only) gave an initial R-factor of 0.37 for the hkO re-flections. Two-dimensional Fourier and differencemaps confirmed the general features of the framework

MINERALOGICAL SOCIETY OF AMERICA, SPECIAL PAPER 1, 1963INTERNATIONALMINERALOGICALASSOCIATION,PAPERS, THIRD GENERAL MEETING

THE CRYSTAL STRUCTURE OF DACHIARDITE

G. GOTTARDI AND W. M. MEIER

I nstituto di M ineralogia dell' Unioersita, Pisa, Italy, and Technikum, Winterthur andInstitut fur Kristallographie und Petrographie der ETH, Zurich, Switzerland

ABSTRACT

Structural work on mordenite suggested a possible structure for dachiardite, a rare zeolite. This trial structure hasbeen confirmed by 2-dimensional Fourier syntheses.

The crystals are monoclinic with space group B2/m or Bm. The framework structure of dachiardite is closely relatedto the mordenite structure. There are comparatively wide channels along both the band c axes.

Dachiardite is a rare zeolite which occurs in asso-ciation with mordenite on Elba, Italy. Its composi-tion, (K, N a, Cat)5A15Si19048. 12H20, is very nearlythat of mordenite. The crystal data of the two zeolitesare as follows:

Dachardite (D)

monoclinic'

a=18.73Ab=10.30Ac= 7.54A 'Y=107°54'Space group: B2/m or Bm

M ordenite (M)

or thor hom bic

a=18.13Ab=20.49 Ac= 7.52 ACmcm or Cmc2

The relationship between the unit cells of D and M,as illustrated in Fig. 1, was first noted by Gottardi(1960)

The structure of M is based on characteristicchains shown in Fig. 2 (Meier, 1961). These chainscan be linked in two different ways to give the alumi-nosilicate frameworks of 1\1 and D. Figure 3 showsthe resultant frameworks in projection along [001].The trial structure of D (involving framework atoms

i The first monoclinic setting is used in this paper for conven-renee.

M-cell

xFIG. 1. Unit cells of D and M. FIG. 2. Mordenite chain.

291

292 G. GOTTARDI AND W. M. MEIER

M

and indicated probable positions for the cations andsome of the water molecules. The R-factor for thehkO reflections of the structure with cations and waterhas been reduced to 0.22 in the course of 4 cycles ofrefinement by means of difference maps. (The R-factor for the aluminosilicate framework alone is0.30). Fourier projections along [010] and [100] werealso computed and helped to confirm the structure.

The structure of D is penetrated by a 2-dimen-sional system of comparatively wide channels. Themain channels run parallel to the c axis and are inter-connected by channels parallel to the b axis. The freeopenings of these channels are about 4 A. The ob-served twinning of D can be readily explained on thebasis of the alumino silicate framework.

Three-dimensional refinement using low-temper-ature data of the sodium form of D is in progress. Amore detailed account of our work will be publishedin Zeitschrift fur Kristallographie.

REFERENCES

GOTTARDI G. (1960) SuI dimorfismo mordenite-dachiardite.Period. Mineral. 28, 183.

MEIER W. M. (1961) The crystal structure of mordenite. Zeit.Krist. US, 439.

FIG. 3. Projections of the framework structures of M and D.