Biaxial Color Centers in Amethyst Quartz
Anisotropy of the characteristic absorption bands in a natural amethyst crystal from Brazil is demonstrated by the dependence of the value of the absorption coefficient upo,n the direction of the electric vector of incident light within the 4.13-0.95 eV region, for differently oriented sections of the crystal. The absorption indicatrix is used for interpreting the observed optical absorptions. A new absorption band in the 6.20 eV region is reported. A complex absorption band in the 5.60 eV region, usually attributed to a transition related to ligand-cation charge transfer in Fe"* compounds, is clearly resolved into at least three bands. The biaxiality and pleochroism of amethyst is explained as due to the existence of strongly anisotropic color centers, related to iron, of orthorhombic or lower symmetry. Upon bleaching the amethyst color, either optically or thermally, the biaxiality disappears. This anomalous biaxiality and pleochroism observed in amethyst quartz is a property oi the color center itself and not a property of the c-quartz Crystal structure.
On the Color Centers of Iron in Amethvst Synthetic Quartz : A Reply
grows upon the fading of the 0 bands as mentioned in the aforementioned Table 2. However, if Lehmann considers the Fe8+ to be so labile in the quartz structure that it can switch from substitutional to interstitial sites and precipitate on heating at 500"C, then it is difficult for us to have a meaningful discussion. If this were true then cation vacancies would be produced in quartz at relatively low temperature. This is counter to the evidence found for Al3+ in the substitutional site (Cohen, 1960). The substitutional Al3+ color center can be regenerated even after heating the quartz through the q-0 transition and then cooling, only the interstitial Al3+ in the quartz being precipitated in the process. (3) Lehmann bases interstitial tetrahedral symmetry of Fe3+ on the absence of ligand field bands in the infrared region. These would be difficult to detect with the total iron content at 120 ppm or below (Lehmann, l97l) as shown in his analyses. Total iron in our samples were all well below 100 ppm/wt. Lehmann states "that these band positions would be compatible with octahedral coordination of this ion, but this possibility has already been excluded by the absence of ligand field bands in the red part of the spectrum." (4) For Lehmann's model to be correct, one must accept that holes cannot be stable near trace iron, thus Fe3+ * hole Fen+. Considering the stability of the Fe3+ half-filled 3d shell and the unstable nature of the 3X state, it is hard to accept that Fe3+ cannot have a stable hole on an adjoining oxygen whereas Al3+ can. However, it should be noted that, in the absence of most other transition ion impurity, Mn3+ (3t) can exist in spodumene (Cohen and Anania, in preparation) so it may be that trace Fen+ exists in qvartz. If this is the case, it would not be reasonable to accept the presence of Fe'+ bands in the Fe2+ region of the spectrum. The Fea+ is similar to the model for the 54504 (d, 18,350cm-') band (Schlesinger and Cohen, 1966), and it is suggested that this band would more likely be related to Fea+ than a band in the Fe2+ spin-allowed region. The crit icism of Lehmann (1975) in the preceding paper as to the neglect of recent literature in the two papers of Cohen and Hassan (1974) might have been tempered if he had noticed that the manuscript was received on January 10,1972. Indeed the papers were written during 1970 and l97l with the authors in two widely separated countries. The actual work was contemporaneous with an earlier paper published in Science (Cohen and Hassan, 1970). I am happy, however, that our results support the work of Lehmann and Bambauer (1973 ) . It might be mentioned here that my published work on color centers in smoky or amethyst quartz consists of some fifteen papers dating from 1954, the flrst paper on amethyst being in 1956. Lehmann's papers until now have neglected to reference any of this work, even though all but four of the papers are prior to his publishing in this field of research. In answer to Lehmann's specific criticisms (the numbers refer to Lehmann's): (l) "The intense band at about 5.6 eV" is not a simple band but consists of a complex band found only in rhombohedral growth in both natural and synthetic quartz. On the side of caution in our assignment of Fe3+ to a given site, we stated "that Fe3+ ion preferentially enters the a-quartz structure in the positive rhombohedral growth regions and mainly interstit ially (in a distorted octahedral environment)." The production of small brown microscopic particles on heating at 500'C strengthens the conclusion that the iron is interstit ial and not substitutional (Cohen and Hassan, 1974). (2) Lehmann uses the words "merely suggests" for our heading "Probable Nature of Center" in Table 2 (Cohen and Hassan, 1974). The term "probable" according to Webster: "Supported by evidence strong enough to establish presumption, but not proof". Thus we are again more cautious than Lehmann who states that the spectrum proves that Fe'z+ occupies an interstit ial site. If the Fe2+ is indeed proven to be in an interstit ial site, then the B(5.6eV) bands are indeed due to interstit ial Fe3+ as this interstit ial Fe2+ band
Substitutional and interstitial aluminum impurity in quartz, structure and color center interrelationships∗☆
That color centers in the 460 and 625 m mu regions of the quartz spectrum are related to substitutional aluminum impurity is now well established. Indirect evidence has indicated that interstitisl aluminum may appear in quartz. Buerger has discussed the stuffing of quartz by beta -eucryptite, LiAlSiO/sub 4/ , with cell dimensions a little more than double those of beta -quartz. Recently, other workers have found that beta -spodumene, LiAl(SiO/sub 3/)/sub 2/ , is also isomorphous with beta -quartz. General Electric Co. synthetic quartz grown from a Z-seed was found in this labcratory to contain in ppm Si atoms: 290 Al; 260 Li; 8 Fe; and 131 Na. X irradiation for long pericds gave no change in the optical absorption of this quartz down to 1850 A. It was concluded that the aluminum present was interstitial. An attempt to pre cipitate the interstitial aluminum and lithium was successful. This separate phase was found to ""decorate'' the macromosaics and to indicate the manner of growth of synthetic quartz on various seed orientations. Small regions of rhombchedral growth on General Electric Co. Z-seed crystals did color due to the substitutional aluminum content. There was no precipitation of interstitial aluminum in these regions.more » The increase of the spin-lattice relaxation time of the Si/sup 29/ nucleus after precipitation indicates that the paramagnetic trace impurity in Z- seed crystals is also interstitial. Precipitation experiments on Y-bar synthetic quartz coupled with x irradiation indicate that the trace of aluminum present in the Z-zones is interstitial and that the major portion of the aluminum in the X/ sub fast/ zone is substitutional, while the aluminum in the X/sub slow/ zone is both substitutional and interstitial. The color banding in x-rayed Y-bar crystals is discussed as well as the dependence of the intensity of the smoky color (due to the aluminum color center system) on the crystallographic orientation of the growing zone and how this can be used to identify the different vicinal faces and their "birth" during growth of the crystal. Mention is made of the effects of germanium (IV) ion on the substitutional aluminum content of the Z-zones. (auth)« less