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Kircherite, a new mineral of the cancrinite-sodalite group with a 36-layer stacking sequence; occurrence and crystal structure

Fernando Camara, Fabio Bellatreccia, Giancarlo Della Ventura, Mickey E. Gunter, Marco Sebastiani and Andrea Cavallo
Kircherite, a new mineral of the cancrinite-sodalite group with a 36-layer stacking sequence; occurrence and crystal structure
American Mineralogist (August 2012) 97 (8-9): 1494-1504

Abstract

This paper reports on the occurrence and the crystal structure of kircherite, a new member of the cancrinite-sodalite group of minerals from Valle Biachella, Sacrofano community (Rome, Latium, Italy). The mineral occurs in association with sodalite, biotite, iron oxides, titanite, fluorite, and a pyrochlore-group mineral. The groundmass of the ejectum consists essentially of K-feldspar with subordinate plagioclase. Kircherite (3 mm as largest size) is observed within miarolitic cavities of the rock and typically occurs as parallel associations of hexagonal, thin, tabular colorless to light-gray transparent crystals; it is non-pleochroic and uniaxial negative, with omega = 1.510(2) and epsilon = 1.502(2). D (sub calc) is 2.457 g/cm (super 3) . Kircherite is trigonal with a = 12.8770(7), c = 95.244(6) Aa, V = 13677(1) Aa (super 3) , Z = 1. The structure has been refined in the trigonal space group R32, obtaining a R-value of 8.5% on 8131 reflections with I/sigma I > 2. The strongest seven reflections in the X-ray powder pattern are [d in Aa (I %) (hkl)]: 3.717 (100) (3 0 0), 2.648 (100) (2 1 28; 0 0 36), 3.232 (65) (2 1 19), 3.584 (60) (1 2 14), 3.604 (53) (1 0 25), 3.799 (52) (1 2 11), 3.220 (38) (2 2 0). The single-crystal FTIR spectrum rules out OH groups and shows the presence of H (sub 2) O and CO (sub 2) molecules in the structural cages of the mineral. Chemical analysis gives (in wt%): SiO (sub 2) 32.05, Al (sub 2) O (sub 3) 27.13, FeO 0.07, K (sub 2) O 4.38, CaO 8.75, Na (sub 2) O 13.62, MgO 0.01, MnO 0.02, TiO (sub 2) 0.01, SO (sub 3) 12.87, Cl 0.35, F 0.05, total 99.82. The empirical formula calculated on the basis of Sigma (Si+Al) = 216 apfu is (Na (sub 89.09) Ca (sub 31.63) K (sub 18.85) Fe (sub 0.20) Mn (sub 0.06) Mg (sub 0.05) Ti (sub 0.03) ) (sub S=139.91) [(Si (sub 108.13) Al (sub 107.87) ) (sub Sigma =216.00) O (sub 430.00) ](SO (sub 4) ) (sub 32.58) Cl (sub 2.00) F (sub 0.53) .6.86H (sub 2) O, which corresponds to the ideal formula [Na (sub 90) Ca (sub 36) K (sub 18) ] (sub Sigma =144) (Si (sub 108) Al (sub 108) O (sub 432) )(SO (sub 4) ) (sub 36) .6H (sub 2) O. The structure can be described as a stacking sequence of 36 layers of six-membered rings of tetrahedra along the c axis. The stacking sequence is ACABCABCABCACBCABCABCABCBABCABCABCAB..., where A, B, and C represent the positions of the rings within the layers. This sequence gives rise to cancrinite, sodalite, and losod cages, alternating along c. Sulfate groups occur within the sodalite and losod cages associated by Na, K, and Ca. H (sub 2) O groups occur within the cancrinite cages, bonded to Ca and Na cations. Anion groups (SO (sub 4) (super 2-) ) in sodalite cages show positional disorder, and so do consequently the extraframework cation sites related to them.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 97
Serial Issue: 8-9
Title: Kircherite, a new mineral of the cancrinite-sodalite group with a 36-layer stacking sequence; occurrence and crystal structure
Affiliation: Universita di Torino, Dipartimento di Scienze della Terra, Turin, Italy
Pages: 1494-1504
Published: 201208
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 41
Accession Number: 2012-079917
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
N41°13'60" - N42°49'60", E11°30'00" - E14°00'00"
Secondary Affiliation: Universita Roma Tre, ITA, ItalyUniversity of Idaho, USA, United StatesIstituto Nazionale di Geofisica e Vulcanologia, ITA, Italy
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201242
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