Chemistry:Tumchaite

Tumchaite
General
Category	Phyllosilicate
Formula (repeating unit)	Na 2(Zr,Sn)Si 4O 11 · H2O
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 9.144 Å, b = 8.818 Å c = 7.537 Å, β = 113.22°; Z = 2
Identification
Formula mass	462.51 g/mol
Color	Colorless to white
Crystal habit	Prismatic, tabular, massive, granular
Twinning	on {100}
Cleavage	Perfect on {100}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	Close to 4.5
|re|er}}	Vitreous
Streak	Colorless to white
Diaphaneity	Translucent to transparent
Specific gravity	2.78
Optical properties	Biaxial (-)
Refractive index	nα = 1.570, nβ = 1.588, nγ = 1.594
Birefringence	δ = 0.0240
Pleochroism	Colorless to greenish-gray
2V angle	60 (5)°
Ultraviolet fluorescence	None
Solubility	Does not dissolve with dilute HCl
References

Tumchaite, Na
2(Zr,Sn)Si
4O
11 · H₂O, is a colorless to white monoclinic phyllosilicate mineral. It is associated with calcite, dolomite, and pyrite in the late dolomite-calcite carbonatites. It can be transparent to translucent; has a vitreous luster; and has perfect cleavage on {100}. Its hardness is 4.5, between fluorite and apatite. Tumchaite is isotypic with penkvilksite. The structure of the mineral is identified by silicate sheets parallel {100}, formed by alternation of clockwise and counterclockwise growing spiral chains of corner-sharing SiO
4 tetrahedra. Tumchaite is named for the river Tumcha near Vuoriyarvi massif.

Occurrence

Tumchaite is found in carbonatites of the alkaline-ultramafic Vuoriyarvi massif, the Murmansk Region, Russia. This is located in the North Karelia on the north shore of the Vuoriyarvi lake and occupies an area of about 19.5 km². The mineral has been found as lens-like segregation of .5 x 1.0 x 1.5 cm³ in size in core sample from the bore hole. Tumchaite is associated with calcite, dolomite, a mineral of the serpentine group, and pyrite, and is formed during the hydrothermal alteration of carbonatites.

Physical properties

Tumchaite occurs as tabular monoclinic crystals, but other habits include massive and granular. The size of individual crystals is approximately 0.2 x 1.2 x 2.5 mm³. It is colorless to white, transparent to translucent. The streak is white and the luster is vitreous. Cleavage on {100} is perfect, no parting is observed, and fracture is uneven as tumchaite is very brittle. The Mohs hardness is 4.5, between fluorite and apatite. The Vickers hardness test yielded 365 to 445- averaging 410- kg/mm² with a 40 g load. The density, which was determined by flotation of the mineral using a dilute Clerici-H₂O solution, was 2.78 (2) g/cm³ versus the 2.77 g/cm³ determined by the empirical formula.

Chemical composition

Subbotin et al., performed quantitative electron microprobe analysis with MS 46 CAMECA instrument operated at 20kV (30 for Zr) and sample current 15-30 nA. The following standards were used for the elements shown in parentheses: lorenzenite (for Na and Ti), diopside (Ca and Si), wadeite (Zr), hematite (Fe), MnCO
3 (Mn), Y
3Al
5O
12 (Y), metallic Sn, Hf, and Nb.

Water contents could not be determined due to little amount of homogeneous material. The presence of molecular water in the mineral was confirmed by the refinement of the crystal structure. The empirical formula, calculated on the basis of 13 O atoms is (Na
2.03Ca
0.01)(Zr
0.76Sn
0.17Ti
0.02Hf
0.01)Si
4.02O
11 · 2H₂O, simplified to Na
2(Zr,Sn)Si
4O
11 · H₂O.

X-ray crystallography

A single crystal with the approximate dimensions of 0.20 x 0.20 x 0.20 mm³ was selected for the X-ray study. This study was performed using a Siemens P4 four circle diffractometer, using graphite monochromatized MoKα radiation (λ=0.71073 Å). Unit cell parameters were found through a least square fit using 25 medium θ reflections: a=9.144 (4), b=8.818 (3), c=7.537 (3) Å, β=113.22 (3)°, V=558.49 ų, Z=2. The full diffraction pattern can be found in Table 2 of Subbotin, et al.

Structure

Tumchaite is isotypic with penkvilksite-1M and is chemically related to vlasovite. The dominant structural feature of tumchaite is the silicate sheet [Si
4O
11] parallel to {100}, which can be considered as a result of condensation of the tetrahedral spiral chains running along [010] with six tetrahedra in the repeat unit. Adjacent spirals are oriented in alternate clockwise and counterclockwise ways. Tumchaite contains two symmetrically independent SiO
4 tetrahedra: Si1 and Si2 have two and three bridging O atoms, respectively.

The tetrahedral silicate sheets are connected by cationic octahedra with a disordered distribution of Zr and Sn (at 0, 1/2, 0), with the Zr/Sn ratio in these octahedra being 4. Figure 2 of Subbotin, et al., shows an illustration of this structure.