Cervandonite
Encyclopedia
Cervandonite is a rare arseonosilicate mineral, it has a chemical formula
(Ce,Nd,La)(Fe3+,Fe2+,Ti4+,Al)3SiAs(Si,As)O13 or (Ce,Nd,La)(Fe3+,Fe2+,Ti,Al)3O2(Si2O7)(As3+O3)(OH). 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. 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. Cervandonite is named after the location where it was first described, Pizzo Cervandone (Scherbadung), Italy
in the Central Alps.
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)3SiAs(Si,As)O13, was rewritten to,(Ce,Nd,La)(Fe3+,Fe2+,Ti4+,Al)3 O2(Si2As7)1-x+y(As03)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. 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. However, 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.
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, 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}. 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. This mountain is well known for Alpine excursionists and mineral collectors. It was known the 1960s as a site for rare and new minerals.
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
(Ce,Nd,La)(Fe3+,Fe2+,Ti4+,Al)3SiAs(Si,As)O13 or (Ce,Nd,La)(Fe3+,Fe2+,Ti,Al)3O2(Si2O7)(As3+O3)(OH). It has a monoclinic crustal structure with supercell (Z=6), the crystal structure was established as a trigonal subcell, with space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
R3m and a = 6.508(1)Ǻ
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
, c = 18.520(3) Ǻ, V 679.4(2) Ǻ3, and Z=3. 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. Cervandonite is named after the location where it was first described, Pizzo Cervandone (Scherbadung), Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
in the Central Alps.
Structure
The current work is based on a single- crystal fragment of cervandonite measuring 0.08 X 0.03 X 0.02 mm. The exact fragment was re-examined with MoKα radiation using BRUKER Apex II diffractometerDiffractometer
A diffractometer is a measuring instrument for analyzing the structure of a material from the scattering pattern produced when a beam of radiation or particles interacts with it....
equipped with a 2K CCD detector. Through the use of X-ray diffraction, the unusual nature of the As to Si substitution found arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
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
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
,(Ce,Nd,La)(Fe3+,Fe2+,Ti4+,Al)3SiAs(Si,As)O13, was rewritten to,(Ce,Nd,La)(Fe3+,Fe2+,Ti4+,Al)3 O2(Si2As7)1-x+y(As03)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. 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. However, 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
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
, 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
Crystal twinning
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals....
microdomains. Since cervandonite contains different values of As/Si and unit- cell parameters, has observed one type of superstructure.
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. 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 groupSpace group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
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
Cleavage (crystal)
Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
, conchoidal fracture
Conchoidal fracture
Conchoidal fracture describes the way that brittle materials break when they do not follow any natural planes of separation. Materials that break in this way include flint and other fine-grained minerals, as well as most amorphous solids, such as obsidian and other types of glass.Conchoidal...
with a brownish black streak, 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
Morphology
Morphology may mean:*Morphology , the study of the structure and content of word forms*Morphology , the study of the form or shape of an organism or part thereof...
, fetiasite has a thin brown-red alteration layer with the perfect cleavage is on {100}. Cervandonite was discovered in the east region of Pizzo Cervandone, Alpe Devero, on the border of Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
and Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
, and on the west region of Cherbadung, Switzerland. This mountain is well known for Alpine excursionists and mineral collectors. It was known the 1960s as a site for rare and new minerals.