Hillite

Chemical formula: Ca<sub>2</sub>Zn<sup>2+</sup>(PO<sub>4</sub>)<sub>2</sub>·2H<sub>2</sub>O

Hillite is a rare, hydrated zinc calcium phosphate, forming small, colorless or pale pink crystals.

## Characteristics Hillite is a mineral from the roselite group, being its zinc analog. It forms small, tabular or prismatic crystals, usually less than one millimeter in size. It most often occurs as single, well-formed crystals or small, radial aggregates growing on a rock matrix within small voids and fissures. ## Physical Properties Hillite crystals are transparent and characterized by a vitreous luster. This mineral is relatively soft, with a Mohs hardness of 3.5. It is quite brittle and exhibits perfect cleavage in one direction. Its calculated density is approximately 3.55 g/cm³. ## Colors and Varieties Hillite is most often colorless, but it can also exhibit a delicate, pale pink tint. No named varieties have been distinguished. ## History and Name The mineral was approved by the IMA in 2009. Its name honors Roderick "Rod" J. Hill (born 1949), an Australian crystallographer and mineralogist from CSIRO, for his contributions to mineralogy. It was described based on material from the Block 14 mine in Broken Hill, Australia. ## Applications Due to its rarity and small crystal size, hillite has no industrial applications. It is solely an object of interest for scientists and specialized collectors of rare minerals.

Properties

Mohs hardness
3.5
Luster
Vitreous
Streak
White
Density
3.55
Cleavage
Perfect on {010}
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Identifying hillite under collecting conditions is difficult. It relies on observing small, tabular crystals with a vitreous luster, pale pink color or its absence, and the locality. Association with other rare, secondary phosphates from Broken Hill or Tsumeb is crucial. ## Distinguishing from Similar Minerals Hillite is visually indistinguishable from other minerals of the roselite group, such as roselite (Co) or brandtite (Mn). Roselite is usually more intensely colored pink or red, but color is not a diagnostic feature. Definitive differentiation requires advanced chemical analyses (e.g., EDS). ## Crystal Forms It forms flattened, tabular crystals with a prismatic habit. They usually occur as single, overgrown crystals or small, divergent groups in vugs and rock fissures.

Geological environment

## Genesis Hillite is a secondary mineral, forming in the oxidation zone of zinc and lead ore deposits. It crystallizes at low temperatures in voids and fissures of highly mineralized rocks, as a result of the alteration of earlier minerals. ## Mineral Associations It most often co-occurs with other secondary phosphates. In the type locality (Broken Hill), these include scholzite, phosphogartrellite, switzerite, parascholzite, and zincian brandtite. ## Localities The most important and type locality is the Block 14 open pit in Broken Hill, New South Wales, Australia. This mineral has also been identified in the famous Tsumeb mine in Namibia.

Rarity

Very rare

For collectors

## Quality Criteria The most highly prized specimens have sharply terminated, transparent crystals with a distinct, pale pink coloration, set on a contrasting rock matrix. Association with other rare and well-formed minerals is also of great importance. Due to the microscopic size of the crystals, their quality and abundance on the specimen's surface are key. ## Popular Localities Specimens from the type locality – Broken Hill, Australia – are by far the most sought after by collectors, considered exemplary for this species. Material from the Tsumeb mine is also highly valued due to the historical significance of this locality.

Care and storage

## Cleaning Hillite specimens should be cleaned with the utmost care, using only distilled water and a very soft brush to remove dust. Ultrasonic cleaners should be avoided, as they could damage the fragile crystals. ## What to Avoid The mineral is soft, brittle, and has perfect cleavage, making it very susceptible to mechanical damage. Contact with acids and other chemicals should be avoided. As a hydrated mineral, it is sensitive to high temperatures, which can lead to its dehydration and structural damage. ## Storage It is recommended to store specimens in separate, padded "micromount" boxes to protect them from shocks, friction, and dust. Stable humidity and temperature conditions should be ensured.

Sources

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