The crystal structure of tetranatrolite from Mont Saint-Hilaire, Quebec, and its chemical and structural relationship to paranatrolite and gonnardite

The structure of tetranatrolite from Mont Saint-Hilaire, Quebec (U.S. Natio nal Museum sample R1830) with a = 13.197(7) Angstrom, c = 6.630(9) Angstrom , and space group I (4) over bar 2d, was refined using single-crystal X-ray data. A representative formula of tetranatrolite determined from electron microprobe analysis is Na12.50K0.01Ca2.93Sr0.11Al19.09Si20.91O79.74.nH(2)O. The structure has the basic natrolite Si-Al-O framework configuration with Na, Ca, Sr, and K residing within inter-framework cages. Aluminum is disor dered over the T1 and T2 tetrahedral sites, with T2 > T1. Water molecules O 4 and O5 coordinate the intercage atoms and have high displacement paramete rs, indicating disorder within the cages. The Mont Saint-Hilaire tetranatro lite structure is compared to four previously determined structures, two te tranatrolite samples from Khibiny and Lovozero, Russia and two "gonnardite" samples from Tvedalen, Norway and Gignat, France. Observations are given t o indicate that the Norwegian sample deduced to be tetranatrolite rather th an gonnardite. Although the crystal structures of tetranatrolite and gonnar dite are very similar, it is shown that the tetranatrolite compositions dif fer significantly from those of gonnardite. The tetranatrolite composition series varies along the join Na(16)A(16)Si(24)O(80)-Na12Ca4Al20Si20O80, and is represented by the formula Na16-xCaxAl16+xSi24-x O-80.nH(2)O, where x e xtends from approximately 2.4 to 3.9. In contrast, gonnardites from Arkansa s and Austria have compositions that vary along the join Na16Al16Si24O80-Na 4Ca8Al20Si20O80, which are represented by the formula square xNa16-3xCa2xAl 16+xSi24-xO80.nH(2)O and where square indicates vacant intercage cation sit es and x varies from approximately 0.3 to 3.2. Tetranatrolite is a dehydrat ion product of paranatrolite and probably does not have a true stability fi eld.