Slawsonite Identification Guide

What Slawsonite Looks Like

Slawsonite is a rare strontium-rich feldspar, SrAl₂Si₂O₈, the strontium analogue of paracelsian and a relative of celsian (the barium feldspar). It is a collector and research mineral, not a gemstone, and most specimens are small. Crystals and grains are colorless to white or pale gray, with a vitreous (glassy) luster and transparent to translucent appearance. It typically occurs as small tabular to prismatic crystals or as anhedral grains intergrown with other minerals in metamorphosed manganese- or strontium-bearing rocks.

Because it is so rare and so similar to other pale feldspars, slawsonite is almost never identified confidently by eye alone — field identification leans heavily on its unusual geological context and is normally confirmed in a lab.

Step-by-Step Field ID Checklist

1. Establish the setting. Slawsonite is found in strontium-enriched metamorphic and altered igneous rocks, often associated with manganese minerals. The host rock is the first clue.
2. Look for pale, glassy grains. Search for colorless to white vitreous crystals or grains with the blocky, feldspar habit.
3. Check for cleavage. Like other feldspars it shows good cleavage in two directions meeting at close to 90°, producing flat reflective faces.
4. Note associated minerals. Common companions include strontium and barium minerals, manganese silicates, and other feldspars.
5. Bag and confirm. Because of its rarity, suspected slawsonite should be confirmed by XRD, microprobe, or SEM analysis.

Key Diagnostic Tests

• Hardness: About 6–6.5, typical of feldspar; scratches glass.
• Streak: White.
• Cleavage: Good cleavage in two directions intersecting near 90°.
• Density: Elevated for a feldspar (around 3.1 g/cm³) due to strontium content — heavier in the hand than common feldspar.
• Acid: No reaction to dilute hydrochloric acid.
• Optics/chemistry: Definitive identification requires detection of strontium, which standard field tools cannot do.

Common Look-Alikes and How to Tell Them Apart

• Paracelsian: Its barium-free polymorph relationship is complex; the two are chemically distinguished (Sr vs structural details) only in the lab.
• Celsian (barium feldspar): Also pale and vitreous; celsian carries barium instead of strontium, and the two cannot be reliably separated by eye.
• Common plagioclase/orthoclase feldspar: Lighter (lower density), far more abundant, and lacking the strontium-rich, manganese-associated host rock.
• Quartz: Higher hardness (7), no cleavage (conchoidal fracture), and lower density distinguish it from slawsonite.

Where Slawsonite Is Found

Slawsonite was first described from Australia and is reported from a small number of localities worldwide, including strontium-bearing metamorphic rocks in Japan and altered volcanic rocks elsewhere. It is a genuinely rare species, so most collectors encounter it only as labeled museum or micromount specimens rather than as field finds.