Cervandonite

Cervandonite is a rare arsenosilicate mineral. It has a chemical formula (Ce,Nd,La)(Fe3+
,Fe2+
,Ti4+
,Al)
3
SiAs(Si,As)O
13
or (Ce,Nd,La)(Fe3+
,Fe2+
,Ti,Al)
3
O
2
(Si
2
O
7
)(As3+
O
3
)(OH)
.[4] It has a monoclinic crustal structure with supercell (Z=6), the crystal structure was established as a trigonal subcell, with space group R3m and a = 6.508(1)Ǻ, c = 18.520(3) Ǻ, V 679.4(2) Ǻ3, and Z=3.[4] It was first described by Buhler Armbruster in 1988, but it has proven to be problem due to the extreme scarcity of single crystals and its unusual replacement of silicon and arsenic.[6] Cervandonite is named after the location where it was first described, Pizzo Cervandone (Scherbadung), Italy in the Central Alps.[2]

Cervandonite
Cervandonite is a classic and very sought after mineral from Binntal.
General
CategorySilicate mineral
Formula
(repeating unit)
(Ce,Nd,La)(Fe3+
,Fe2+
,Ti,Al)
3
O
2
(Si
2
O
7
)(As3+
O
3
)(OH)
Strunz classification9.BE.09.2
Crystal systemTrigonal
Space groupR3m
Identification
ColorBlack
Crystal habitAggregates and rosettes of tabular crystals
Cleavage{001} Indistinct
FractureConchoidal
TenacityBrittle
LusterAdamantine
StreakBrownish black
DiaphaneityTranslucent to opaque
Specific gravity4.9 (calculated)
Optical propertiesBiaxial
Refractive indexnα = 1.990 nγ = 2.000
PleochroismYellowish, reddish brown to black
References[1][2][3][4][5]

Structure

The current work is based on a single- crystal fragment of cervandonite measuring 0.08 X 0.03 X 0.02 mm.[6] The exact fragment was re-examined with MoKα radiation using BRUKER Apex II diffractometer equipped with a 2K CCD detector. Through the use of X-ray diffraction, the unusual nature of the As to Si substitution found arsenic to be present as As3+ instead of As5+, with the presence of sorosilicate Si2076− anions were also established. The original description of the cervandonite mineral, (Ce,Nd,La)(Fe3+
,Fe2+
,Ti4+
,Al)
3
SiAs(Si,As)O
13
, was rewritten to (Ce,Nd,La)(Fe3+,Fe2+,Ti4+,Al)3O2(Si2As7)1−x+y(AsO3)1+x−y(OH)3x−3y. The x and y values are 0.47 and 0.31. Even though the values were derived from a refined and disordered crystal, the new formula matches the charge balance [13 negative versus 12.75 positive charges] for the average chemical composition for the M(1)+M(2) sites.[7] The As-containing silicates, none of which contained the Si2As7 composition, or arsenic in the 3+ state, where considered to be examples of As to Si diadochy. After accurate determination of the crystal structure the As and Si atoms where shown to occupy distinct cell sites. So far, the As-Si disorder is unique for the cervandonite mineral, and accounts for the As:Si ratio. Due to the unusual diadochy, the variable composition of cervandonite might consist of an assembly of more or less twinned microdomains. Since cervandonite contains different values of As/Si and unit- cell parameters, has observed one type of superstructure.[8]

Physical properties

Cervandonite- (Ce) has a monoclinic cell with Z=6. The monoclinic structure was refined as a trigonal subcell using 411 reflections with I > 2σ (I), R1=0.320, wR2=0.0887.[6] The R- centered cell can be transformed with, a 6.508 (1), c 18.520 (3) Ǻ, V 679.4(2) Ǻ3, and Z=3. The structure has a space group of R3m, the solution of the cervandonite structure instantly revealed the presence of one eight-coordinated structure, REE-containing M(1) lying on the threefold axis and M(2) which includes Fe, Ti, or Al. The color of the mineral is a black, with transmitted light it will reflect a yellowish, reddish brown to black color. The mineral is brittle, porous, rosettelike aggregate, with adamantine luster, poorest at {001} cleavage, conchoidal fracture with a brownish-black streak,[9] and a hardness= 5.0. Although cervandonite was found in Pizzo Cervandone, it is not the only rare mineral discovered there, much like the crystal fetiasite, which shares common morphology, fetiasite has a thin brown-red alteration layer with the perfect cleavage is on {100}.[10] Cervandonite was discovered in the east region of Pizzo Cervandone, Alpe Devero, on the border of Italy and Switzerland, and on the west region of Cherbadung, Switzerland.[7] This mountain is well known for Alpine excursionists and mineral collectors. It was known the 1960s as a site for rare and new minerals.[8]

References

  1. Mineralienatlas
  2. http://rruff.geo.arizona.edu/doclib/hom/cervandonitece.pdf Handbook of Mineralogy
  3. http://webmineral.com/data/Cervandonite-(Ce).shtml Webmineral data
  4. http://www.mindat.org/min-935.html Mindat.org
  5. Demartin, F.; Gramaccioli, C. M.; Graeser, S. (1 April 2008). "THE CRYSTAL STRUCTURE OF CERVANDONITE-(Ce), AN INTERESTING EXAMPLE OF As3+ -> Si DIADOCHY". The Canadian Mineralogist. 46 (2): 423–430. doi:10.3749/canmin.46.2.423.
  6. Poirier, G., Ercit, T., Tait, K., Piilonen, P., and Rowe, R. (2009) New mineral names. American Mineralogist 94, 399-408.
  7. Anthony, J., Bideaux, R., Bladh, K., and Nichols, M. (2003) Cervandonite. Handbook of Mineralogy 2, 1
  8. Demartin, F., Gramaccioli, C., and Graeser, S. (2008) The crystal structure of cervandonite-(Ce), an interesting example of As (super 3+) -->Si diadochy. Canadian Mineralogist 46, 423-430
  9. Jambour, J., and Grew, E. (1990) New mineral names. American Mineralogist vol. 75, pp. 931-937
  10. Graeser, S., Schwander, H., Demartin, F., Gramaccioli, C., Pilati, T., and Reusser., E., (1994) Fetiasite (Fe2+, Fe3+, Ti)3O2 [As2O5], a new arsenite mineral: its description and structure
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