Atomic Positional Parameters
Si1 8a  0.0000 0.0000 0.0000
Si2 16b 0.1532 0.2112 0.6181
Al1 16b 0.0376 0.0936 0.6119
O1  16b 0.0227 0.0683 0.8594
O2  16b 0.0704 0.1824 0.6011

O3  16b 0.0986 0.0346 0.4997
O4  16b 0.2063 0.1526 0.7166
O5  16b 0.1799 0.2272 0.3860
Na1 16b 0.2208 0.0305 0.6120

[water]
O6  16b 0.0564 0.1893 0.1085
H1  32 atoms not located in unit cell

Reference
WM Meier (1960) Zeitschrift fuer Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie, 113, 430.

BH Torrie, ID Brown and HE Petch (1964) Canadian Journal of Physics, 42, 229
Torrie et al. refine hydrogen positions in the natrolite structure

Mineral Chemistry
Natrolite is a hydrous framework aluminosilicate and a member of the zeolite ('boil stone') group of minerals. Zeolites are formed in nature by low-grade regional metamorphism. In natrolite, which derives its name from natrium (Latin for sodium), the SiO₄ and AlO₄ tetrahedra form interconnected chains with large pore spaces accommodating sodium (Na⁺) ions and water. Natrolite is termed a fibrous zeolite and the structural feature which is common to fibrous zeolites is a small cage made up of six tetrahedra.

Each sodium ion is coordinated to six oxygens, two of which are oxygens of water, and four from the SiO₄ tetrahedra. Water in the crystal structure is weakly held by hydrogen bonding.

Other zeolites with crystal structures related to natrolite include mesolite, scolecite, thomsonite and edingtonite.

The CHIME figure shows sodiums and waters in the large pores of the alumino-silicate framework as blue and pink spheres respectively. Aluminiums are grey, oxygens red, and silicons ochre. Clicking on the waters gives 'W' in the status line - these water molecules are identified in 'select - atom' as 'W'. In the upper right hand corner of the CHIME figure, one sodium is shown with bonds to six nearest oxygens.