Slawsonite

Chemical formula: SrAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>

Slawsonite is a rare strontium and aluminum silicate from the feldspar group, forming colorless or white, vitreous crystals and granular aggregates.

## Characteristics Slawsonite is a mineral belonging to the feldspar group, specifically to the celsian series. It is a strontium and aluminum silicate. It most often occurs in the form of small, tabular or prismatic crystals, which can be aggregated into granular or massive forms. Crystals are usually colorless, white, or grayish, often with milky turbidity. They are characterized by a vitreous luster. ## Physical Properties This mineral is relatively hard, with a Mohs hardness of about 6-6.5. It has a vitreous luster, and on cleavage planes, it can be pearly. It is transparent to translucent. Its density is higher than that of most feldspars, approximately 3.23 g/cm³. ## Colors and Varieties Slawsonite does not form colored varieties. Its coloration is limited to white, gray, and colorless. It has no trade or gemological names. ## History and Name The mineral is named in honor of Donald B. Slawson (1933-2014), a professor of mineralogy at the University of California, Santa Barbara (USA), who first found this mineral. It was described and approved as a new mineral species in 1977 by R.C. Erd, A.C. Gossard, D.B. Slawson, G.E. Dunning, and J.F. Cooper, Jr. ## Uses Slawsonite has no industrial application. Its significance is purely scientific and collectible, as a rare representative of the feldspar group.

Properties

Mohs hardness
6-6.5
Luster
Vitreous, Pearly
Streak
White
Density
3.23
Cleavage
Perfect on {001}, good on {010}
Fracture
Uneven, Conchoidal
Transparency
Transparent to Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Slawsonite is difficult to identify visually without specialized tests. Its characteristic features include its occurrence as small, tabular crystals with a vitreous luster, relatively high hardness (approx. 6-6.5), and a higher density than other feldspars. It also exhibits a characteristic, light blue fluorescence under shortwave UV light. ## Differentiation from Similar Minerals It can be confused with other feldspars, such as celsian, orthoclase, or albite. Differentiation usually requires chemical analysis (detection of strontium) or X-ray diffraction (XRD) studies. It is distinguished from quartz by the presence of cleavage. Compared to other feldspars, slawsonite has a distinctly higher density. ## Crystal Forms Slawsonite crystals are usually small, with a tabular or short-prismatic habit. They often form twins, similar to other feldspars. It also occurs as granular, massive, or embedded aggregates in the host rock.

Geological environment

## Genesis Slawsonite is a hydrothermal mineral. It forms in veins cutting low-grade metamorphic rocks, such as rodingites, serpentinites, or glaucophane schists. Its crystallization occurs under relatively low temperatures and pressures, from solutions rich in strontium, aluminum, and silica. ## Mineral Associations This mineral often co-occurs with minerals such as: pectolite, prehnite, valentinite, natrolite, thomsonite, garnets (andradite), diopside, vesuvianite, titanite, and calcite. ## Localities The most important and classic slawsonite localities worldwide are: - United States: Santa Cruz County, California (type locality), Martin Marietta Quarry in Iowa, and Wallowa-Whitman National Forest in Oregon. - Japan: Wessels Mine on Honshu Island. - Sweden: Jakobsberg Mine in the Värmland region.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, sharp, and transparent crystals that are clearly visible on the rock matrix are most valued by collectors. Specimens from rare localities and those that form aesthetic compositions with other, color-contrasting associated minerals are also highly prized. Crystal size is a key factor – specimens with crystals exceeding a few millimeters are considered exceptional. ## Popular Localities Specimens from the type locality in California (USA) and from the Jakobsberg Mine in Sweden are most sought after by collectors due to their historical significance and crystal quality.

Care and storage

## Cleaning Slawsonite specimens can be cleaned with lukewarm water and a mild soap, using a soft brush. Strong detergents should be avoided. After washing, the specimen should be thoroughly rinsed, preferably in distilled water, and left to dry completely. ## What to Avoid This mineral is sensitive to acids, especially hydrofluoric acid. Contact with household and laboratory chemicals should be avoided. It should not be subjected to sudden temperature changes or cleaned in ultrasonic cleaners. ## Storage Slawsonite is a relatively hard but brittle mineral. It is best stored in separate, padded boxes or display cases to prevent scratching by harder minerals (e.g., quartz, beryl) and mechanical damage. It is stable under household conditions and does not require special protection from light or humidity.

Sources

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