Piemontite-(Sr)
Chemical formula: CaSr(Al<sub>2</sub>Mn<sup>3+</sup>)(Si<sub>2</sub>O<sub>7</sub>)(SiO<sub>4</sub>)O(OH)
Piemontite-(Sr) is a rare sorosilicate of the epidote group, distinguished by strontium dominance and a characteristic reddish-purple color.
Description
## Characteristics Piemontite-(Sr) is the strontium analog of piemontite and belongs to the epidote group. It forms elongated, prismatic crystals that rarely exceed a few millimeters in size. It typically occurs as radial or granular aggregates embedded in the host rock. Its most distinctive feature is an intense, reddish-purple to reddish-black color, resulting from the presence of trivalent manganese (Mn³⁺). ## Physical Properties This mineral is characterized by a Mohs hardness of 6 and a vitreous luster. It is brittle, and its density, 3.77 g/cm³, is slightly higher than that of piemontite due to the presence of heavier strontium. It is usually translucent, becoming opaque in thicker sections or darker varieties. ## Colors and Varieties The color of piemontite-(Sr) is its main visual distinguishing feature, ranging from purplish-red, through red, to reddish-black. The intensity and hue depend on the manganese concentration. There are no commercial or color varieties distinguished. ## History and Name The mineral's name refers to its chemical composition – it is piemontite with dominant strontium (Sr) in one of the structural positions. It was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 1990. Its type locality is the Prabornaz mine in the Aosta Valley, Italy, a historical source of many manganese minerals. ## Applications Piemontite-(Sr) has no industrial applications. Its significance is purely scientific and collectible, being an object of interest for mineral systematics specialists and collectors of rare species.
Diagnostic features
## Identification Visual identification of piemontite-(Sr) is based on its characteristic reddish-purple color, prismatic crystal form, and occurrence in manganese-rich metamorphic rocks. Vitreous luster and relatively high hardness are auxiliary features. ## Distinguishing from Similar Minerals It is visually indistinguishable from **piemontite**. The only reliable method of identification is advanced chemical analysis, such as X-ray microanalysis (EDS/WDS), which can confirm the dominance of strontium over calcium in the appropriate structural position. From other red minerals, such as almandine, it is distinguished by crystal form and occurrence environment. ## Crystal Forms It forms columnar, elongated crystals with a rhombic cross-section. It often occurs as radial or disordered aggregates, as well as inclusions in quartzites.
Geological environment
## Genesis Piemontite-(Sr) forms under conditions of regional metamorphism, in rocks rich in manganese and silica, such as manganiferous quartzite. Its formation is associated with processes occurring under relatively high pressure and low temperature. ## Mineral Associations In its type locality, it coexists with minerals such as braunite, quartz, albite, hematite, hollandite, cryptomelane, and other rare manganese minerals, such as ardennite-(As) or piemontite. ## Localities The most important and historical occurrence, from which the best specimens originate, is the Prabornaz mine, Saint-Marcel, in the Aosta Valley, Italy. This is its type locality. It is also known from several other, few localities worldwide, including Japan (Shiromaru mine, Tokyo).
Rarity
Very rare
Collector aspects
## Quality Criteria The quality of piemontite-(Sr) specimens is primarily assessed based on the intensity and attractiveness of its reddish-purple color. Well-formed, sharply terminated crystals are highly valued, especially when they form contrasting compositions on a light quartz matrix. Due to its rarity, even small but distinct crystals in rich aggregates are considered valuable. ## Popular Localities The vast majority of the best collectible specimens come from a single location in the world – the historic Prabornaz mine in Italy. Specimens from this locality are the standard for this mineral.
Care and storage
## Cleaning Specimens should only be cleaned mechanically, using a soft brush and distilled water. Ultrasonic cleaners should be avoided, as they can cause fractures in brittle crystals. ## What to Avoid The mineral is sensitive to strong acids, which can etch it. It should be protected from sudden temperature changes and impacts due to its brittleness. ## Storage Store in separate, padded display boxes to prevent scratches and mechanical damage. The label should clearly state that it is piemontite-(Sr) to avoid confusion with ordinary piemontite.