Portlandite

Chemical formula: Ca(OH)<sub>2</sub>

Portlandite is calcium hydroxide, a rare mineral formed mainly in anthropogenic environments, e.g., in old concrete, or as a result of natural metamorphic processes.

## Characteristics Portlandite is calcium hydroxide, known in chemistry as slaked lime. It is rare in nature, forming colorless or white, hexagonal crystals with a tabular habit. It is often found in fibrous, spherulitic aggregates or earthy masses. Crystals can be transparent and exhibit a vitreous or pearly luster on cleavage surfaces. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2.5, meaning it can be scratched with a fingernail. It has a low density, about 2.23 g/cm³. It is characterized by perfect cleavage in one direction, which causes it to easily split into thin lamellae. It is transparent to translucent. ## Colors and Varieties Portlandite is usually colorless or white. It may take on pale green, yellowish, or brownish hues due to impurities. There are no named color varieties or commercial varieties. ## History and Name The mineral's name comes from Portland cement, in whose hardened form it is a common component. It was first described in 1933 by Cecil Edgar Tilley based on material found at Scawt Hill in Northern Ireland. ## Uses Due to its rarity and small crystal size, portlandite has no industrial application as a mineral. Its synthetic counterpart, calcium hydroxide, however, is a key component of building mortars, cement, and plasters, and is also used in the chemical and food industries.

Properties

Mohs hardness
2.5
Luster
Pearly, Vitreous
Streak
White
Density
2.23
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification Key diagnostic features of portlandite are its low hardness (2.5), perfect cleavage in one direction, pearly luster on cleavage planes, and hexagonal crystal outline. It reacts with acids, but less vigorously than calcite. Its instability in humid air is also a characteristic feature. ## Distinguishing from Similar Minerals Portlandite can be confused with brucite (Mg(OH)₂), which has a very similar appearance, crystal habit, cleavage, and hardness. Differentiation requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS/WDS). It differs from gypsum by the absence of water in its structure and a different crystal form. It differs from calcite by significantly lower hardness and the lack of vigorous effervescence with hydrochloric acid. ## Crystal Forms Portlandite crystals are typically hexagonal, thin tablets. It also occurs as fibrous, radial, spherulitic aggregates, as well as massive, earthy, or compact masses.

Geological environment

## Genesis Portlandite is a mineral formed under specific conditions of low-temperature contact metamorphism of calcareous and marly rocks in contact with magmatic intrusions. It also forms in volcanic environments, in lavas and tuffs. It is a product of the hydration of calcium oxide (quicklime) or dehydration of ettringite group minerals. Importantly, it is a common product of human activity (anthropogenic mineral), crystallizing in old concrete, masonry mortars, and coal combustion waste heaps. ## Mineral Associations Minerals co-occurring with portlandite in natural environments include calcite, larnite, brownmillerite, ettringite, afwillite, and hydrocalumite. In anthropogenic environments, it is accompanied by other cement phases. ## Localities The most important natural portlandite localities are Scawt Hill in Northern Ireland (type locality), Hatrurim in Israel, Jebel Awq in Oman, Kopeisk in Russia (Southern Urals), and Mayen in Germany. It also occurs in volcanic exhalations of Vesuvius in Italy.

Rarity

Rare

For collectors

## Quality Criteria The most valued by collectors are well-formed, sharp, transparent hexagonal crystals, especially those set on a contrasting rock matrix. Rich crystalline aggregates are also attractive. Due to the mineral's instability, specimens preserved in perfect condition, without traces of alteration to calcite, are particularly sought after. Crystal size rarely exceeds a few millimeters, so larger specimens are exceptional. ## Popular Localities Classic and most desirable specimens come from localities such as the Hatrurim Complex in Israel, where well-formed crystals have been found. Specimens from Kopeisk in Russia and Mayen in Germany are also valued in the collector's market.

Care and storage

## Cleaning Portlandite specimens are very delicate and sensitive to water. They should be cleaned only dry, using a soft brush or compressed air from a safe distance to remove dust. ## What to Avoid Contact with water, which can cause dissolution or chemical alteration of the mineral, must be absolutely avoided. It is unstable in the presence of carbon dioxide from the air, with which it reacts to form calcite – this process causes clouding and disintegration of crystals. It should be protected from acids and humid air. ## Storage Portlandite requires storage in dry, airtight conditions, preferably in a tightly sealed container (a "perky box") with a desiccant (silica gel). It should be displayed away from sources of moisture and areas with high air circulation.

External references

Sources

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