Harkerite

Chemical formula: Ca₄₈Mg₁₆[AlSi₄O₁₅(OH)]₄(BO₃)₁₆(CO₃)₁₆·₂(H₂O,HCl)

Harkerite is a rare, complex borosilicate of calcium, magnesium, and aluminum, forming characteristic white or colorless octahedral crystals.

## Characteristics Harkerite is a complex borosilicate from the ettringite group, occurring as well-formed, though usually small, crystals. They most often adopt an octahedral form, frequently with visible modifications of other faces, which gives them a complex, geometric appearance. Crystals can be flattened or slightly distorted. They are typically colorless, white, or grayish, and their surface may have a vitreous or slightly greasy luster. Due to its complex chemical structure and rarity, harkerite is a mineral valued mainly in scientific circles and among advanced collectors. ## Physical Properties This mineral is characterized by a Mohs hardness of 5, placing it in the medium hardness range. Its density is relatively low, approximately 2.95 g/cm³. Crystals are usually translucent, less often transparent. They do not exhibit cleavage, and their fracture is uneven to conchoidal. ## Colors and Varieties Harkerite does not form colored varieties. Its coloration is usually uniform and limited to white, shades of gray, and colorless crystals. Any coloration, if present, is most often caused by minute inclusions of other minerals. ## History and Name The mineral was discovered on the Scottish Isle of Skye and first described in 1951 by Cecil Edgar Tilley. The name "harkerite" was given in honor of Alfred Harker (1859–1939), a distinguished English petrologist who made immense contributions to the study of igneous and metamorphic rocks, including those from the Isle of Skye. ## Uses Due to its rarity and small crystal size, harkerite has no industrial applications. It is solely an object of scientific and collecting interest.

Properties

Mohs hardness: 5
Luster: Vitreous to greasy
Streak: White
Density: 2.95
Cleavage: None
Fracture: Uneven to sub-conchoidal
Transparency: Transparent to translucent
Crystal system: Trigonal

Diagnostic features

## Identification A characteristic feature of harkerite is its crystal habit – most often well-formed, sharp octahedra, less commonly cubes. It occurs in a specific geological environment – in contact metamorphic zones of carbonate rocks (skarns). It reacts with acids, which is helpful for identification but destroys the sample. ## Distinguishing from Similar Minerals Due to its habit, harkerite can be confused with other minerals that form octahedral crystals, such as magnetite, franklinite, or spinel. It is distinguished from them by its significantly lower hardness, lower density, light color, and lack of magnetic properties. It also occurs in a completely different geological environment than most of them. ## Crystal Forms Harkerite crystals are typically isometric, with an octahedral habit {111}, sometimes modified by cube faces {100} or rhombic dodecahedron faces {110}. Crystals rarely exceed a few millimeters in size, although centimeter-sized specimens are known. They occur as single, overgrown crystals or as inclusions in calcite and other skarn minerals.

Geological environment

## Genesis Harkerite is a mineral typical of contact metamorphic environments. It forms as a result of the interaction of silica- and boron-rich hydrothermal solutions with impure dolomites and limestones in contact zones with magmatic intrusions. It forms at relatively high temperatures, in rocks known as skarns. ## Mineral Associations This mineral often coexists with other minerals typical of skarns, such as calcite, diopside, vesuvianite, monticellite, wollastonite, spinel, perovskite, as well as other rare borates and silicates. ## Localities The most important and classic harkerite localities are in Scotland, on the Isle of Skye (type locality at Camas Malag) and on the Isle of Muck. It is also known from Crestmore in California (USA), from the vicinity of Bajan-Cagan in Russia, and from several localities in Italy, including the Lazio region.

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with sharp, well-formed crystals of a distinct, octahedral habit. Transparent, colorless, or pure white crystals without mechanical damage are highly valued. Specimens where harkerite crystals are contrastingly set on a rock matrix, for example, in association with calcite or diopside, are particularly attractive. Crystal size is a key factor – specimens with crystals exceeding 5 mm are already considered exceptional. ## Popular Localities Classic and most sought-after specimens come from historical localities in Scotland (Isle of Skye). Good quality crystals, though usually smaller, have also been found in the Crestmore quarries in California, USA. Specimens from other localities are much rarer on the collector's market.

Care and storage

## Cleaning Harkerite specimens should be cleaned very carefully. It is best to use a soft brush to remove dust. If necessary, the specimen can be rinsed in distilled water and then thoroughly dried. Ultrasonic cleaners should be avoided, as they can damage delicate crystals. ## What to Avoid Harkerite is sensitive to acids, which can damage or completely dissolve it. Contact with all household and laboratory chemicals should be avoided. The mineral should also not be subjected to sudden temperature changes. ## Storage Collector's specimens of harkerite are best stored in separate, sealed boxes or display cases to prevent abrasions and mechanical damage. Protect from dust and direct contact with other, harder minerals.

Sources

IMA Mineral List (CNMNC)