High-temperature vesuvianite: crystal chemistry and surface considerations

• Published in: Physics and chemistry of minerals Vol. 38; no. 6; pp. 459 - 468
• Main Authors: Elmi, Chiara, Brigatti, Maria Franca, Pasquali, Luca, Montecchi, Monica, Laurora, Angela, Malferrari, Daniele, Nannarone, Stefano
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01-06-2011; Springer Nature B.V
• Subjects: Aluminum; Boron; Calcium; Cations; Coordination; Crystallography and Scattering Methods; Crystals; Earth and Environmental Science; Earth Sciences; Geochemistry; High temperature; Magnesium; Mineral Resources; Mineralogy; Occupancy; Organic chemistry; Original Paper; Photoelectrons; Silica; Silicon dioxide; Surface structure; Unit cell; Volcanoes; X ray photoelectron spectroscopy; Surface structure and chemistry; Vesuvianite; Crystal structure and chemistry
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-011-0419-2

A multi-methodical approach has been applied for characterizing the bulk and surface crystal chemical features of a high-temperature vesuvianite crystal from skarns of Mount Somma-Vesuvius Volcano (Naples, Italy). Vesuvianite belongs to the space group
P
4/
nnc
with unit cell parameters
a
 = 15.633(1) Å,
c
 = 11.834(1) Å and chemical formula (Ca
18.858
Na
0.028
Ba
0.004
K
0.006
Sr
0.005
□
0.098
)
19.000
(Al
8.813
Ti
0.037
Mg
2.954
Mn
0.008
Fe
0.114
2+
Fe
1.375
3+
Cr
0.008
B
0.202
)
13.511
Si
18.000
(O
0.261
F
0.940
OH
7.799
)
9.000
. Structure refinement, which converges at
R
 = 0.0328, demonstrates a strong positional disorder down the fourfold axes, indicating that the Y1 site is split into two positions (Y1A and Y1B) alternatively occupied. However, because of X4 proximity to Y1B and Y1A, X4 cannot be occupied if Y1B or Y1A are. Overall Y1 occupancy (Y1A + Y1B) reaches approximately 0.5, as common in vesuvianite and occupancy of Y1B site is extremely limited. Moreover, T1 position, limitedly occupied, accommodates the excess of cations generally related to Y position. A small quantity (0.202 apfu) of boron is sited at the T2 site that, like T1, is poorly occupied. The determination of the amount of each element on the (100) vesuvianite surface, obtained through X-ray photoelectron spectroscopy high-resolution spectra in the region of the Si
2p
, Al
2p
, Mg
1s
, and Ca
2p
core levels, evidences that a greater amount of aluminum and a smaller amount of calcium characterize the surface with respect to the bulk. Although both of these features require further investigation, we may consider the Al increase can be related to preferential orientation of Al-rich sites on the (100) plane. Furthermore, the surface structure of vesuvianite suggests that Al, Ca, and Mg cations maintain coordination features at the surface similar to the bulk. Silica, however, while presenting fourfold coordination, shows also a [1]-fold small coordinated component at binding energy 99.85 eV, due to broken Si–O bonds at the surface. The presence of eight- and nine-coordinated Ca cations is suggested by a large XPS feature resulting from the merging of Ca
2p3/2
and Ca
2p1/2
peaks at 348.45 and 352.05 eV, respectively.