A new KAlSiO 4 polymorph was found in a granulite facies gneiss from the Punalur district, southern India. The structure was solved and refined on a twinned crystal to an R index value of 1.98% for 265 independent reflections. Metamorphic kalsilite is trigonal, space group P31c with a = 5.157 (1) Aa, b = 8.706 (3) Aa, V = 200.52 (9) Aa 3 , Z = 2. The overall diffraction symmetry 6/mmm exhibited from all the crystals examined arises from a {0001} twinning, related to a mistake in the ordered Al-Si-Al-Si sequence along the c axis. The crystal structure is a stuffed derivate of tridymite, and is characterized by six-membered tetrahedral rings with ditrigonal shape. Individual layers of this structure are the same as those of P6 3 kalsilite, but are stacked along the c axis in an eclipsed manner rather than in the staggered manner of the P6 3 structure.

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