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.
Spectroscopic and nanoscale characterization of blue-coloured smithsonite (ZnCO3) from Lavrion historical mines (Greece)
Abstract Spectroscopic and microscopic (particularly HRTEM) techniques were used to investigate the origin of the colour of natural blue Zn-carbonate (smithsonite). Blue smithsonite is rich in copper, but substitution of zinc cations by copper cations, as proposed in the past for the origin of the colour, is questionable considering the absence of anhydrous divalent copper carbonates in nature. In this work, optical microscopy, SEM-EDS, XRD and laser micro-Raman could not resolve distinct phases either than Zn-carbonate, while NIR spectra excluded known chromophore Cu-hydroxycarbonate minerals. HRTEM studies however could clearly resolve nano-sized (3-7 nm) Cu-rich inclusions (specifically Si/Ca/Cu/As-rich inclusions of at least one phase), which are organised in bands with no topotaxial relation to bulk smithsonite. Electron-beam sensitivity of the samples, even at low electron current densities, did not allow the exact identification of the inclusions. However, it can be safely suggested, for the first time in the literature, that they are the cause of the blue colour in smithsonite.
A trapped-hole center causing rose coloration of natural quartz
In rose-colored natural quartz crystals two new hole centers were detected and analyzed by electron paramagnetic resonance (EPR). A hole center with characteristic doublet and sextet hyperfine structures was identified as an O ~ ion bridging between a substitutional aluminum and phosphorus atom each. This center slowly bleaches in UV light and can be enhanced by X-ray irradiation. It is most likely responsible for the rose coloration and also explains the repeatedly reported bleaching of "rose" quartz in daylight. Light-induced transfer of the hole in the P0 4 , not the A104 unit is assumed to cause this strong visible absorption. Several features of this center are quite unexpected: The localization of the hole between Al and Ρ contradicts electrostatic considerations. It is evidently caused by the longer A l . . . O distance resulting from substitution of Si by the smaller P. This longer distance also explains why this center is still observable by EPR at room temperature in contrast to the unperturbed Al . . . O _ (smoky quartz) center. It also has an unexpectedly high thermal stability compared to aluminum-related hole centers with nearby positive ions. All the phosphorus present is obviously associated with aluminum, in agreement with estimates of the equilibrium distribution based on a purely Coulomb attraction between Al and P 5 + on sites of Si + . The isotropic A1 superhyperfine structure is four times smaller than in the unperturbed smoky quartz center, evidently as a result of the strong electron attraction by the pentavalent phosphorus opposite to Al which also gives rise to a large and practically isotropic 3 P doublet superhyperfine splitting.