+ leaves a transmission window in the blue. Fe3+ charge transfer is centered at the edge of the When there is much more Fe3 + than Fez + , the ultraviolet, and extends into the visible region, 02- +Fe3 + charge transfer also absorbs part of the generally causing yellow to brown hues. Finally, blue and results in a more greenish color. Heat 02- +Fe4+ transitions absorb light in the middle treatment in a reducing atmosphere is used to of the visible region, giving amethyst, for example, change part of the Fe3+ to Fez+ (Nassau, 1984). its purple color (Cox, 1977). This moves the transmission window back into Even ions that are not lilzely to generate color the blue, and produces a more saleable stone. by themselves can do so with oxygen + metal Intervalence charge transfers also occur be- charge transfer. For instance, the chromate group tween metal ions of different chemical elements. (Cr6+O4)gives the mineral crocoite its bright Such is the case for the FeZ++Ti4+ charge transfer orange-red color (see figure 1). The oxygen ions that gives sapphire its blue color (figure 3). The transfer some of their electrons to the chromium heat treatment of near-colorless "geuda" sapphires ion in such a way that transitions absorbing in the produces blue stones because naturally occurring near ultraviolet and in part of the violet and blue inclusions of rutile and spinel in the corundum are possible (Loeffler and Burns, 1976). dissolve at high temperatures. Fe and Ti from the Transitions associated with a single metal ion inclusions are released and, through random diffu- have a much lower probability of occurring than do sion, form pairs of Fez+ and Ti4+ ions which charge-transfer transitions. In the case of Fe3+, the interact through Fe2++Ti4+ IVCT to yield the absorption resulting from one charge-transfer blue coloration (Harder and Schneider, 1986). The Color in Gems, Part 2 GEMS 8: GEMOLOGY Spring 1988 5 As one can appreciate from the former exam- ples, charge-transfer processes generally create a strong pleochroism. To understand why the ab- sorption is more intense in one direction than the other, it is necessary to know the exact location of the ions involved (see, e.g., figures 4 and 5).