Rubicline Identification Guide

What Rubicline Looks Like

Rubicline is an extremely rare feldspar — the rubidium-dominant analogue of microcline (ideally RbAlSi3O8). It is a true rarity, known from only a few pegmatite occurrences, and is not a commercial gem.

• Color: Colorless to white, pale shades; not visually distinctive.
• Luster: Vitreous to pearly on cleavage surfaces.
• Transparency: Transparent to translucent.
• Habit: Occurs intergrown with other feldspars (microcline, albite) in cesium-rich pegmatites; rarely as discrete crystals.

Step-by-Step Field-ID Checklist

1. Confirm feldspar traits. Two cleavages meeting at about 90 degrees and a vitreous-to-pearly luster point to the feldspar group.
2. Test hardness — scratches glass (Mohs ~6-6.5).
3. Note the setting — only highly evolved, rubidium- and cesium-rich granitic pegmatites (e.g., with pollucite).
4. Recognize the limits. Rubicline cannot be identified by eye; it is confirmed only by chemical analysis showing rubidium dominance over potassium.
5. Treat any "rubicline" specimen skeptically without analytical data.

Key Diagnostic Tests

• Mohs hardness: ~6-6.5.
• Streak: White.
• Cleavage: Two good cleavages at ~90 degrees (typical feldspar).
• Density: Slightly higher than common K-feldspar due to heavy rubidium.
• Acid: Inert.
• Definitive ID: Electron microprobe / X-ray analysis is mandatory — Rb must dominate the alkali site. This is a laboratory-only determination.

Common Look-Alikes and How to Tell Them Apart

• Microcline/orthoclase (K-feldspar): Visually identical; only chemistry (Rb vs K) separates rubicline. This is the central look-alike problem.
• Albite (Na-feldspar): Plagioclase with fine twinning striations on cleavage; rubicline lacks the plagioclase albite-twin lamellae.
• Quartz: No cleavage and harder (7); feldspar shows two cleavages at right angles.
• Pollucite: Associated cesium mineral, but isometric with different habit.

Where It Is Found

Rubicline was first identified from cesium-rich pegmatites — notably the Elba (Italy) pollucite-bearing pegmatites and similar rare-element pegmatites. It is one of the rarest feldspars and is essentially a specimen and research mineral.

Field Tips and Common Mistakes

The honest field determination for rubicline is "alkali feldspar, species undetermined." You can confirm the feldspar group from two cleavages near 90 degrees, Mohs around 6-6.5, and a vitreous-to-pearly luster, but nothing visible distinguishes the rubidium-dominant rubicline from ordinary potassium microcline. If a specimen is labeled rubicline, ask for the supporting microprobe or X-ray data; without it, the identification is unverifiable.

A common mistake is expecting rubicline to look exotic because it is rare. It does not — it is a plain pale feldspar that happened to incorporate enough rubidium to cross a compositional threshold. Collectors should also avoid confusing it with quartz: quartz has no cleavage and is harder (Mohs 7), so a piece that splits cleanly along two flat planes at right angles is feldspar, not quartz, narrowing things to the alkali feldspar group before lab work decides the rest.