Millisite

Chemical formula: NaCaAl₆(PO₄)₄(OH)₉·₃H₂O

Millisite is a rare sodium, calcium, and aluminum phosphate mineral, typically occurring as radial or fibrous aggregates.

## Characteristics Millisite is a sodium, calcium, and aluminum phosphate with a hydroxyl group and water. It typically forms radial or fibrous aggregates, and less commonly occurs as small, tetragonal crystals. Millisite specimens are usually white or light gray, and colorless in transmitted light. ## Physical Properties The Mohs hardness of millsite is 5.5. This mineral has a density of 2.83 g/cm³. It is transparent to translucent. It does not exhibit distinct cleavage, and its fracture is uneven. ## Colors and Varieties Millisite occurs mainly in white and light gray colors. In transmitted light, it is colorless. No specific color varieties or commercial names are known. ## History and Name Millisite was discovered and described in 1930. The mineral is named after Frank Millis, an American mineralogist and collector, who discovered this mineral in the Tip Top mine in South Dakota, USA. ## Uses Millisite has no industrial applications. It is a mineral primarily of interest to collectors due to its rarity and specific structure.

Properties

Mohs hardness: 5.5
Luster: Szklisty
Streak: Biała
Density: 2.83
Cleavage: Brak
Fracture: Nierówny
Transparency: Przezroczysty do przeświecającego
Crystal system: Tetragonal

Diagnostic features

## Identification Millisite can be identified by its radial or fibrous aggregates and its white or light gray color. Its Mohs hardness of 5.5 and tetragonal crystal system are also helpful. ## Distinguishing from Similar Minerals Millisite can be confused with other white, fibrous phosphate minerals. X-ray diffraction and chemical analysis are key to distinguishing it, confirming its composition and crystal structure. ## Crystal Forms Millisite most commonly occurs as radial or fibrous aggregates. Less frequently, it forms small, euhedral crystals with a tetragonal habit.

Geological environment

## Genesis Millisite is a secondary mineral, forming in the oxidation zones of phosphate deposits, often in pegmatites, where it replaces other phosphate minerals. It forms as a result of hydrothermal processes or weathering. ## Mineral Associations Millisite often co-occurs with other phosphates, such as apatite, lazulite, wardite, and with minerals such as quartz and muscovite. ## Localities The most important occurrences of millsite are in the Tip Top mine in South Dakota, USA, where it was discovered. It also occurs in other localities in the USA, Brazil, Australia, and Namibia.

Rarity

Very Rare

For collectors

## Quality Criteria The most prized millsite specimens are those that exhibit well-formed, radial aggregates with an intense white or light gray color. Large, aesthetic specimens are particularly sought after by collectors. ## Popular Localities The most sought-after millsite specimens come from the Tip Top mine in South Dakota, USA, which is its type locality. Specimens from other localities are also valued if they exhibit good aesthetic quality.

Care and storage

## Cleaning Millisite can be cleaned gently using a soft brush and lukewarm water. For more stubborn dirt, distilled water can be used to avoid hard water deposits. ## What to Avoid Avoid using strong chemical agents that can damage the mineral's surface. Prolonged exposure to direct sunlight may lead to slight color changes. Also, avoid sudden temperature changes. ## Storage Millisite is best stored in a dry place, away from dust and direct light. It is ideally suited for display in a closed cabinet, which protects it from mechanical damage and contamination.