Kalborsite |K6 B(OH)4Cl|[Al4Si6O20]
  Equant, cubic-appearing crystals to 2 mm.
Physical properties:  
  Cleavage: perfect on {110}.
Hardness:  5 - 6.
D = 2.5 gm/cm3.
Luster: vitreous.
Streak: white.
Optical properties:            
  Color: Colorless to faint pinkish brown.
Uniaxial (+).  ω = 1.525, ε = 1.525, δ= <0.001
  Unit cell data:
a   9.851,  c  13.060 Å.
Z = 2, Space group  P 21m. (Malinovskii and Belov 1980)
  Kalborsite was described and named by Khomyakov et al. (1980). The type material comes from Mount Rasvumchorr, Khibina massif, Kola Peninsula, Russia, and the name is for the composition.
Crystal structure:  
  The kalborsite framework (Malinovskii and Belov 1980), like that of tetragonal edingtonite, is highly ordered
despite the Si/Al ratio being slightly different from 1.5 (1.54 and 1.45 for the two analyzed samples), but has K+, Cl-, and B(OH)4- in the channels. In the figure projected along [001], SiO4 tetrahedra white, AlO4 are gray, B(OH)4 are yellow, and K cations are orange and Cl anions are green. Doubling of the c-axis relative to edingtonite occurs because of ordering of [B(OH)4] tetrahedra and Cl-. Both are bonded to non-framework K, along the [001] channels. [B(OH)4] and Cl- sites correspond to the Ba sites in edingtonite. The different size of [B(OH)4] and Cl- anions causes different distortions of two adjacent cages.

Chemical composition:
  Kalborsite occurs in the pegmatitic veins in two localities, Mt. Rasvumchorr and Mt. Kukisvumchorr, in the Khibina massif of the Kola Peninsula, Russia. At Mt. Rasvumchorr kalborsite is disseminated in lovozerite-rich pegmatite, containing potassium feldspar, nepheline, aegerine, eudialyte, lomosovite, lamprophyllite, shcherbakovite, and magnesian astrophyllite. It is mostly confined to clusters of pectolite that form rims around lovozerite masses. Khomyakov et al. (1980) suggest that kalborsite was originally associated with zirsinalite, which developed from eudialyte through reactions with late fluids supersaturated with alkalis, boron, and volatiles.

On Mt. Kukisvumchorr kalborsite occurs in an alkaline, cancrinite pegmatitic vein as part of a hyperagpaitic assemblage of villiaumite, pirssonite, rasvumite, thermonatrite, among others (Pekov and Chukanov 1996).
  Khomyakov, A.P., Sandomirskaya, S.M. and Malinovskii, Yu. A. 1980. Kalborsite, K6BAl4Si6O20(OH)4Cl, a new mineral. Dokl. Akad. Nauk SSSR 252, 131-134 (translated).

Malinovskii, Yu. A. and Belov, N.V. 1980. Crystal structure of kalborsite. Sov. Phys. Dokl. 25, 331-334 (translated).

Pekov, I.V. and Chukanov, N.V. 1996. New data on kalborsite. Zap. Vses. Mineral. Obshchest. 125, 55-59 (in Russian).