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CategoryFeldspar group
(repeating unit)
(Rb, K)[AlSi3O8]
Strunz classification9.FA.30
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Formula mass313.11 g/mol
Density2.8 g/cm3
Optical propertiesbiaxial
Refractive indexnα = 1.520 nβ = 1.524 nγ = 1.527
Birefringenceδ = 0.007
Other characteristicsRadioactive

Rubicline, also referred to as Rb-microcline, is the rubidium analogue of microcline, an important tectosilicate mineral. Its chemical formula is (Rb, K)[AlSi3O8] with an ideal composition of RbAlSi3O8. Chemical analysis by electron microprobe indicated the average weight of the crystal is 56.66% SiO2, 16.95% Al2O3, and 23.77% Rb2O,[3] along with trace amounts of caesium oxide (Cs2O) and iron(III) oxide (Fe2O3).[4]

Rubicline was first discovered in 1998 in Elba, Italy, by a team from the University of Manitoba. It was the first mineral to have been discovered with rubidium as an essential constituent.[4] It has also been found in Mozambique[5] and the Kola Peninsula in Russia.[6] Rubicline occurs as small, abundant, rounded grains found within veins of rubidian microcline. Pure rubicline with an ideal potassium-free composition has never been found in nature.[3] Rubicline was synthesized in 2001 by placing powdered albite in a solvent of RbCl. This mixture was then placed in a silver tube containing H2O, heated to 400 °C and pressurized to 60 MPa.[3]

Unlike microcline, which can be yellow, red, or green, rubicline is colorless. It is also transparent, brittle,[4] and has a vitreous luster.[7] Rubicline has been classified as both triclinic and monoclinic.[3][7] The crystal does not show twinning. Other minerals in this group include adularia, anorthoclase, buddingtonite, celsian, hyalophane, microcline, monalbite, orthoclase, and sanidine.[7]

Like all potassium compounds, rubicline is mildly radioactive. Activity and dose rate of various amounts of rubicline are listed in the table below.[1]

Specimen weight/size Calculated
activity (Bq)
activity (Ci)
activity GR(api)
exposure (mRem)/hr*
1000 g / 8.79 cm 183,355 4.96×10−6 8,449.31 2.78
100 g / 4.08 cm 18,336 4.96×10−7 844.93 0.28
10 g / 1.89 cm 1,834 4.96×10−8 84.49 0.03
1 g / 8.79 mm 183 4.96×10−9 8.45 0.00
0.1 g / 4.08 mm 18 4.96×10−10 0.84 0.00
0.01 g / 1.89 mm 2 4.96×10−11 0.08 0.00
0.001 g / 0.88 mm 0 4.96×10−12 0.01 0.00
  • If held in hand for one hour.
  • Government estimate of average annual exposure (360 mRem)
  • Max permissible adult dose 50,000 mRem/yr (hands), 15,000 mRem/yr (eyes)
  • Lethal exposure 400,000 to 500,000 mRem


  1. ^ a b Rubicline at Webminerals
  2. ^ Rubicline at Mindat
  3. ^ a b c d Kyono, A. & Kimata, M. (August 2001). "Refinement of the crystal structure of a synthetic non-stoichiometric Rb-feldspar" (PDF). Mineralogical Magazine. 65 (4): 523–531. CiteSeerX doi:10.1180/002646101750377542.
  4. ^ a b c Teertstra, David K.; et al. (December 2008). "Rubicline, a new feldspar from San Piero in Campo, Elba, Italy". American Mineralogist. 83 (11–12): 1335–1339. doi:10.2138/am-1998-11-1223.
  5. ^ Teertstra, D. K.; Cerny, P.; Hawthorne, F. C. (1999). "Subsolidus rubidium-dominant feldspar from the Morrua pegmatite, Mozambique: paragenesis and composition". Mineralogical Magazine. 63 (3): 313–320. CiteSeerX doi:10.1180/002646199548538. ISSN 0026-461X.
  6. ^ "Rubicline R070044". RRUFF. Retrieved 15 February 2010.
  7. ^ a b c Ralph, Jolyon & Chau, Ida (2010). "Rubicline". Retrieved 15 February 2010.