Pentagonite

Chemical formula: CaV₄+OSi₄O₁₀·₄H₂O

A hydrated calcium vanadium silicate, forming characteristic radial crystal aggregates of an intense blue color.

## Characteristics Pentagonite is a mineral from the silicate group, chemically a hydrated calcium vanadium silicate. Its most recognizable feature is its vibrant, saturated shades of blue, from azure to almost navy. It forms slender, prismatic crystals that almost always occur as radial aggregates, resembling starfish or sea urchins. These specimens are often set on a contrasting white matrix, composed of minerals from the zeolite group, which further enhances their color. Pentagonite is dimorphous with cavansite, meaning both minerals have the same chemical composition but crystallize in different crystallographic systems. ## Physical Properties This mineral is characterized by a vitreous luster and is usually transparent to translucent. Its Mohs hardness is 3-4, making it relatively soft and susceptible to scratching. The density of pentagonite is low, around 2.33 g/cm³, which is typical for hydrated minerals. ## Colors and Varieties The color of pentagonite is its main asset and is very uniform – almost exclusively intensely blue. The hue depends on the content and oxidation state of vanadium. There are no named color varieties or commercial varieties. ## History and Name Pentagonite was first described in 1973 by Lloyd W. Staples, Howard T. Evans Jr., and James R. Lindsay. Its name comes from the Greek word *pente* meaning "five" and refers to the frequent occurrence of characteristic twins, in which five crystals intergrow at an angle of approximately 72°, forming a shape resembling a five-pointed star or a pentagonal cross-section. The type locality (where the mineral was first identified) is the Owyhee Dam in Malheur County, Oregon, USA. ## Uses Due to its rarity, fragility, and small crystal sizes, pentagonite has no industrial application. However, it is a highly valued and sought-after collector's stone.

Properties

Mohs hardness: 3-4
Luster: Vitreous
Streak: Bluish white
Density: 2.33
Cleavage: Good on {010}
Fracture: Uneven
Transparency: Transparent to Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification The most important diagnostic feature of pentagonite is the combination of intense blue color with the characteristic habit of its crystals – slender, prismatic, forming radial aggregates. The often observed fivefold twinning is unique and allows for certain identification. ## Distinguishing from Similar Minerals Pentagonite is most often confused with its dimorph, cavansite. Both have identical color and occur in the same environment. However, pentagonite usually forms longer, more prismatic, and acicular crystals that form "spiky" aggregates, while cavansite more often forms spherical aggregates (rosettes) composed of smaller, tabular crystals. It is distinguished from the also blue azurite by its lack of reaction with hydrochloric acid, lower density, and different crystal habit. ## Crystal Forms Prismatic, elongated crystals, almost always gathered in radial, stellate, or divergent aggregates. Penetration twins are characteristic, in which five individuals intergrow at an almost constant angle, forming five-pointed stars.

Geological environment

## Genesis Pentagonite is a secondary mineral, formed at low temperatures as a result of hydrothermal processes. It crystallizes in voids and fissures (amygdules) in volcanic rocks, such as basalts and andesitic tuffs. ## Mineral Associations It most often co-occurs with minerals from the zeolite group, such as heulandite and stilbite (which often form a white matrix for it), as well as with apophyllite and calcite. Its most common companion is its dimorphous form – cavansite. ## Localities The vast majority of the most beautiful collector's specimens come from one place in the world: the Wagholi quarries in the Pune district, Maharashtra state, India. The type locality in Oregon (USA) has provided very few specimens and has no commercial significance.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued specimens are those with an intense, deeply saturated color, without signs of dulling. The form of the aggregates is crucial – ideal, fully formed, symmetrical "stars" or "sea urchins" fetch the highest prices. The contrast with the matrix is also very important – snow-white heulandite or stilbite provides an ideal background for the blue crystals. The specimen must be free of damage, with sharp, unchipped crystal terminations. ## Popular Localities For collectors, practically only one locality matters: the Wagholi quarries in India. Specimens from this location are considered the global standard for pentagonite.

Care and storage

## Cleaning Pentagonite specimens should be cleaned very carefully. It is best to use a soft brush and compressed air to remove dust. For heavier soiling, distilled water can be used, but prolonged soaking should be avoided. Ultrasonic cleaners should not be used, as they can damage delicate crystals. ## What to Avoid Pentagonite is sensitive to high temperatures, which can cause dehydration, leading to loss of color and dulling. It should be protected from prolonged exposure to direct sunlight, which can cause fading. It is also susceptible to acids and other strong chemicals. Due to its low hardness and fragility, it must be protected from impacts and scratches. ## Storage Collector's specimens of pentagonite are best stored in separate, padded boxes to avoid contact with harder minerals. For display, enclosed display cases are recommended to protect against dust and sudden changes in humidity.