_{4}, is investigated. The directionality of (Al,Si)O

_{4}tetrahedra within single six-membered tetrahedral ring building units (S6R) could not be defined. With equal probability for the directionality of each tetrahedra within one S6R [free apex pointing up (U) or down (D)], an undefined sequence of U and D directionalities is needed to describe the S6R building units. The extinction conditions of disordered kalsilite are also different compared to ordered kalsilite within the space group

*P*6

_{3}. In disordered kalsilite,

*h*0

*l*and

*hhl*reflections with

*l*= 2

*n*+ 1 are systematically absent.

### Supporting information

Crystallographic Information File (CIF) https://doi.org/10.1107/S2053229614002423/cu3046sup1.cif | |

Rietveld powder data file (CIF format) https://doi.org/10.1107/S2053229614002423/cu3046Isup2.rtv |

CCDC reference: 984799

Kalsilite, a feldspatoid group mineral, can be found in basic plutonic rocks
rich in potassium, as well as in metamorphic rocks (Woolley *et al.*,
1996). In addition, it is a well-known ceramic material. Claringbull &
Bannister (1948) carried out the first structural investigations and
suggested
a tridymite structural type for kalsilite. Perrotta & Smith (1965)
proved
their conclusion by giving a precise structure description: potassium ions
coordinate with nine nine O atoms (O^{2-}), three apical O atoms connect
neighbouring tetrahedral layers and two groups of three basal O atoms form two
different tetrahedral layers. The average K—O distance is 2.90 Å. In
addition, they also found disorder of apical O atoms which are shifted \sim
0.25 Å from the threefold axis. Consequently, the 180° T—O—T (T = Si,
Al) angle is reduced to a more energetically favourable value of 163°. The
tetrahedral layers are perpendicular to the *c* axis and are assembled
from six-membered rings (S6R) of (Al,Si)O_{4} tetrahedra in which the sequence
of the directionality [free apex pointing up (U) or down (D)] of the
tetrahedra within one ring is UDUDUD.

The changes of diffraction intensities or reflection extinction conditions in
kalsilite patterns are explained by twinning (double or triple), a polydomain
structure or modulations of the structure. Andou & Kawahara (1982) and
Kawahara *et al.* (1987) reported different intensities and the
disappearance of *hhl* (*l* = 2*n*+1) reflections, which are
associated with different ratio of twinning domains in a crystalline phase
described by the space group *P*6_{3}*mc*. Dollase & Freeborn
(1977)
reported the same phenomenon for 11*l* (*l* = 2*n*+1)
reflections found in phase–antiphase boundaries on the domain contacts. They
suggested that a great number of nuclei of both kinds are formed with equal
probability during fast nucleation. Abbot (1984) pointed out the great
influence of domain structure on the diffraction pattern in all KAlSiO_{4}
polymorphs and suggested that synthetic kalsilite reported by Tuttle & Smith
(1958) is probably the so-called intermediary kalsilite phase with two
domains
that may be described in *P*6_{3}*/mmc* and *P*6_{3}*/mc*.
Xu & Veblen (1996) investigated the superstructure reflections and
domain
structure in synthetic and natural (with 0.5–5 atomic% Na) kalsilites. In
natural kalsilite, with more than 2.5% Na, they found superstructure
reflections. These superstructure reflections correspond to the three
orthorhombic domains (*P*2_{1}) which are rotated 120° around the
*c* axis of unit cell. In synthetic kalsilite, they found a complex
microstructure with large number of `fine' twinning lamellas parallel to
*c* axis. They concluded that twin domains correspond to *P*6_{3} and
*P*31*c* polymorph modifications. Further studies of kalsilite
involving temperature-induced changes and the use of transmission electron
microscope (TEM) and microstructure analysis have revealed the true complexity
of this problem (Henderson & Taylor, 1988; Andou & Kawahara,
1982; Kawahara
*et al.*, 1987; Capobianco & Carpenter, 1989; Carpenter &
Cellai, 1996;
Cellai *et al.*, 1992, 1997, 1999; Dollase &
Freeborn, 1977; Abbot 1984;
Artioli & Kvick, 1990; Kosanovic *et al.*, 1997,
Dimitrijevic & Dondur,
1995; Barbier & Fleet, 1988; Xu & Veblen, 1996).

Polymorphism of phases at the KAlO_{2}–SiO_{2} join of the
K_{2}O—Al_{2}O_{3}–SiO_{2} phase diagram was investigated by the ZTIT (zeolite
thermally induced phase transformation) method by Dimitrijevic & Dondur
(1995). They reported five KAlSiO_{4} polymorphs stable at room
temperature,
two of them having powder patterns close to kalsilite framework topology. In
contrast to known kalsilite (space group *P*6_{3}) synthesized at 1373 K
[*a* = 5.160 (1) Å and *c* = 8.632 (6) Å], the X-ray powder
diffraction (XRPD) pattern of a new polymorph synthesized at 1273 K [*a*
= 5.197 (1) Å and *c* = 8.583 (5) Å] is characterized by systematic
disappearance of *h0l* and *hhl* reflections with *l* =
2*n*+1. The latter we will hereafter denote as disordered kalsilite.

In ordered kalsilite, the layers of tetrahedra are perpendicular to the
*c* axis and are assembled from six-membered rings (S6R) of (Al,Si)O_{4}
tetrahedra, where the sequence of the directionality [free apex pointing up
(U) or down (D)] of the tetrahedra within one ring is UDUDUD. However, in the
disordered kalsilite, the directionality of (Al,Si)O_{4} tetrahedra within one
S6R could not be defined. With equal probability for the directionality of
each tetrahedra within one S6R, an undefined sequence of letters U and D are
needed to describe the S6R building units. Such disorder structure model is
characterized by systematic disappearance of *h0l* and *hhl*
reflections with *l*=2*n*+1 (Fig. 1). The disordered kalsilite
structure model could be described as two substructures, denoted **a** and
**b**, each with 50% population (Fig. 2). In Fig. 3, simulated XRPD
patterns for models composed from substructrures are presented: 100% **a**,
90% **a** and 10% **b**, 75% **a** and 25% **b**, and 50% **a**
and 50% **b**. Evidently only the model composed of 50% **a** and 50%
**b** substructures results in systematic disappearance of (*h0l*) and
(*hhl*) reflections with *l* = 2*n*+1.

Interatomic distances and angles for disordered kalsilite are in agreement with
previously published data for known kalsilite structures: space group
*P*6_{3} (Andou & Kawahara, 1982), *P*31*c* (Cellai *et
al.* 1997) and *P*6_{3}*mc* (Dollase & Freeborn,
1977). The
electrostatic valence balance calculated according to the method of Brown &
Altermatt (1985) is satisfactory. In the structure, Si^{4+} and Al^{3+}
are
fully ordered, which is consistent with the ^{29}Si and ^{27}Al MAS NMR results
obtained by Dimitrijevic & Dondur (1995).

Cation exchanged zeolites are confirmed as excellent precursors for the
preparation of aluminosilicate ceramics (Dondur & Dimitrijevic, 1986).
Numerous phases, such as β-eucriptite, nepheline, carnegite, kalsilite,
anorthite, celsian or α-cordierite, were synthesized using the ZTIT method
(Dondur & Dimitrijevic, 1986; Norby, 1990; Newsam,
1988). We applied this
method in order to produce different phases on the KAlO_{2}–SiO_{2} join of the
K_{2}O–Al_{2}O_{3}—SiO_{2} system, particularly KAlSiO_{4} polymorphs stable at
room temperature. As a starting material, the sodium form of synthetic zeolite
LTA (Meier & Olson, 1992) manufactured by Union Carbide Co was used. A
fully
exchanged K^{+} form of LTA zeolite was prepared from KCl solution, using
several successive exchanges. The disordered kalsilite was synthesized from
K-LTA zeolite by the ZTIT route after heating for 1 h at 1273 K. A detailed
procedure of the synthesis is explained by Dimitrijevic & Dondur
(1995).
Diffraction data were collected on a Phillips PW-1710 diffractometer equipped
with a graphite monochromator (Cu *K*α) and an Xe-filled proportional
counter. Divergence and receiving slits were fixed to 1° and 0.1 mm, and the
generator was set at 40 kV and 32 mA. The diffractometer alignment was checked
using a reference material of powdered crystalline silicon. Data for Rietveld
refinements were collected, in scan-step mode, between 4 and 90° 2θ with
0.02° 2θ step and 15 s per step.

Data collection: Philips PC-APD PW1877 (Philips, 1989); program(s) used to refine structure: FULLPROF (Rodriguez-Carvajal, 1990); molecular graphics: *DIAMOND* (Brandenburg & Putz, 2005); software used to prepare material for publication: *publCIF* (Westrip, 2010).

KAlSiO_{4} | Z = 2 |

M = 158.16_{r} | D_{x} = 2.614 Mg m^{−}^{3} |

Hexagonal, P6_{3} | Cu Kα_{1}, Cu Kα_{2} radiation, λ = 1.540562, 1.544390 Å |

Hall symbol: P 6c | T = 295 K |

a = 5.19817 (15) Å | white |

c = 8.5865 (3) Å | flat sheet, 25 × 25 mm |

V = 200.93 (1) Å^{3} |

Philips PW1710 diffractometer | Data collection mode: reflection |

Equatorial mounted graphite monochromator | Scan method: step |

Specimen mounting: packed powder pellet | 2θ_{min} = 4.001°, 2θ_{max} = 89.961°, 2θ_{step} = 0.020° |

R_{p} = 7.001 | Profile function: pseudo-Voigt |

R_{wp} = 9.735 | 45 parameters |

R_{exp} = 4.507 | 6 restraints |

R_{Bragg} = 8.182 | |

χ^{2} = 4.666 | Background function: linear, extrapolation,, points, were, determined, by, visual, estimation, and, refined |

4299 data points |

KAlSiO_{4} | V = 200.93 (1) Å^{3} |

M = 158.16_{r} | Z = 2 |

Hexagonal, P6_{3} | Cu Kα_{1}, Cu Kα_{2} radiation, λ = 1.540562, 1.544390 Å |

a = 5.19817 (15) Å | T = 295 K |

c = 8.5865 (3) Å | flat sheet, 25 × 25 mm |

Philips PW1710 diffractometer | Scan method: step |

Specimen mounting: packed powder pellet | 2θ_{min} = 4.001°, 2θ_{max} = 89.961°, 2θ_{step} = 0.020° |

Data collection mode: reflection |

R_{p} = 7.001 | χ^{2} = 4.666 |

R_{wp} = 9.735 | 4299 data points |

R_{exp} = 4.507 | 45 parameters |

R_{Bragg} = 8.182 | 6 restraints |

^{2}) top

x | y | z | U_{iso}*/U_{eq} | Occ. (<1) | |

Sia | 0.33333 | 0.66666 | 0.45815 (8) | 0.04750 (2)* | 0.50000 |

Ala | 0.33333 | 0.66666 | 0.07424 (8) | 0.042 (3)* | 0.50000 |

O2a | 0.61394 (13) | 0.019 (2) | 0.02309 (8) | 0.0214 (18)* | 0.50000 |

O1a | 0.33333 | 0.66667 | 0.27785 (8) | 0.060 (3)* | 0.50000 |

Sib | 0.33333 | 0.66666 | 0.95815 (8) | 0.04750 (2)* | 0.50000 |

Alb | 0.33333 | 0.66666 | 0.57425 (8) | 0.042 (3)* | 0.50000 |

O2b | 0.61394 (13) | 0.019 (2) | 0.52310 (8) | 0.0214 (18)* | 0.50000 |

O1b | 0.33333 | 0.66667 | 0.77785 (8) | 0.060 (3)* | 0.50000 |

K | 0.00000 | 0.00000 | 0.27354 | 0.048 (2) |

^{2}) top

U^{11} | U^{22} | U^{33} | U^{12} | U^{13} | U^{23} | |

K | 0.067 (3) | 0.067 (3) | 0.0099 (18) | 0.033 (3) | 0.00000 | 0.00000 |

Sia—O2a^{i} | 1.614 (6) | K—O2a^{viii} | 2.971 (3) |

Sia—O2a^{ii} | 1.614 (10) | K—O2a^{ix} | 2.977 (7) |

Sia—O2a^{iii} | 1.614 (4) | K—O2a^{x} | 2.971 (4) |

Sia—O1a | 1.5481 (10) | K—O2a^{vi} | 2.977 (3) |

Ala—O2a^{iv} | 1.733 (7) | K—O2a^{iii} | 2.971 (7) |

Ala—O2a^{v} | 1.733 (3) | K—O1a^{xi} | 3.0014 (1) |

Ala—O2a^{vi} | 1.733 (10) | K—O1a^{xii} | 3.0014 (1) |

Ala—O1a | 1.7483 (10) | K—O1a | 3.0014 (1) |

Sib—O2b^{i} | 1.614 (6) | K—O2b^{vii} | 2.971 (4) |

Sib—O2b^{ii} | 1.614 (10) | K—O2b^{xiii} | 2.977 (3) |

Sib—O2b^{iii} | 1.614 (4) | K—O2b^{ix} | 2.971 (7) |

Sib—O1b | 1.5481 (10) | K—O2b^{xiv} | 2.977 (4) |

Alb—O2b^{iv} | 1.733 (7) | K—O2b^{vi} | 2.971 (3) |

Alb—O2b^{v} | 1.733 (3) | K—O2b^{xv} | 2.977 (7) |

Alb—O2b^{vi} | 1.733 (10) | K—O1b^{xvi} | 3.0014 (1) |

Alb—O1b | 1.7482 (10) | K—O1b^{xvii} | 3.0014 (1) |

K—O2a^{vii} | 2.977 (4) | K—O1b^{xviii} | 3.0014 (1) |

Symmetry codes: (i) x−y, x, z+1/2; (ii) −x+1, −y+1, z+1/2; (iii) y, −x+y+1, z+1/2; (iv) x, y+1, z; (v) −y, x−y, z; (vi) −x+y+1, −x+1, z; (vii) x−1, y, z; (viii) x−y−1, x−1, z+1/2; (ix) −y, x−y−1, z; (x) −x+1, −y, z+1/2; (xi) x−1, y−1, z; (xii) x, y−1, z; (xiii) x−y−1, x−1, z−1/2; (xiv) −x+1, −y, z−1/2; (xv) y, −x+y+1, z−1/2; (xvi) x−y, x−1, z−1/2; (xvii) x−y, x, z−1/2; (xviii) x−y+1, x, z−1/2. |

Experimental details

Crystal data | |

Chemical formula | KAlSiO_{4} |

M_{r} | 158.16 |

Crystal system, space group | Hexagonal, P6_{3} |

Temperature (K) | 295 |

a, c (Å) | 5.19817 (15), 8.5865 (3) |

V (Å^{3}) | 200.93 (1) |

Z | 2 |

Radiation type | Cu Kα_{1}, Cu Kα_{2}, λ = 1.540562, 1.544390 Å |

Specimen shape, size (mm) | Flat sheet, 25 × 25 |

Data collection | |

Diffractometer | Philips PW1710 diffractometer |

Specimen mounting | Packed powder pellet |

Data collection mode | Reflection |

Scan method | Step |

2θ values (°) | 2θ_{min} = 4.001 2θ_{max} = 89.961 2θ_{step} = 0.020 |

Refinement | |

R factors and goodness of fit | R_{p} = 7.001, R_{wp} = 9.735, R_{exp} = 4.507, R_{Bragg} = 8.182, χ^{2} = 4.666 |

No. of data points | 4299 |

No. of parameters | 45 |

No. of restraints | 6 |

Computer programs: Philips PC-APD PW1877 (Philips, 1989), FULLPROF (Rodriguez-Carvajal, 1990), *DIAMOND* (Brandenburg & Putz, 2005), *publCIF* (Westrip, 2010).