Rubicline
Rubidium aluminosilicate (RbAlSi3O8)
An extremely rare rubidium-dominant alkali feldspar, the rubidium analogue of microcline, found in rare-element granitic pegmatites.
- Mohs hardness
- 6-6.5
- Color
- Colorless to white or pale pink
- Type
- mineral
Got a rock like this?
Identify any rock from a photo, free.
Overview
Rubicline is the rubidium analogue of microcline, with rubidium (Rb) occupying the large alkali site normally filled by potassium. It is one of the rarest members of the feldspar group and was only recognized as a distinct mineral species in the 1990s.
Like its potassium cousin it is a triclinic framework aluminosilicate, but its existence depends on rubidium becoming concentrated enough during the final, most fractionated stages of pegmatite crystallization to dominate the structure.
Because rubidium is geochemically dispersed, rubicline never forms large crystals or massive bodies. It occurs as tiny grains, lamellae, or exsolution intergrowths within other alkali feldspars and pollucite, making it primarily a mineral of scientific rather than commercial interest.
Formation & geology
Rubicline forms in the latest stages of crystallization of highly evolved, rare-element granitic pegmatites, particularly the lithium-cesium-tantalum (LCT) family. As a magma fractionates, incompatible large-ion elements such as cesium and rubidium become enriched in the residual melt and fluids.
When rubidium concentrations become sufficient, it substitutes for potassium in feldspar, locally producing the rubidium-dominant phase. Rubicline is closely associated with pollucite (a cesium mineral) and often occurs as exsolution lamellae or grains within or alongside pollucite and cesian feldspars.
The type locality is the pollucite-bearing pegmatite at San Piero in Campo on the island of Elba, Italy; similar occurrences are reported from other cesium-rich pegmatites worldwide.
How to identify it
Rubicline is essentially impossible to identify in the field by eye. It is colorless to white or faintly pink, has a vitreous luster, a white streak, and a Mohs hardness near 6-6.5 like other feldspars.
It cannot be reliably distinguished from microcline, orthoclase, or pollucite without laboratory analysis (electron microprobe or X-ray methods) to detect its high rubidium content.
Key clue to its possible presence: an association with pollucite and cesium-rich feldspars in a highly evolved pegmatite. Any positive identification requires chemical analysis rather than optical or hardness tests.
Uses & significance
Rubicline has no commercial or ornamental use. It is far too rare and fine-grained to be mined as an ore or cut as a gem.
Its importance is purely scientific: it documents the extreme geochemical fractionation that concentrates rare alkalis like rubidium and cesium, helping mineralogists understand pegmatite evolution and the behavior of large-ion lithophile elements.
The rubidium and cesium economically recovered for industry (catalysts, specialty glass, atomic clocks) come from more abundant minerals such as lepidolite and pollucite, not from rubicline itself.
Frequently asked questions
Is rubicline a type of feldspar?
Yes. It is a triclinic alkali feldspar, the rubidium-dominant analogue of microcline, with rubidium replacing potassium in the crystal structure.
Where is rubicline found?
Its type locality is the pollucite-bearing pegmatite at San Piero in Campo on Elba, Italy, and it occurs in other cesium-rich rare-element pegmatites.
Can you buy rubicline jewelry?
No. Rubicline is microscopic, extraordinarily rare, and has no gem or ornamental value; it is only of scientific interest.
How is rubicline identified?
Only by laboratory chemical analysis such as electron microprobe, because it looks identical to ordinary potassium feldspars.
Rubicline guides
In-depth guides for identifying, valuing, and understanding Rubicline.
