Plombièrite

Chemical formula: Ca<sub>5</sub>Si<sub>6</sub>O<sub>16</sub>(OH)<sub>2</sub>·7H<sub>2</sub>O

A hydrated calcium silicate forming white, porcelain-like aggregates and incrustations, sensitive to dehydration.

## Characteristics Plombièrite is a mineral of the tobermorite group, a hydrated calcium silicate. It most commonly occurs as porcelain-like masses, spherical aggregates, stalactitic incrustations, or as coatings and fissure fillings. It rarely forms microscopic, lath-like crystals. It is an unstable mineral that readily transforms into tobermorite upon water loss, retaining its external form (this is a pseudomorph). ## Physical Properties Its Mohs hardness is approximately 3.5, making it a relatively soft mineral. It has a pearly luster, and in earthy aggregates and porcelain-like masses, it is dull. It is translucent to opaque. The mineral's density is about 2.45 g/cm³. ## Colors and Varieties Plombièrite is usually white, but can also be grayish, yellowish, or slightly brownish, especially when it contains impurities. No distinct color varieties or commercial names are recognized. ## History and Name The mineral's name comes from its discovery locality – the hot springs in Plombières-les-Bains in the Vosges (France). It was first described in 1879, where it was observed in ancient Roman concrete structures that had contact with thermal waters. ## Uses Plombièrite has no industrial applications. It is primarily an object of scientific interest, particularly in the context of cement chemistry and the durability of historical concretes, as well as for collectors.

Properties

Mohs hardness
3.5
Luster
Pearly to Dull
Streak
White
Density
2.45
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Translucent to Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification A characteristic feature of plombièrite is its porcelain-like appearance in compact aggregates, low hardness, and occurrence in the form of incrustations and coatings. It often has a slightly pearly luster on fracture surfaces. Its instability and ease of transformation into tobermorite are also diagnostic features, although difficult to ascertain without laboratory tests. ## Distinguishing from Similar Minerals Plombièrite can be confused with chalcedony or opal, which are, however, much harder (hardness 6.5-7). It differs from calcite and aragonite by its lack of reaction with hydrochloric acid. Visually, it is almost identical to tobermorite, into which it transforms; definitive differentiation of these two minerals requires X-ray diffraction (XRD) analysis. ## Crystal Forms Well-formed crystals are extremely rare and appear as small laths or needles. It usually forms compact, massive, radial, fibrous, reniform, and stalactitic aggregates.

Geological environment

## Genesis Plombièrite is a mineral of low-temperature hydrothermal processes. It forms in fissures and druses of basic and ultrabasic rocks (e.g., serpentinites), in contact metamorphic zones (skarns), and in altered limestones. It is also found in anthropogenic products, such as old concrete and mortars. ## Mineral Associations It most commonly co-occurs with tobermorite, calcite, aragonite, apophyllite, and minerals from the zeolite group (e.g., laumontite, stilbite, thomsonite). ## Localities Key localities worldwide include: Plombières-les-Bains in France (type locality); Crestmore quarry in California, USA; Ballycraigy, Larne in Northern Ireland; areas around Mayen in the Eifel Mountains, Germany; numerous localities in Japan.

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed incrustation shapes (stalactites, reniform aggregates) and a pure, white color. Associations with other, color-contrasting minerals, such as green apophyllite, enhance their attractiveness. Due to its rarity, even small but well-documented samples have collector's value. ## Popular Localities The best quality specimens come from the Crestmore quarry in California and from localities in Northern Ireland, where it formed attractive incrustation shapes.

Care and storage

## Cleaning Plombièrite specimens should be cleaned very carefully, preferably dry with a soft brush. If necessary, a small amount of distilled water can be used, but prolonged soaking should be avoided. Ultrasonic cleaners must absolutely not be used. ## What to Avoid The mineral is very sensitive to heat and dry air, which cause irreversible water loss and transformation into tobermorite. It should be protected from direct sunlight, proximity to heaters, and sudden temperature changes. It is also susceptible to acids. ## Storage It is recommended to store specimens under stable humidity conditions, preferably in closed, padded boxes or display cases, to limit fluctuations in air humidity. Exposure should be away from heat sources and strong lighting.

Sources

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