Scawtite

Chemical formula: Ca₇(Si₃O₉)₂(CO₃)·₂H₂O

Scawtite is a rare calcium silicate, forming colorless or white, vitreous crystals and aggregates, found in contact metamorphic zones of carbonate rocks.

## Characteristics Scawtite is a complex, hydrated calcium silicate and carbonate. It most commonly occurs as small, tabular or prismatic crystals, rarely exceeding a few millimeters in length. It usually forms compact, granular, or fibrous aggregates, as well as crusts and veins filling fractures in the host rock. It is a colorless, white, or light gray mineral, often transparent with a vitreous luster. ## Physical Properties This mineral is characterized by a Mohs hardness ranging from 4.5 to 5. Its luster is vitreous, and on cleavage planes, it can be pearly. It is transparent to translucent. The density of scawtite is approximately 2.77 g/cm³. It exhibits perfect cleavage in one direction. ## Colors and Varieties Scawtite does not have colored varieties. Its coloration is constant and limited to white, shades of gray, and colorless. No commercial or gemological varieties are known. ## History and Name The mineral's name comes from its first discovery locality - Scawt Hill in County Antrim, Northern Ireland. It was first described by Cecil Edgar Tilley in 1929, who was also the discoverer of many other contact metamorphic minerals from the same locality. ## Uses Scawtite has no industrial application. Its significance is limited to the scientific sphere as an indicator of specific metamorphic conditions and for collecting rare minerals.

Properties

Mohs hardness: 4.5-5
Color: Colourless
Luster: Vitreous, Pearly
Streak: White
Density: 2.71
Cleavage: {010} fair {100} poor
Fracture: Uneven
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification A key diagnostic feature of scawtite is its vigorous reaction with hydrochloric acid, during which carbon dioxide is released and a silica gel forms. In field conditions and in a collection, it can be recognized by its occurrence in a characteristic geological environment (contact rocks), white color, vitreous luster, and relatively low hardness. ## Distinguishing from Similar Minerals Scawtite is sometimes confused with other white minerals from contact zones, such as wollastonite, calcite, or zeolites. - It differs from **calcite** by its slightly higher hardness (calcite has a hardness of 3) and the formation of silica gel after reaction with acid. - It differs from **wollastonite** by its vigorous reaction with acid (wollastonite reacts very weakly or not at all). - It differs from **zeolites** (e.g., natrolite) by the absence of characteristic acicular crystal forms and strong effervescence in acid. ## Crystal Forms Scawtite crystals are monoclinic, usually tabular or short-prismatic. However, it is more often found in the form of compact, fibrous, or granular aggregates, as well as fillings of small veins.

Geological environment

## Genesis Scawtite is a product of low-grade contact metamorphism. It forms in contact zones of hot, basic magmatic intrusions (e.g., diabases, gabbros) with carbonate rocks, such as limestones or chalk. It forms at relatively low temperatures, typically below 200°C, in calcium- and silica-rich conditions. ## Mineral Associations This mineral often co-occurs with other products of contact metamorphism of carbonate rocks. Its typical associated minerals include larnite, spurrite, tobermorite, calcite, portlandite, hydrocalumite, and various minerals from the zeolite group. ## Localities The most important and classic locality for scawtite is its discovery site - Scawt Hill in Northern Ireland. Other known localities include the Crestmore mine in California (USA), the Hatrurim area in Israel, Boisséjour in France, Mayen in Germany, and some occurrences in Japan and Russia (Siberia).

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, transparent crystals, even if small, are most valued by collectors. Distinct contrast with darker host rock (matrix) and co-occurrence with other rare contact minerals, such as larnite or spurrite, enhance attractiveness. Large, rich fibrous or granular aggregates are also sought after. ## Popular Localities Specimens from the type locality, Scawt Hill in Northern Ireland, hold particular historical and scientific value. High-quality crystals also come from the Crestmore mine in California, which is known for providing many rare contact metamorphic minerals.

Care and storage

## Cleaning Scawtite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Due to its reactivity with acids, no chemical cleaning agents should be used. ## What to Avoid Contact with acids (even weak ones, like vinegar) must be strictly avoided, as they cause vigorous effervescence and decomposition. The mineral is relatively soft and brittle, so it must be protected from impacts and scratching. It should not be exposed to high temperatures. ## Storage Collector's specimens of scawtite are best stored in separate, padded boxes or display cases to avoid contact with harder minerals. They should be protected from dust and moisture. Display should be in a closed cabinet.