Rubicline

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,^[4] along with trace amounts of caesium oxide (Cs₂O) and iron(III) oxide (Fe₂O₃).^[5]

Rubicline
General
Category	Tectosilicate minerals, feldspar group
Formula	(Rb, K)[AlSi3O8]
IMA symbol	Rub[1]
Strunz classification	9.FA.30
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Identification
Formula mass	313.11 g/mol
Color	Colorless
Cleavage	{001}
Luster	Vitreous
Diaphaneity	Transparent
Density	2.8 g/cm3
Optical properties	biaxial
Refractive index	nα = 1.520 nβ = 1.524 nγ = 1.527
Birefringence	δ = 0.007
Dispersion	Strong
Other characteristics	Radioactive
References	[2][3]

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.^[5] It has also been found in Mozambique^[6] and the Kola Peninsula in Russia.^[7] 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.^[4] 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.^[4]

Unlike microcline, which can be yellow, red, or green, rubicline is colorless. It is also transparent, brittle,^[5] and has a vitreous luster.^[8] Rubicline has been classified as both triclinic and monoclinic.^[4][8] The crystal does not show twinning. Other minerals in this group include adularia, anorthoclase, buddingtonite, celsian, hyalophane, microcline, monalbite, orthoclase, and sanidine.^[8]

Like all rubidium compounds, rubicline is mildly radioactive. Activity and dose rate of various amounts of rubicline are listed in the table below.^[2]

Specimen weight/size	Calculated activity (Bq)	Calculated activity (Ci)	Estimated activity GR(api)	Estimated 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