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.
Co-ordination of boron in sillimanite
Ion-Microprobe analyses of six sillimanites 
associated with kornerupine show that the sillimanite 
can incorporate from 0.035 to 0.43 wt. % 
B_2O_3 (Grew and Hinthorne, 1983). Boron appears 
to substitute for silicon concomitantly with Mg 
substitution for Al such that the atomic Mg/B ratio 
is close to 0.5. This substitution results in a 
deficiency of cationic charge, which Grew and 
Hinthorne (1983) attributed to a submicroscopic 
rearrangement of the sillimanite structure involving 
loss of oxygen. A possible substitution scheme is 
2(B + xMg) → 2(Si + xAl) + (1 + x)O, where x ≃ 0.5. 
In the present study, we have addressed the question 
of co-ordination of boron in sillimanite. As 
boron can occur in trigonal or tetrahedral coordination 
with oxygen, there is no compelling 
reason that B substitution for Si implies tetrahedral 
co-ordination for B.
Fe2+-Ti4+ charge transfer between face-sharing octahedra : polarized absorption spectra and crystal chemistry of ellenbergerite
L 'ellenbergerite, [Mg,(Ti,Zr),⃞]2 Mg6Al6Si8O28(OH)10, est hexagonale et tres pleochroique, incolore selon n0, pourpre a lilas avec zonation d'intensite selon ne. Les spectres d'absorption en lumiere polarisee ont ete obtenus sur 27 points analyses a la microsonde electronique dans 5 cristaux provenant de 3 echantillons. Les teneurs [Fe] et [Ti] varient respectivement entre 0,02 et 0,11, 0,18 et 0,59 atome par unite formulaire. Les spectres E//c montrent une bande unique et completement polarisee dont le maximum se situe entre 18 500 et 19 500 cm-1, dont la largeur a mi-hauteur est 6 500 cm-1 et dont l'intensite est une fonction lineaire du produit des concentrations [Fe].[Ti]. Les cristaux provenant d'echantillons differents definissent des droites de pentes differentes, suggerant un rapport Fe2+/Fe3+ variable d'un echantillon a l'autre. La bande d'absorption est attribuee a un transfert de charge entre Fe2+ et Ti4+ dans les octaedres accoles par les faces constituant des chaines simples le long des axes senaires. La force de l'oscillateur est 480. 10-4. L'unicite de la bande implique la presence exclusive de Ti et preferentielle de Fe dans les chaines simples et la succession ordonnee de Mg(Fe), Ti(Zr) et des sites vacants le long de ces chaines, Ti etant excentre vers le site vacant adjacent. Un epaulement vers 12 500 cm-1 dans le spectre E//c des cristaux supposes les plus riches en Fe3+ est attribue a un transfert de charge Fe2+-Fe3+ dans les memes sites, par comparaison avec un nouveau phosphate de Mg ayant la structure de l' ellenbergerite (incolore selon n0, bleu-vert selon ne, [Fe] = 0,28, [Ti] = 0,005) dont le spectre E//c montre une unique bande a 12 400 cm-1 due au transfert de charge entre Fe2+ et Fe3+ dans les chaines simples. La parente de structure et de colorations avec la dumortierite suggere que Ti et/ou Fe sont aussi excentres dans les chaines simples de ce mineral.