Kirchhoffite		|Cs| [BSi2O6]
Morphology:
	Equant grains 10 to 80 ?m in diameter.
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Physical properties:
	Cleavage: none. Fracture: conchoidal. Hardness: 6 - 6½. Density = 3.622 gm/cm3. Luster: vitreous, sub-vitreous. Streak: white.
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Optical properties:
	Color: Colorless. Uniaxial (+) ω = 1.592 ε = 1.600, δ = 0.008
Crystallography:
	Unit cell data: a  13.019,   c  12.900 Å. Z =  16,  Space group  I41/acd.
Name:
	The mineral is named kirchhoffite after Gustav Robert Kirchhoff (1824–1887), the great German physicist and professor of physics at the universities of Breslau and Berlin in the second half of the 19th century, one of the founders of spectral analysis and codiscoverer (with Robert Bunsen) of cesium and rubidium.
Crystal structure:
	In the kirchhoffite structure, there are two tetrahedrally coordinated sites: Si is occupied by silicon, Si–O = 1.610 Å, B is occupied by boron, B–O = 1.465 Å, and Cs is [12]-coordinated, Cs–O = 3.301 Å. Tetrahedra form a [BSi2O6] framework, the topology of which is identical to that of tetragonal pollucite, CsAlSi2O6. The [12]-coordinated Cs atoms occupy the channels along [111] formed by six-membered rings of tetrahedra. Kirchhoffite, CsBSi2O6, is a B analogue of the tetragonal modification of pollucite.
	Crystal structure of kirchhoffite projected along (001). The Si and B tetrahedra are white and striped, Cs atoms are shown as black spheres. Modified from Agakhanov et al., 2012.
Chemical composition:
	Chemical analysis by electron and ion microprobes gave SiO2 40.47, B2O3 11.27, K2O 0.11, Cs2O 48.16, Rb2O 0.09, for a sum of 100.10 wt.%. The resulting empirical formula on the basis of six atoms of oxygen is (Cs1.02K0.01)Σ1.03B 0.96Si2.02O6, ideally CsBSi2O6.
Occurrences:
	At the type locality (Darai-Pioz alkaline massif, Darai-Pioz River, Tadzhikistan), kirchhoffite is associated with quartz, pectolite, baratovite, polylithionite, aegirine, fluorite, leucosphenite, pyrochlore, neptunite and reedmergnerite.
References:
	Agakhanov, A.A., Pautov, L.A., Sokolova, E., Hawthorne, F.C. and Karpenko, V.Yu. (2010) Kirchhoffite, IMA 2009-094, CNMNC Newsletter 2, April 2010, page 376; Mineral. Mag., 74, 375-377. Updated: August 2025.