0012-5008/05/0008- © 2005 Pleiades Publishing, Inc.0148

Doklady Chemistry, Vol. 403, Part 2, 2005, pp. 148–151. Translated from Doklady Akademii Nauk, Vol. 403, No. 5, 2005, pp. 636–639.Original Russian Text Copyright © 2005 by Rastsvetaeva, Rozenberg, Khomyakov.

The mineral kentbrooksite is considered as an indi-vidual mineral species [1], which differs from typicalrepresentatives of the eudialyte group in both the com-position and the structure. Investigations of georgbar-sanovite [2], taseqite [3, 4], ferrokentbrooksite [5], andcarbokentbrooksite [6], which are respectively theFe,Cl; Fe,Sr; Fe,Na; and

ëé

3

analogues of kentbrook-site, demonstrated that all these minerals have the fol-lowing common features: the location of manganeseand iron atoms predominantly in the five-coordinated

å

(2)

position rather than in a “square” position, anacentric arrangement of the structural fragments, and ahighly ordered distribution of the cations in the zeolitecavities. These minerals differ primarily in the compo-sition of the most isomorphically capacious

A

(4)

position. This position can be occupied predominantlyby Na [1, 5, 6], Mn [2], or Sr [3, 4]. In the mineral zir-silite-(Ce), structurally similar to kentbrooksite, REEsare predominantly located in the

A

(4)

position [6].

The present paper is a continuation of investigations ofkentbrooksite analogues characterized by a variablecomposition of the

A

(4)

position.

In the present study, we characterized a potassium-rich eudialyte-like mineral with an unusual composi-tion. This mineral was found in pegmatites of KoashvaMountain (the Khibiny massif, the Kola Peninsula) asporcelain-like pale yellow rims of transparent cherryred eudialyte crystals. The mineral is optically uniaxialand negative. The average refractive index is 1.620. Theexperimental density (2.93 g/cm

3

) is lower than the the-oretical value (3.01 g/cm

3

), due, apparently, to numer-ous gas–liquid inclusions.

The chemical composition of the mineral was deter-mined by electron probe microanalysis and corre-sponds to the following empirical formula calculatedfor the sum of the high-valence cations (Si + Zr + Ti +Nb + Hf = 29,

Z

= 3):

Na

12.2–12.7

K

1.2–1.45

Ca

5.8–6.0

Sr

0.75-0.95

Ba

0.04

Fe

0.9–1.1

Mn

1.8

(Ce,La,Nd)

0.6–0.7

Y

0.1

Al

0.01–0.02

Si

25.3–25.4

Ti

0.05–0.1

Zr

2.95

Hf

0.06

Nb

0.68

Cl

0.18–0.29

.

The water and

ëé

2

contents (which were deter-mined by wet chemistry methods using microweightsof samples) correspond to

~ 2.7

H atoms and

~0.42

Catoms. The IR spectrum of the mineral shows strongabsorption bands of

ëé

3

groups.

X-ray diffraction data were collected from an isometricsingle crystal. The characteristics of the crystal and detailsof the X-ray diffraction study are given in Table 1.

Taking into account that the chemical compositionof the new mineral is similar to that of taseqite, we usedthe coordinates of the framework atoms of the latter [3]

as the starting model for the structure investigation.Other positions were revealed from a series of differ-ence electron density maps. The last map showed elec-tron density peaks corresponding to the C and O atomsof the carbonate group. The refinement of these posi-tions confirmed the presence of the

ëé

3

group in themineral. Some positions were refined with the involve-ment of mixed atomic scattering curves.

The atomic coordinates and characteristics of thecoordination polyhedra of the new mineral are given inTables 2 and 3, respectively.

The main compositional and structural features ofthe mineral are reflected in its crystal-chemical formula(

Z

= 3), which is in good agreement with the results ofchemical analysis:

[Na

11.68

Ce

0.23

Ba

0.09

][K

1.45

Sr

1.05

Ce

0.5

][Ca

5.7

Ce

0.3

]

×

[Zr

2.94

Hf

0.06

][ ][(Mn

1.85

Fe

0.55

)

[5]

][Nb

0.78

][Si

0.95

]

×

[Si

3

O

9

]

2

[Si

9

O

27

]

2

O

2.4

(OH,O)

1

(CO

3

)

0.46

Cl

0.31

· 1.43H

2

O,

where the compositions of the key positions areenclosed in brackets. The ideal formula is

Fe0.64[ ]

Crystal Structure of a K Analogue of Kentbrooksite

R. K. Rastsvetaeva*, K. A. Rozenberg**, and A. P. Khomyakov***

Presented by Academician L.N. Kogarko February 14, 2005

Received February 14, 2005

* Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119991 Russia

** Faculty of Geology, Moscow State University, Vorob’evy gory, Moscow, 119992 Russia

*** Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements, ul. Veresaeva 15, Moscow, 121357 Russia

CHEMISTRY

DOKLADY CHEMISTRY

Vol. 403

Part 2

2005

CRYSTAL STRUCTURE OF A K ANALOGUE OF KENTBROOKSITE 149

Na

12

(K,Sr,Ce)

3

Ca

6

Mn

3

Zr

3

NbSi[Si

3

O

9

]

2

[Si

9

O

27

]

2

(O,OH)

4

(H

2

O,CO

3

,Cl)

2

.

The new mineral is, on the whole, structurally simi-lar to other representatives of the eudialyte group andconsists of three- and nine-membered silicon–oxygenrings and six-membered rings of (Ca,O) octahedra,

which are linked to each other by discrete (Zr,O) octa-hedra to form a heterogeneous framework. The cavitiesin the framework are occupied by alkali, alkaline-earth,and transition-metal cations and additional largeanions. Of the

ï

(2)

anions, the

ëé

3

groups lie on three-fold axes. The oxygen atoms of these groups are ran-domly involved in coordination of the

A

(3‡)

position ata distance of

2.58

Å from the central Na atom, thusincreasing its coordination number from 7 to 9,whereas these groups are located at a shortened dis-tance from the

A

(3b)

position (

1.87

Å) and are notinvolved in its environment. The carbonate groups,which were found in approximately the same amounts inthe carbonate analogues of eudialyte (0.43 and 0.58 apfu,where apfu is the number of atoms per formula unit, incarbokentbrooksite and zirsilite-(Ce), respectively),occupy the

X

(1)

position.The structure investigation demonstrated that the

new mineral is structurally similar to kentbrooksite andits analogues. All these minerals are characterized bythe occupied five-coordinated

M

(2)

position and the

M

(3)

and

M

(4)

positions occupied predominantly byNb and Si, respectively. However, the five-coordinatedposition in ferrokentbrooksite, georgbarsanovite, andtaseqite is occupied predominantly by Fe, whereas Mnis predominantly located in this position in the newmineral. Hence, this mineral is structurally related tokentbrooksite, carbokentbrooksite, and zirsilite-(Ce).

The above-considered six minerals differ primarilyin the composition of the

A

(4)

position (figure). In kent-brooksite, ferrokentbrooksite, and carbokentbrooksite,

A

(3)

M

(4)

A

(4)

M

(2)

M

(3)

M

(1)

b

a

Fragment of the structure of the K-analogue of kentbrooksite projected onto the (001) plane. The positions discussed in the text arelabeled by letters.

Table 1.

Characteristics of the crystal and details of X-raydiffraction study

Characteristic Parameter

Unit cell parameters, Å

a

= 14.281(4)

c

= 30.243(7)

Unit cell volume, Å

3

V

= 5341.6

Space group

R

3

m

Radiation,

λ

, Å Cu

K

α

, 1.54

Experimental density, g/cm

3

2.93

Crystal dimensions, mm 0.3 × 0.25 × 0.20

Diffractometer Syntex P21

Index ranges 0 < h < 14; 0 < k < 14;0 < l < 35

sinθ/λ <0.58

Total number of reflections 2960 F > 3σ(F)

Number of independent reflections 1060 F > 3σ(F)

Merging R factor for equivalentreflections

0.049

R factor after anisotropic refinement 0.038

Program used for refinement AREN [7]

Program for absorption correction DIFABS [8]

150

DOKLADY CHEMISTRY Vol. 403 Part 2 2005

RASTSVETAEVA et al.

Table 2. Atomic coordinates, equivalent thermal displacement parameters, multiplicities (Q), and occupancies (q) of positions

Atom x/a y/b z/c Q q Beq, Å2

Zr 0.3302(1) 0.1650(1) 0.1667(1) 9 1 0.43(2)M(1) 0.4051(1) 0.3318(1) 0.3331(1) 18 1 0.39(3)Si(1) 0.6090(1) 0.6039(1) 0.0976(1) 18 1 0.13(4)Si(2) 0.1408(2) 0.0703(2) 0.0813(1) 9 1 0.33(6)Si(3) 0.2696(1) 0.3256(1) 0.2365(1) 18 1 0.18(4)Si(4) 0.2085(1) 0.4168(2) 0.0753(1) 9 1 0.19(6)Si(5) 0.5265(2) 0.2633(1) 0.2506(1) 9 1 0.10(7)Si(6) 0.4593(1) 0.5407(1) 0.2556(1) 9 1 0.35(6)O(1) 0.2099(8) 0.6050(5) 0.2510(3) 9 1 1.7(2)O(2) 0.4043(5) 0.3037(5) 0.1272(2) 18 1 0.8(1)O(3) 0.6249(5) 0.0338(5) 0.0463(1) 18 1 0.4(1)O(4) 0.0445(5) 0.3000(5) 0.2881(2) 18 1 0.8(1)O(5) 0.1045(5) 0.3852(6) 0.1076(2) 18 1 0.9(1)O(6) 0.5706(5) 0.6099(5) 0.2267(2) 18 1 1.0(1)O(7) 0.2539(6) 0.2257(5) 0.2068(2) 18 1 1.2(2)O(8) 0.4448(7) 0.2221(5) 0.2903(3) 9 1 0.9(2)O(9) 0.1786(3) 0.3572(5) 0.2191(2) 9 1 0.4(2)O(10) 0.1868(8) 0.0933(6) 0.1296(3) 9 1 1.2(2)O(11) 0.1796(4) 0.3591(5) 0.0295(2) 9 1 0.6(2)O(12) 0.6044(3) 0.3959(3) 0.2530(3) 9 1 0.4(2)O(13) 0.2302(9) 0.1151(6) 0.0437(3) 9 1 0.9(2)O(14) 0.4840(3) 0.5159(3) 0.3043(3) 9 1 0.6(2)O(15) 0.0207(9) 0.5104(6) 0.1145(3) 9 1 1.2(2)O(16) 0.0606(3) 0.1212(5) 0.0751(3) 9 1 0.9(2)O(17) 0.2750(4) 0.5498(6) 0.0689(3) 9 1 1.4(2)O(18) 0.4717(7) 0.2359(5) 0.2018(2) 9 1 0.8(2)M(2a) 0.1838(1) 0.3674(1) 0.3306(1) 9 0.80(1) 0.79(5)M(2b) 0.014(1) 0.5058(8) 0.0015(5) 9 0.20(1) 1.9(2)M(3) 0.3333 0.6667 0.2959(1) 3 0.78(1) 0.53(5)M(4a) 0.3333 0.6667 0.0906(2) 3 0.75(1) 0.5(1)M(4b) 0.3333 0.6667 0.048(1) 3 0.20(4) 1.0(7)A(1) 0.1105(3) 0.2211(5) 0.1528(2) 9 1 2.8(1)A(2a) 0.5691(8) 0.138(1) 0.1739(5) 9 0.56(4) 4.8(3)A(2b) 0.5513(3) 0.1026(5) 0.1820(2) 9 0.44(4) 1.8(2)A(3a) 0.234(1) 0.1170(7) 0.2825(4) 9 0.67(4) 3.2(2)A(3b) 0.1692(9) 0.0846(6) 0.2904(4) 9 0.33(3) 2.1(1)A(4) 0.4715(1) 0.2357(1) 0.0482(1) 9 1 2.08(3)A(5) 0.4759(7) 0.7379(5) 0.1875(3) 9 1 3.1(2)Cl 0.6667 0.3333 0.094(2) 3 0.31(5) 5.9(5)O(19)* 0.6032(5) 0.2064(8) –0.0036(4) 9 1 2.8(3)OH(1) 0.3333 0.6667 0.1433(7) 3 0.80(3) 2.9(5)OH(2) 0.3333 0.6667 –0.003(4) 3 0.20(4) 1.2(9)H2O(1) 0 0 0.225(2) 3 0.45(5) 4.6(9)H2O(2) 0.6667 0.3333 0.132(2) 3 0.38(5) 2.9(9)C 0 0 0.292(2) 3 0.46(8) 2.9(9)Oc 0.046(2) 0.091(2) 0.302(1) 9 0.46(9) 8.1(4)

* The composition of the position O(19) = 2.4O + 0.6H2O.

DOKLADY CHEMISTRY Vol. 403 Part 2 2005

CRYSTAL STRUCTURE OF A K ANALOGUE OF KENTBROOKSITE 151

this position is occupied predominantly by Na. In tase-qite and zirsilite-(Ce), this position is occupied pre-dominantly by Sr and REE, respectively. In georgbar-sanovite, Mn, Sr, Ca, and some other cations are locatedin this position, with Mn predominating. In the newmineral, the A(4) position is occupied by K, Sr, and Ce,with the former predominating. In many minerals ofthis group (for example, in kentbrooksite, taseqite, andgeorgbarsanovite), the coordination polyhedron con-sists of 11 vertices and has distances in the range2.471–2.943 Å, whereas the coordination polyhedronin the structure of the new mineral consists of eight ver-tices and has larger distances (2.557–2.98 Å). Thereduction in the coordination number is, apparently,associated with the fact that the X(2) anion position ona threefold axis is predominantly vacant. These miner-als differ also in the composition of the X(1) and X(2)positions of the extraframework anions. These posi-tions are occupied predominantly by F and ç2é inkentbrooksite; by F, Cl, and H2O in ferrokentbrooksite;and by Cl and ç2é in georgbarsanovite and taseqite. Incarbokentbrooksite, zirsilite-(Ce), and the new mineral,these positions are occupied by ëé3 groups, Cl atoms,and ç2é molecules.

Therefore, the new mineral is most similar to kent-brooksite and its carbonate analogues and differs fromthem in the composition of the A(4) position, where Kprevails over other cations in the mineral under study.Hence, the new mineral can be considered as a K ana-logue of kentbrooksite.

ACKNOWLEDGMENTS

This work was supported by the federal grant pro-gram “Leading Scientific Schools” (grant nos. NSh-1087.2003.5 and NSh-1642.2003.5) and the RussianFoundation for Basic Research (project nos. 02–05–64152 and 04–05–64192).

REFERENCES

1. Johnsen, O., Grice, J.D., and Gault, R.A., Eur. J. Min-eral., 1998, vol. 10, no. 3, pp. 207–219.

2. Ekimenkova, I.A., Rastsvetaeva, R.K., and Khomya-kov, A.P., Dokl. Akad. Nauk, 2000, vol. 370, no. 4,pp. 477−480 [Dokl. Chem. (Engl. Transl.), vol. 370,nos. 4–6, pp. 17–20].

3. Ekimenkova, I.A., Rastsvetaeva, R.K., and Kho-myakov, A.P., Kristallografiya, 2000, vol. 45, no. 6,pp. 1010−1013.

4. Petersen, O.V., Johnsen, O., Gault, R.A., et al., NeuesJahrb. Mineral. Monatsh., 2004, pp. 83–96.

5. Khomyakov, A.P., Dusmatov, V.D., Ferraris, Dzh., et al.,Zap. Vseross. Mineral. O-va, 2003, vol. 132, no. 5,pp. 40–51.

6. Johnsen, O., Grice, J.D., and Gault, R.A., Can. Mineral.,2003, vol. 41, no. 1, pp. 55–60.

7. Andrianov, V.I., Kristallografiya, 1987, vol. 32, no. 1,pp. 228–231.

8. Walker, N. and Stuart, D., Acta Crystallogr., Sect. A:Found. Crystallogr., 1983, vol. 39, no. 2, pp. 158–166.

Table 3. Characteristics of coordination polyhedra

Position Composition (Z = 3) Coordination number

Cation–anion distances, Å

min max av.

Zr 2.94Zr + 0.06Hf 6 2.046(9) 2.09(1) 2.08

M(1) 5.7Ca + 0.3Ce 6 2.30(1) 2.444(6) 2.37

M(2a) 1.85Mn + 0.55Fe 5 2.13(1) 2.148(7) 2.14

M(2b) 0.6Fe 4 2.08(1) 2.12(1) 2.10

M(3) 0.78Nb 6 1.872(9) 2.042(9) 1.96

M(4a) 0.75Si 4 1.586(7) 1.59(2) 1.59

M(4b) 0.2Si 4 1.53(3) 1.57(1) 1.56

A(1) 3Na 8 2.59(1) 2.75(1) 2.65

A(2a) 1.68Na 9 2.54(1) 3.00(1) 2.68

A(2b) 1.19Na + 0.13Ce 8 2.51(1) 2.726(8) 2.62

A(3a) 2.01Na 7 2.48(1) 2.89(1) 2.63

A(3b) 0.8Na + 0.1Ce + 0.09Ba 6 2.68(1) 3.07(1) 2.83

A(4) 1.45K + 1.05Sr + 0.5Ce 8 2.557(7) 2.98(1) 2.75

A(5) 3Na 6 2.21(1) 3.008(8) 2.59

C 0.46C 3 1.16(4) 1.16(4) 1.16