Study of the thermal decomposition of the cancrinite-kyanoxalite solid-solution series mineral

Olysich L.V.*, Vigasina M.F.*, Melchakova L.V.*, Pekov I.V.*, Chukanov N.V**.

*Geological Faculty, Moscow State University by M.V. Lomonosov, Moscow, Russia

** Institute of Chemical Physics Problems RAS, Chernogolovka, Russia

 milusia@mail.ru

 

This work followed the thermal and thermochemical studies of the cancrinite-group minerals (CGM) has been started from the investigations of the cancrinite and cancrisilite from the Khibiny-Lovozero alkaline complex at Kola Peninsula, Russia (Ogorodova et al., 2009). Recently the new mineral kyanoxalite, Na₇[Si_6-7Al_6-5O₂₄](С₂О₄)_0.5-1∙5H₂O, a water-rich oxalate-dominant  analog of cancrinite, was described from the Lovozero alkaline complex (Chukanov et al., 2009). The oxalate groups occur as the extra-framework admixture anion of the different genetic types CGM. The most (С₂О₄)^2--enriched minerals were found in magmatic rocks and pegmatites from Lovozero alkaline complex (Olysych et al., 2009). Our IR spectroscopy data reveals the continuous cancrinite-kyanoxalite solid-solution series with gradual change of the CO₃/С₂О₄ extra-framework anions ratio.

The thermal evolution of the intermediate member of the kyanoxalite-cancrinite series (Mountain Flora, Lovozero) was studied by means of DTA method and IR spectroscopy.

The average composition of studied mineral (wt. %) is: Na₂O 20.8, K₂O 0.6, CaO 0.6, Al₂O₃ 26.5, SiO₂ 38.6, SO₃ 0.6, CO₂ 5.4, H₂O 6.7, ∑= 99.8. The empirical formula [calculated on 12 (Si+Al)] of the intermediate member of the kyanoxalite-cancrinite series is: Na_6.9K_0.1Ca_0.1Al_5.4Si_6.6O₂₄(C₂O₄)_0.4(CO₃)_0.4(SO₄)_0.1∙3.6H₂O.

Our thermal study was carried out within a temperature range from room temperature to 1300^оС on Q-1500D derivatograph. The temperature was increased with a rate of 15^оС/min. The thermal gravimetric plot displayed mass losses at two continuing stages followed one another: at 100-875^оС - 10.2% and at 875-1300^оС – 2.2% (∑=12.4%).

The IR absorption spectrum was obtained using a FSM-1201 Fourier spectrometer. It is similar to the cancrinite and Si-rich kyanoxalite spectra (Chukanov et al., 2009). The IR spectra of the mineral heated to 300^оС, 450^оС, 700^оС show gradual decrease of the absorption bands intensities of H₂O and oxalate ion. Simultaneously rise of the intensities of carbonate-ion absorption bands was detected. Above 900^оС these bands disappeared, cancrinite structure was destroyed and nepheline-like phase was formed at 1300^оС (Moenke, 1962).

The thermal and IR spectroscopy studies showed that thermal evolution on the air of the oxalate-bearing mineral can be considered as superposition of the three overlapping processes with volatiles: 1) at 100-875^оС H₂O molecules  loss; 2) from 200-250 to 500-600^оС oxalate ion to carbonate ion oxidation; 3) above 500^оС  CO₃^2- to CO₂ transformation and process was completed at about 1300^оС.

Oxalate ion to carbonate ion oxidation happens without elimination of these anions from the crystal. This process takes place with air oxygen participation and includes the C-C-bond break in oxalate group with two carbonate groups formation: C₂O₄^2- + O₂ = 2CO₃^2-. The detectable shift of the stretching vibration frequencies of C-O bond in C₂O₄^2--group to lower values occurs in temperature region from 20 to 300^оС and shows that oxalate ion irreversibly changed at the heating and following rapid cooling. C₂O₄^2--groups move in the channel along the c axis to more energetically advantageous position (Chukanov et al., 2009), or change their configuration.  The process of the oxalate ion oxidation to carbonate ion occurs at the same temperature range. In the IR spectra this process was registered by decrease of the bands intensities of C-O vibrations in the oxalate ion with simultaneous increase of the bands intensities of C-O vibrations in the carbonate ion. It was found that among all absorption bands of CO₃^2- group the intensity of only one band increases. It is concluded that “new” CO₃² ions more preferably located into only one structure position after migration along the zeolitic cannel. The process of the oxalate ion to carbonate ion oxidation was finished before 700^оС.

This study was financially supported by RFBR grants 08-05-00077-a,  09-05-00143-а and 10-05-91333-NNIO_а.

 

References:

Ogorodova L.P., Melchakova L.V., Vigasina M.F., Olysych L.V., Pekov I.V. Cancrinite and Cancrinite in the khibina-Lovozero Alkaline Complex: Thermochemical and thermal data // Geochemistry International. 2009. V. 47. № 3. P. 260-267.

Chukanov N.V., Pekov I.V., Olysych L.V., Massa W., Yakubovich O.V., Zadov A.E., Rastsvetaeva R.K., Vigasina M.F. Kyanoxalite, a new cancrinite-group mineral with oxalate extra-framework anion from the Lovozero alkaline massif (Kola Peninsula) // Zapiski RMO. 2009. V. 138. № 6. P. 18-35 (in Russian).

Olysych L.V., Vigasina M.F., Chukanov N.V., Pekov I.V. Oxalate-bearing cancrinite-group minerals: diversity and occurrence. Abstract // V International symposium “Mineral Diversity: Research and Preservation”. Sofia, 2009. P. 45.

Moenke H.H. Mineralspektren. B. 2. Berlin; Akad. Verlag, 1962.