Calciolangbeinite-O, a natural orthorhombic modification of K₂Ca₂(SO₄)₃, and the langbeinite–calciolangbeinite solid-solution system

Abstract

Calciolangbeinite, ideally K 2 Ca 2 (SO 4 ) 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 K 2 Mg 2 (SO 4 ) 3 . The most part of the system is represented by cubic phases, and only members with compositions K 2 (Ca 2.0-1.9 Mg 0.0-0.1 )(SO 4 ) 3 have orthorhombic symmetry under room conditions. The crystal structure of calciolangbeinite-O was studied on a single crystal, chemically very close to K 2 Ca 2 (SO 4 ) 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) Å 3 and Z = 4; space group is P2 1 2 1 2 1 . Calciolangbeinite-O is a low-temperature modification of K 2 Ca 2 (SO 4 ) 3 belonging to the K 2 Cd 2 (SO 4 ) 3 structure type whereas calciolangbeinite-C (space group P2 1 3), 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 K 2 Ca 2 (SO 4 ) 3 undergoes phase transition to calciolangbeinite-O whilst the Mg-enriched varieties of the mineral remain calciolangbeinite-C.

In situ synchrotron PXRD investigation of fresh and aged Equisetum ramosissimum Desf. (horsetail grass)

“…[22] At temperatures greater than 400 C, the diffraction intensity of syngenite became weak, and a new diffraction pattern gradually grew, which was consistent with that of cubic langbeinite (K 2 Mg 2 [SO 4 ] 3 ) maintained till 550 C. Ideal K 2 Ca 2 (SO 4 ) 3 is a new mineral named calciolangbeinite, which is the Ca-dominant or Mg-replaced analog of langbeinite, also named as calcium-langbeinite. [23][24][25] After cooling to room temperature under forced convection, cubic calciolangbeinite and the orthorhombic phase also co-existed. [26] Kloprogge, J.T.…”

“…Ideal K 2 Ca 2 (SO 4 ) 3 is a new mineral named calciolangbeinite, which is the Ca‐dominant or Mg‐replaced analog of langbeinite, also named as calcium‐langbeinite. [ 23–25 ] After cooling to room temperature under forced convection, cubic calciolangbeinite and the orthorhombic phase also co‐existed. [ 26 ] Kloprogge, J.T.…”

Minerals, crystal structures and geochemistry

“…Pictures show colourless to white calciolangbeinite- O overgrowing sanidine and crystallised together with hematite and iron-black tenorite. Many other minerals are also described from the localities where the samples analysed were collected (Pekov et al ., 2022). Another sulfate mineral [CaCe 2 Cu 6 (SO 4 ) 4 (OH) 12 ⋅8H 2 O] is described by Mike S. Rumsey, Frank C. Hawthorne, John Spratt, Jens Najorka and Wren Montgomery in the article ‘Bridgesite-(Ce), a new rare-earth element sulfate, with a unique crystal structure, from Tynebottom Mine, Cumbria, United Kingdom'.…”