Rankinite

Chemical formula: Ca₃Si₂O₇

Rankinite is a rare calcium silicate, occurring as colorless to white, fine grains or aggregates in contact metamorphic rocks.

## Characteristics Rankinite is a mineral belonging to the sorosilicate group, chemically defined as a calcium silicate. It typically occurs as very fine, anhedral (not exhibiting its own crystal form) grains, which can form compact or earthy aggregates. It is rarely observed as small, platy crystals. It is colorless or white, with a vitreous luster. ## Physical Properties This mineral is characterized by a Mohs hardness of approximately 5.5. Its density is similar to that of quartz, around 2.96 g/cm³. It exhibits a white streak and vitreous luster. It is a brittle mineral with an uneven fracture. Some sources indicate possible, though indistinct, cleavage. ## Colors and Varieties Rankinite is usually colorless or white. Due to its rarity and mode of occurrence, no colored or commercial varieties are distinguished. ## History and Name Rankinite was first described in 1942 by Cecil Edgar Tilley. The mineral is named in honor of George Rankin, a chemist working at the Geophysical Laboratory of the Carnegie Institution for Science in Washington, D.C., who first synthesized its chemical equivalent (Ca₃Si₂O₇) in 1915. The type locality from which the first described samples originated is Scawt Hill in County Antrim, Northern Ireland. ## Uses Rankinite has no industrial applications. Its significance is limited to the scientific sphere, as a component of certain metamorphic rocks and industrial products (cement, slag), and to collector interest due to its rarity.

Properties

Mohs hardness
5.5
Color
Colourless
Luster
Vitreous
Streak
White
Density
2.96
Cleavage
(100)
Fracture
Uneven
Transparency
Transparent,Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Amateur identification of rankinite is extremely difficult. Macroscopically, it is most often a white or colorless mineral with a granular texture, occurring in thin layers or aggregates. Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals Rankinite can be easily confused with many other white contact minerals, such as wollastonite, larnite, spurrite, or calcite. It is distinguished from calcite by its lack of reaction with cold, dilute hydrochloric acid and its higher hardness. Differentiation from other calcium silicates is practically impossible without specialized laboratory equipment. ## Crystal Forms Rankinite crystals are extremely rare. If they occur, they take the form of small, elongated blades or needles, reaching at most a few millimeters in length. However, it usually forms fine-grained, compact, or powdery aggregates.

Geological environment

## Genesis Rankinite is a product of high-temperature, low-pressure contact metamorphism. It forms in contact zones of magmatic intrusions (mainly basaltic and diabasic) with calcium carbonate-rich sedimentary rocks, such as limestones and chalk. It can also crystallize in limestone xenoliths entrained by lava. It is also identified as a component of some Portland cements and blast-furnace slags. ## Mineral Associations This mineral often co-occurs with other calcium silicates formed under similar conditions. The most common associated minerals include larnite, spurrite, wollastonite, melilite, grossular, vesuvianite, tilleyite, and calcite. ## Localities The most important and classic rankinite localities are in Northern Ireland (Scawt Hill, Carneal) and Scotland (Isle of Muck). It also occurs in Crestmore, California (USA), in the Vesuvius region in Italy, in the Hatrurim Complex in Israel, in Okayama Prefecture, Japan, and in several locations in Russia (Siberia).

Rarity

Rare

For collectors

## Quality Criteria The collector appeal of rankinite is primarily determined by its rarity and locality. Specimens with well-defined, even if small, bladed crystals are most valued. Rich aggregates from classic localities, such as Scawt Hill, are also highly prized, especially when they occur in association with other rare contact minerals. Purity and intensity of white color are of secondary importance. ## Popular Localities Specimens from Scawt Hill in Northern Ireland and the Isle of Muck in Scotland are considered classics and are most sought after by collectors specializing in systematic mineralogy and contact metamorphic minerals. Material from Crestmore, California, is also significant from a historical and scientific point of view.

Care and storage

## Cleaning Rankinite specimens should be cleaned very carefully, preferably with a soft, dry brush. Due to its occurrence in delicate aggregates, washing under running water, which could damage them, should be avoided. If liquid is necessary, pure distilled water or isopropyl alcohol, applied gently with a cotton swab, is recommended. ## What to Avoid Contact with acids, which can react with the mineral, should be avoided. Rankinite is sensitive to shocks and impacts, so it should be handled gently. It should not be heated or subjected to sudden temperature changes. ## Storage Rankinite specimens, especially those with a delicate structure, are best stored in sealed "perky boxes" to protect them from dust, moisture, and mechanical damage. Storage in areas of high humidity should be avoided.

External references

Sources

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