Slawsonite
Strontium aluminosilicate (SrAl2Si2O8)
A rare strontium-dominant feldspar, the strontium analogue of paracelsian, found in metamorphosed strontium-rich and manganese-bearing rocks.
- Mohs hardness
- 6
- Color
- Colorless, white, to pale gray
- Type
- mineral
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Overview
Slawsonite is a rare feldspar-group mineral in which strontium (Sr) takes the place of calcium or barium in the large cation site. It is structurally the strontium analogue of paracelsian and is closely related to celsian (the barium feldspar).
It is a framework aluminosilicate built of linked silica and alumina tetrahedra, but the strong preference for strontium makes it uncommon, since strontium is rarely concentrated enough in rocks to dominate a mineral.
Slawsonite typically appears as small colorless to white grains and is recognized mainly in specialized metamorphic and ore environments where strontium has been locally enriched.
Formation & geology
Slawsonite forms in low- to medium-grade metamorphic rocks where strontium has been concentrated, commonly in association with strontium- and barium-rich silicates and carbonates. It is also reported from manganese-bearing metamorphosed sediments and from some altered volcanic rocks.
The strontium needed to make the mineral generally derives from strontium-rich protoliths such as celestine-bearing sediments or hydrothermally altered rocks. During recrystallization, strontium partitions into the feldspar structure to form slawsonite rather than ordinary plagioclase.
Reported localities include occurrences in the Wallowa Mountains of Oregon, USA, and metamorphic manganese deposits in Japan, where it accompanies other rare Sr-Ba minerals.
How to identify it
Slawsonite is colorless to white or pale gray, with a vitreous luster, white streak, and Mohs hardness around 6, typical of feldspars. It shows good cleavage like other feldspars.
It cannot be visually separated from paracelsian, celsian, or ordinary feldspar; its strontium dominance is only confirmed by chemical analysis (electron microprobe) or X-ray diffraction.
Look-alikes include celsian and paracelsian (barium-dominant) and common plagioclase (calcium-sodium). The diagnostic difference is strontium content, which requires laboratory testing rather than field methods.
Uses & significance
Slawsonite has no commercial, industrial, or gem use. It is far too rare and fine-grained to be exploited.
Its value lies in mineralogy and petrology, where it serves as an indicator of unusual strontium enrichment during metamorphism and helps researchers study how strontium and barium behave in silicate systems.
Strontium of economic importance is obtained from celestine and strontianite, not from slawsonite.
Frequently asked questions
What is slawsonite?
It is a rare strontium-dominant feldspar, the strontium analogue of paracelsian, with the formula SrAl2Si2O8.
Is slawsonite a gemstone?
No. It occurs as tiny grains in metamorphic rocks and has no ornamental or commercial value.
How is slawsonite different from celsian?
Celsian is barium-dominant (BaAl2Si2O8) while slawsonite is strontium-dominant; the two look alike and need chemical analysis to tell apart.
Where does slawsonite occur?
In strontium-enriched metamorphic and manganese-bearing rocks, including localities in Oregon, USA, and in Japan.
Slawsonite guides
In-depth guides for identifying, valuing, and understanding Slawsonite.
