Chlorine content and crystal chemistry of dellaite from the Birkhin gabbro massif, Eastern Siberia, Russia

Abstract

Dellaite crystals of close to end-member composition, Ca6(Si2O7)(SiO4)(OH)2, and with ∼1.5 wt.% Cl. yielding Ca6(Si2O7)(SiO4)(OH)1.75Cl0.25 have been found in skarns within the gabbroid rocks of the Birkhin complex (Eastern Siberia, Russia). The greatest Cl content analysed in a dellaite domain in this skarn is 5.2 wt.% Cl corresponding to 0.8 Cl p.f.u. Dellaite occurs in altered merwmite-larnite-bredigite-gehlenite skarns and also in calcio-olivine skarns with residual larnite. The crystal structures of Cl-free and Cl-bearing (∼1.5 wt.% Cl) dellaite have been refined, including hydrogen positions, from single-crystal X-ray data to R1 = 3.7 and 3.8%, respectively. In addition, both dellaite varieties were studied by Raman spectroscopy indicating stronger hydrogen bonds for the Cl-bearing sample, which agrees with the structural data. Cl is strongly selective and enriches at one (O6) of the two OH positions allowing for the formation of a stronger hydrogen bond O8—H8…C16 compared to O8—H8…O6. Raman spectra of the domain with ∼0.8 Cl p.f.u. confirm the general enhancement of a low-frequency band in the OH range suggesting the dominance of the O—H…Cl hydrogen bond systems.Dellaite and killalaite, Ca3.2(H0.6Si2O7)(OH), have related modular structures, differentiated only by the Si2O7 units in killalaite and alternating Si2O7 and SiO4 units in dellaite. The similarity in cell dimensions and chemical composition suggests that trabzonite, Ca4Si3Oi0-2H2O, with Si3Oi0 trimers also belongs to the same family of structures.

A Modular Concept of Crystal Structure Applied to the Thermal Transformation of α‐C2SH

“…Modular representation of the dellaite structure as portlandite, killalaite, and x ‐ C 2 S (right). Depending on the occupation of Ca 5 in killalaite (which corresponds to Ca 4 in dellaite after), the dellaite structure could be imagined either as composed as modules of fully dehydrated killalaite ( Ca 3 [ Si 2 O 7 ]), Ca 2 SiO 4 and Ca ( OH ) 2 , or of modules of Ca ‐rich killalaite ( Ca 3.5 [ Si 2 O 7 ]( OH )), Ca 2 SiO 4 , and 0.5 Ca ( OH ) 2 ( Si tetrahedra are represented in pink and red, and calcium polyhedra are shown in blue, green, and yellow, respectively).…”

“…On the other hand, polarization parallel to the b axis of the crystal shows intensity ratios close to those observed in killalaite. For polarization both parallel and perpendicular to the b axis, an additional band at 3511 cm −1 is observed, which (along with a band at 3527 cm −1 ) was described in the Raman spectra of natural dellaite and was interpreted as domains with stronger H‐bonding and different chemical composition (presence of Cl) . Interestingly, a corresponding band is observed in the spectrum of killalaite at 3527 cm −1 .…”

“…Interestingly, a corresponding band is observed in the spectrum of killalaite at 3527 cm −1 . These observations raise some questions about probable polarization effects in the study …”

“…Relationship between the crystal structures of α‐ C 2 SH , killalaite, and dellaite projected along [100], [001], and [100], respectively (top). The highlighted Ca atoms (yellow) in the α‐ C 2 SH structure are “redundant” and could be represented as portlandite according 2 Ca 2 [ HSiO 4 ]( OH ) = Ca 3.5 [ Si 2 O 7 ]( OH ) + 0.5 Ca ( OH ) 2 or 2 Ca 2 [ HSiO 4 ]( OH ) = Ca 3 [ HSi 2 O 7 ]( OH ) + Ca ( OH ) 2 , dependent on the silanol quantity .…”

The Mineralogy of Skarns of the Spurrite–merwinite Subfacies, Sanidinite Facies, Güneyce–ikizdere Area, Eastern Black Sea, Turkey

“…The mineral is close to its ideal composition; like in the case of dellaite, there is the possibility of limited Cl-for-OH substitution, a slight excess of Ca and a light deficiency in Si. Interestingly, Nawaz (1974) gave the formula of killalaite from the type locality in Ireland as Ca 3.2 (H 0.6 Si 2 O 7 )(OH), as did Armbruster et al (2011) for killalaite in the Birkhin suite of skarn xenoliths in eastern Siberia, Russia.…”