Fibrous nanoinclusions in massive rose quartz: HRTEM and AEM investigations
Abstract Pink fibrous crystals within massive rose quartz from localities in California, South Dakota, Brazil, Madagascar, and Namibia were examined with high-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM). This study reveals that the nanofibers in all samples are related to dumortierite. Selected-area electron diffraction (SAED) patterns and HRTEM images indicate that the dumortierite-related fibers have a superstructure with a doubled periodicity along the a and b axes of dumortierite, giving cell parameters a = 2adum = 2.36 nm, b = 2bdum = 4.05 nm, c = cdum = 0.47 nm. Computer simulations suggest that periodic arrangements of two different M1 site occupancies in the octahedral face-sharing chains give rise to the superstructure. One type of M1 site is occupied mainly by Al, whereas the other type is dominated by Ti and Fe. Simulated HRTEM images based on our proposed model match the experimental images. Most of the fibers, elongated along the c axis, are free of defects. AEM analysis shows that the dumortierite-related fibers have a composition similar to well-characterized dumortierite, but that they contain a greater amount of Fe substituting for Al in the M1 sites. Boron was detected in all fibers examined by electron energy loss spectroscopy as well as in sillimanite crystals found as a minor component in one rose quartz from Brazil.
The occurrence of dumortierite in the Espinhaço Range, Minas Gerais, Brazil, and its mineralogical- crystallographic comparison with other specimen
Samples of dumortierite from various localities of Brazil (Serra do Espinhaco-Minas Gerais, Macaubas-Bahia) were analyzed with microprobe and the results were compared with those obtained from samples from Louvincourt dumortierite deposit (Quebec, Canada), Lincoln Hill-Champion Mines dumortierite deposit (Nevada, USA) Jack Creek dumortierite deposits (Montana), Acuelos (Chile) and from Madagascar (locality unknown). Infra-red spectroscopy (FTIR), ERP, Mossbauer effect spectroscopy (MS) were performed on the samples. Fourier transform infrared spectroscopy (FTIR), electron paramagnetic resonance (EPR) and Mossbauer spectroscopy have been used to study dumortierite samples of Brazil and of other origins. FTIR spectroscopy shows the existence of various environments for the OH- groups, which are related to substitutions and/or vacancies in the M1 octahedral site. Mossbauer spectroscopy consistently reveals the presence of divalent iron in 2 different octahedral sites, trivalent iron in one octahedral site and minor IVCT or IMMT iron. EPR also shows that isolated Fe3+ is present in various types of sites. The other signals ascribed to paramagnetic Fe3+ correspond to tetrahedral or more distorted octahedral sites.
Quantitative Spectra and Optics of Some Meteoritic and Terrestrial Titanian Clinopyroxenes
Of all mineral spectra, those of titanian clinopyroxenes are among the most difficult to measure, interpret, and assign. Many overlapping (and often unresolvable) single-ion d-d (Fe2+, Fe3 +, and Ti3+) and intermetallic charge-transfer (Fe2+→Fe3 +, Fe2+→Ti4+, Ti3+→Ti4+) bands contribute to a broad absorption envelope stretching from 420 to beyond 800 nm, which is superimposed on the tail of a strong ligand-metal charge-transfer band centered in the near ultraviolet (Manning and Nickel, 1969; Burns et al, 1976). The measurement and interpretation of these spectra is further complicated by large differences between the orientations of the principal indices and principal absorbances in the (010) plane (Dowty, 1978) and by the occurrence of both spin-allowed (Smith, 1978a) and spin-forbidden (Ferguson et al, 1966) d-d transitions of exchange-coupled ion-pairs. These pair transitions have the energy, width, width/energy ratio, and pressure dependence of single-ion d-d bands, but the polarization-, composition-, and temperature-dependence of charge-transfer absorptions. The result is that such bands have often been assigned by different authors to both charge-transfer and d-d origins, depending on the type of measurement made. Examples are the 725- and 1100-nm bands in tourmaline (Smith, 1978b) and the 620-nm band in the Allende fassaite, assigned to Ti3+→Ti4+ charge transfer by Dowty and Clark (1973) and as a d-d transition of Ti3+ by Burns and Huggins (1973) and Mao and Bell (1974).