Macroscopic, microscopic, goniometric, qualitative spectrographic and quantitative chemical data of kettnerite [2] with some new observations are given. The unit cell dimensions determined from the powder and rotating crystal photographs are
a
0=
b
0=5.36±0.02 Å,
c
0=13.59±0.03 Å. There are four molecules in the unit cell. The crystal structure of kettnerite was studied from the ordinary and generalized projections of the Patterson function along the [010] axis. Direct evidence of tetragonal layers (Ca. 2 F, Ca) and (Bi, 2 O, Bi) parallel to the basal face was found. The spatial arrangement of these layers corresponds to the symmetry of the space group
P 4/
nmm. These layers alternate in the [001] direction being interleaved by single CO
3 layers. Both the biaxial character of the mineral and the uncertainty concerning the rotation of the CO
3 groups indicate a lower, most probably an orthorhombic symmetry. The highest possible symetry is that of
Cmma. The structure is related to that of the type
X
1 found by Sillén et al. for several bismuth oxyhalides and especially to that of bismutite.
,
,
,
[2]
.
,
:
a
0=
b
0=5,36±0,02 Å,
0=13,59±0,03 Å;
Z=4.
[010].
(Ca, 2 F,
)
(Bi, 2 O, Bi),
(010).
4/
.
,
3.
3 .
C.
,
,
1,
.
We wish to express gratitude to our colleagues Dr. A. Línek and Ing. C. Novák (Institute of Technical Physics, Czech. Acad. Sci.) for the use of the M1 equipment [11]. We are also very grateful to Mr. Z. 
iký
for carrying out most of the numerical calculations.