Calciolangbeinite-O, a natural orthorhombic modification, and langbeinite–calciolangbeinite solid-solution

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Marco E. Ciriotti
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Iscritto il: ven 25 giu, 2004 11:31
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Calciolangbeinite-O, a natural orthorhombic modification, and langbeinite–calciolangbeinite solid-solution

Messaggio da Marco E. Ciriotti » ven 28 gen, 2022 11:04

Prossima pubblicazione.

Referenza:
▪ Pekov, I.V., Zubkova, N.V., Galuskina, I.O., Kusz, J., Koshlyakova, N.N., Galuskin, E.V., Belakovskiy, D.I., Bulakh, M.O., Vigasina, M.F., Chukanov, N.V., Britvin, S.N., Sidorov, E.G., Vapnik, Y., Pushcharovsky, D.Yu. (2022): Calciolangbeinite-O, a natural orthorhombic modification of K2Ca2(SO4)3, and the langbeinite–calciolangbeinite solid-solution system. Mineralogical Magazine, 86, (in press).

Abstract:
Calciolangbeinite, ideally K2Ca2(SO4)3, exists in two modifications, cubic and first described in the present paper orthorhombic. They are topologically-similar polymorphs which are considered as the structural varieties of the same mineral species and were named calciolangbeinite-C and calciolangbeinite-O, respectively. Calciolangbeinite-O is the first natural orthorhombic langbeinite-like sulfate. It clearly differs from calciolangbeinite-C in the powder X-ray diffraction pattern, optical data and Raman spectrum. Calciolangbeinite-O is found in sublimates of the active Arsenatnaya fumarole at the Tolbachik volcano, Kamchatka, Far Eastern Region, Russia and in pyrometamorphic rocks of the Hatrurim Complex at Jabel Harmun (Judean Desert, Palestinian Autonomy) and Har Parsa (Negev Desert), both in Israel. Calciolangbeinite-C is known only in fumarole sublimates at Tolbachik. Calciolangbeinite forms a continuous solid-solution system with langbeinite K2Mg2(SO4)3. The most part of the system is represented by cubic phases, and only members with compositions K2(Ca2.0–1.9Mg0.0–0.1)(SO4)3 have orthorhombic symmetry under room conditions. The crystal structure of calciolangbeinite-O was studied on a single crystal, chemically very close to K2Ca2(SO4)3, from Tolbachik (R1 = 2.75%). The unit-cell parameters are: a = 10.3330(2), b = 10.5027(2), c = 10.1763(2) Å, V = 1104.37(4) Å3 and Z = 4; space group is P212121. Calciolangbeinite-O is a low-temperature modification of K2Ca2(SO4)3 belonging to the K2Cd2(SO4)3 structure type whereas calciolangbeinite-C (space group P213), a high-temperature modification, has the langbeinite-type structure. Probably significant Mg admixture in calciolangbeinite-C from Tolbachik can stabilize its cubic structure at room temperature. In both high-temperature fumaroles and pyrometamorphic rocks calciolangbeinite crystallizes definitely in cubic modification and during cooling its chemical variety close to the end-member K2Ca2(SO4)3 undergoes phase transition to calciolangbeinite-O whilst the Mg-enriched varieties of the mineral remain calciolangbeinite-C.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 29464
Iscritto il: ven 25 giu, 2004 11:31
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Re: Calciolangbeinite-O, a natural orthorhombic modification, and langbeinite–calciolangbeinite solid-solution

Messaggio da Marco E. Ciriotti » sab 29 gen, 2022 11:30

In questo caso pur essendo una fase ortorombica e l'altra cubica non ci sono i requisiti perchè la seconda (ortorombica) debba avere un nuovo nome.
Il motivo è lo stesso che ha permesso, insieme a coautori, di proporre il discredito (approvato da IMA CNMNC) di una specie: le strutture cristalline pur essendo relative a due (o più) sistemi cristallini diversi e due (o più) gruppi spaziali diversi sono sostanzialmente simili e pertanto la fase di più recente ritrovamento (nel caso della calciolangbeinite quella ortorombica) sarà soltanto il politipo -O del minerale già noto.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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