Putnisite

Chemical formula: SrCa₄Cr³⁺₈(CO₃)₈S⁶⁺O₄(OH)₁₆·₂₅H₂O

Putnisite is a rare mineral with a unique chemical composition and a purple-pink color, distinguished by an unusual combination of strontium, calcium, chromium, sulfur, and carbonates.

## Characteristics Putnisite is a complex mineral from the carbonate and sulfate groups, notable for its unique chemical composition, unlike any other known mineral. It forms small, pseudocubic (cubic) crystals, reaching up to 0.5 mm in size. It most commonly occurs as single crystals or small aggregates, scattered on a rock matrix. Its characteristic features are an intense, purple-pink color and a vitreous luster. ## Physical Properties The hardness of putnisite on the Mohs scale is low, ranging from 1.5-2, which means it is a very soft mineral. It has a vitreous luster and is translucent. The calculated density is 2.20 g/cm³. It is a brittle mineral with an uneven fracture. ## Colors and Varieties This mineral occurs in a characteristic range of colors from dark pink to purple. No distinct varieties have been identified. ## History and Name Putnisite was discovered in 2011 at the Polar Bear Peninsula in the southern part of Lake Cowan in Western Australia. It was named in honor of mineralogists Andrew and Christine Putnis from the University of Münster in Germany, in recognition of their contributions to mineralogy. The description of the new mineral was approved by the International Mineralogical Association (IMA) in 2012. ## Applications Due to its extreme rarity and small crystal size, putnisite has no industrial applications. It is solely an object of scientific interest and a collector's item for advanced collectors of rare minerals.

Properties

Mohs hardness: 1-2
Luster: Vitreous
Streak: Pink
Density: 2.20
Cleavage: One excellent and two good cleavages parallel to {100}, {010} and {001}.
Fracture: Irregular/Uneven
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Putnisite is relatively easy to identify visually due to its unique appearance. Key features include its characteristic purple-pink color, pseudocubic (cubic) shape of small crystals, and occurrence on a dark, chromite rock matrix. Its extreme softness is also a diagnostic feature, though difficult to verify without damaging the specimen. ## Distinguishing from Similar Minerals Due to its unique combination of color, crystal form, and occurrence environment, putnisite is difficult to confuse with other minerals. Sugilite may have a similar color but forms massive aggregates and occurs in a completely different geological environment. Other pink or purple minerals, such as cobaltoan calcite or some fluorites, have entirely different crystal forms and higher hardness. ## Crystal Forms Putnisite crystals, although crystallizing in the orthorhombic system, have a distinctly pseudocubic appearance, resembling small cubes. They reach a size of up to 0.5 mm and are scattered on the surface of the host rock.

Geological environment

## Genesis Putnisite forms under specific volcanic conditions. In its only known locality, it occurs in volcanic rock (komatiite) that has undergone weathering processes. It is believed to be a product of secondary alteration of primary minerals, such as sulfides (e.g., millerite) and chromite, in an environment rich in strontium and calcium derived from weathering silicates. ## Mineral Associations This mineral co-occurs with chromite, quartz, magnesite, millerite, nickel, and gypsum. ## Localities The only confirmed locality of putnisite in the world is its type locality – the "Polar Bear" outcrop, on the Polar Bear Peninsula, south of the town of Norseman in Western Australia.

Rarity

Extremely rare

For collectors

## Quality Criteria The collector's appeal of a putnisite specimen depends on several factors. The most important is the intensity and saturation of the purple-pink color. Specimens with numerous, well-formed, and sharp crystals of pseudocubic form are more highly valued. Contrast with the dark chromite matrix, on which the crystals are clearly visible, is also important. The size of individual crystals, although always small, also affects the value – the larger they are, the more valuable the specimen. ## Popular Localities All specimens available on the collector's market come from a single location in the world: the "Polar Bear" outcrop in Western Australia. This is the only known locality for this mineral.

Care and storage

## Cleaning Putnisite specimens should be cleaned with the utmost care due to their extreme softness. For dust removal, it is best to use compressed air or a very soft brush (e.g., a makeup brush). Contact with water is not recommended. ## What to Avoid Avoid all chemicals, acids, and detergents. The mineral is very soft and brittle, so it must be protected from any physical contact, impacts, and scratching. It should not be subjected to ultrasonic cleaning. High temperatures should also be avoided. ## Storage Putnisite specimens should be stored in separate, closed display boxes to prevent abrasion and contact with harder minerals. It is best to keep them in stable room conditions, away from dust, humidity, and direct sunlight.