Strontium Feldspar Identification Guide
Identifying strontium feldspar (slawsonite), a rare strontium-rich feldspar, by its feldspar habit, two cleavages, and tests against common feldspars.
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What Strontium Feldspar Looks Like
Strontium feldspar refers to strontium-bearing members of the feldspar group, the rare species slawsonite (SrAl2Si2O8) being the strontium analogue of the calcium feldspar anorthite. It typically appears as colorless, white, or pale grayish to creamy grains and small crystals with a vitreous to slightly pearly luster, transparent to translucent. Like other feldspars it shows blocky habit, two good cleavage directions meeting at close to 90 degrees, and often occurs intergrown with other feldspars in metamorphic or alkaline igneous rocks.
Step-by-Step Field ID
- Recognize feldspar habit. Blocky, prismatic grains with flat reflective cleavage surfaces.
- Find two cleavages. Two good cleavages intersecting at about 90 degrees give stepped, flat flashes when rotated in light.
- Test hardness. Feldspar hardness ~6-6.5; it scratches glass but is scratched by quartz.
- Check color/luster. Colorless to white/creamy, vitreous to pearly.
- Note the setting. Look for occurrence in strontium-rich metamorphic rocks or alkaline assemblages (definitive ID needs lab analysis).
Key Diagnostic Tests
- Mohs hardness: ~6-6.5, scratches glass.
- Streak: white.
- Cleavage: two good directions near 90 degrees (typical feldspar).
- Specific gravity: elevated for feldspar (~3.0+) due to heavy strontium, higher than common ~2.55-2.76 feldspars.
- Definitive ID: because it looks like ordinary feldspar, confirmation requires chemical analysis (EDS/XRF for Sr) or XRD; field ID is provisional.
Common Look-Alikes
- Plagioclase (albite-anorthite) / orthoclase: visually nearly identical; common feldspars are lighter (SG ~2.55-2.76). Strontium feldspar's higher density and Sr chemistry (lab-detected) distinguish it.
- Celsian (barium feldspar): the barium analogue, also dense; only chemistry separates Ba from Sr feldspar.
- Quartz: harder (Mohs 7), no cleavage (conchoidal fracture), so quartz will scratch the feldspar and lacks flat cleavage flashes.
- Scapolite or zeolites: different cleavage/habit and often associated minerals; testing required.
Where It Is Found
Slawsonite and strontian feldspars are rare, occurring in strontium-enriched metamorphic and alkaline rocks. The type locality for slawsonite is in Oregon (USA); strontian feldspars are also reported from manganese-rich metamorphic deposits and alkaline complexes elsewhere, generally as minor or accessory phases.
Frequently asked questions
What is strontium feldspar?
It is a strontium-rich feldspar; the named species slawsonite (SrAl2Si2O8) is the strontium analogue of anorthite, occurring rarely in strontium-enriched metamorphic and alkaline rocks.
How can you identify strontium feldspar in the field?
You can recognize feldspar (blocky habit, two ~90-degree cleavages, Mohs 6-6.5, white streak) and note an unusually high density, but confirming strontium requires chemical analysis (XRF/EDS) or XRD.
How is strontium feldspar different from ordinary feldspar?
It shares feldspar properties but contains strontium in place of calcium or potassium, giving a higher specific gravity (~3.0+); only chemistry reliably separates it from common feldspars.
Strontium feldspar vs celsian: what is the difference?
Both are dense, rare feldspars, but celsian is the barium feldspar (BaAl2Si2O8) while strontium feldspar (slawsonite) carries strontium; laboratory analysis distinguishes Ba from Sr.