Cámaraite, Ba₃NaTi₄(Fe²⁺,Mn)₈(Si₂O₇)₄O₄(OH,F)₇. II. The crystal structure and crystal chemistry of a new group-II Ti-disilicate mineral

Abstract

Abstract-The mechanism for the kaolinization of smectite is extremely complex. The purpose of this study was to explore this mechanism by providing more microscopic information about kaolinite-smectite (K-S) intermediate phases. Crystal-chemical changes were investigated and integrated in a model of the transformation mechanism. Eight K-S samples from three localities, derived from volcanic ash beds, were studied using transmission and analytical electron microscopy (TEM, AEM) and high-resolution TEM (HRTEM). The study completes a previous investigation, using several analytical techniques. The samples cover the range of K-S composition available from the previously studied sample set. Analysis by TEM indicated the preservation of particle morphology throughout the process. Most K-S particles had anhedral, smectite-like morphology, and only the most kaolinitic specimen revealed the coexistence of anhedral and euhedral, hexagonal particles. Analytical electron microscopy showed large chemical variations within samples, corresponding to various degrees of smectite kaolinization. Comparison of chemical results (Si/Al) and d 060 values (proxy for octahedral composition) with the extent of kaolinization from thermogravimetry (TG) indicates that chemical changes in the octahedral sheet occur mainly when the proportion of kaolinite is 40À70%. The results above are consistent with kaolinization occurring via layerby-layer transformation through the progressive loss of individual tetrahedral sheets in smectite layers and subsequent chemical changes in the octahedral sheet. Such a mechanism would produce the results observed in this study: (1) most particles preserve their original morphology; (2) significant variation in terms of the extent of transformation of particles within samples, and (3) formation of crystal structures intermediate between those of smectite and kaolinite, with parts of the tetrahedral sheets missing (kaolinite-like patches). Such structures become least stable at kaolinite~50%, when the perimeter of the kaolinite-like patches is largest and chemical changes in the octahedral sheet can occur more easily. Kaolinite layers could not be resolved by HRTEM in most cases and showed lattice fringes corresponding to superstructures. A model was established to quantify kaolinite and smectite layers in the HRTEM images with results which matched TG-derived values.

Fluoroleakeite, NaNa2(Mg2Fe3+2Li)Si8O22F2, a new mineral of the amphibole group from the Verkhnee Espe deposit, Akjailyautas Mountains, Eastern Kazakhstan District, Kazakhstan: description and crystal structure

“…References: (1) Hawthorne et al (1992); 2 3 mm long, and occur as isolated crystals, as small aggregates, and as inclusions in cámaraite (Sokolova et al, 2009a;Cámara et al, 2009). Fluoroleakeite has a Mohs hardness of~6 and is brittle with a splintery fracture; it has the characteristic perfect {110} cleavage of monoclinic amphiboles, intersecting at~56º.…”

“…During work on the new Ti-silicate mineral, cámaraite, from the Verkhnee Espe deposit in Eastern Kazakhstan (Sokolova et al, 2009a;Cámara et al, 2009), another new amphibole was discovered coexisting with cámaraite, the 'fluoro' equivalent of leakeite, ideally NaNa 2 (Mg 2 Al 2 Li)Si 8 O 22 F 2 ; a formal description of the amphibole is given here. The new species and the new name (Leake et al, 1997(Leake et al, , 2003 have been approved by the International Mineralogical Association Commission on New Minerals and Mineral Classification (2009-085).…”

Jinshajiangite: structure, twinning and pseudosymmetry

“…The bafertisite layers in the structure could also be responsible for the minor excessive of anion (F or OH) in the chemical analysis (Table 1), as the bafertisite has one extra F in the chemical formula compared with jinshajiangite. An ordered stacking of bafertisite and jinshajiangite layers has also been reported for the derivative structure of the new mineral species camaraite (Cá mara et al, 2009;Sokolova & Cá mara, 2013).…”

From structure topology to chemical composition. XXVI. Crystal structure and chemical composition of a possible new mineral of the murmanite group (seidozerite supergroup), ideally Na2CaTi4(Si2O7)2O4(H2O)4, from the Lovozero alkaline massif, Kola Peninsula, Russia

“…We suggest that {P 0.03 } belongs to other phases which form intergrowths with MRM. Intimate intergrowths are very common for TS-block minerals: our high-resolution transmission electron microscopy work on murmanite-group minerals lomonosovite, Na 10 Ti 4 (Si 2 O 7 ) 2 (PO 4 ) 2 O 4 , and betalomonosovite, Na 6 Ti 4 (Si 2 O 7 ) 2 [PO 3 (OH)][PO 2 (OH) 2 ]O 3 F (Sokolova et al ., 2015); zvyaginite, Na 2 ZnTiNb 2 (Si 2 O 7 ) 2 O 2 (OH) 2 (H 2 O) 4 , a lamprophyllite-group mineral (Sokolova et al ., 2017), and cámaraite, NaBa 3 Fe 2+ 8 Ti 4 (Si 2 O 7 ) 4 O 4 (OH) 4 F 3 , a bafertisite-group mineral (Cámara et al ., 2009), shows that these four TS-block minerals contain intergrown phases.…”