Macaulayite

Chemical formula: Fe³⁺₂₄Si₄O₄₃(OH)₂

Macaulayite is an extremely rare, blood-red iron silicate, occurring as microscopic, earthy aggregates in weathered granite.

## Characteristics Macaulayite is a mineral from the silicate group, characterized by an intense red color. It occurs almost exclusively in the form of very fine-grained, earthy or powdery aggregates that cover the surfaces of other minerals, mainly quartz and feldspars. Due to the microscopic size of its crystals, individual, well-formed specimens are not observed with the naked eye. This mineral is extremely rare and known from only one location in the world. ## Physical Properties Due to its occurrence as microcrystalline aggregates, many physical properties of macaulayite have not been precisely determined. Hardness is difficult to measure but can be described as very low. Luster is described as earthy. It is translucent in very thin fragments. ## Colors and Varieties This mineral is monochromatic and occurs exclusively in shades of intense red, from red to reddish-orange. No varieties are distinguished. ## History and Name The name macaulayite comes from the Macaulay Institute for Soil Research (now the James Hutton Institute) in Aberdeen, Scotland, where the mineral was first identified and described in 1979 by Dr. Jeffrey Wilson. The discovery was made during studies of weathered granite from Bennachie hill in Aberdeenshire, Scotland. ## Uses Due to its extreme rarity and occurrence only as microscopic coatings, macaulayite has no industrial application. Its significance is purely scientific and collectible, although it is primarily sought after by specialized collectors of rare minerals.

Properties

Luster: Earthy
Density: 0
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification A characteristic feature of macaulayite is its intensely red color and earthy, powdery form of occurrence on granite or weathered granite. Field identification is possible based on these features, but certainty is only provided by laboratory analysis. It occurs as extremely small, hexagonal platelets visible only under an electron microscope. ## Distinguishing from Similar Minerals It can be confused with other secondary iron minerals, such as hematite (Fe₂O₃) or goethite (FeO(OH)), which also form red and orange coatings. However, hematite usually has a cherry-red streak and often a metallic luster, while macaulayite is dull (earthy). Final differentiation from powdery hematite requires advanced analytical methods (e.g., XRD). ## Crystal Forms Macaulayite crystals are microscopic, appearing as thin, hexagonal platelets with a diameter not exceeding a few micrometers. They form dense, felted aggregates with the appearance of an earthy coating.

Geological environment

## Genesis Macaulayite is a secondary mineral, formed as a result of the weathering of iron-rich minerals, such as biotite, under temperate climate conditions. It forms in weathered granite (known as granitic grus) in the near-surface zone. ## Mineral Associations This mineral occurs in association with rock-forming minerals of the granite from which it weathered. It is most often accompanied by quartz, feldspars (mainly orthoclase), and biotite. It also co-occurs with other weathering products, such as hematite, goethite, and clay minerals. ## Localities The only confirmed locality of macaulayite in the world is its type locality – weathered granite on the slopes of Bennachie hill, near Inverurie, in Aberdeenshire, Scotland, United Kingdom.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a macaulayite specimen is primarily assessed based on the intensity and saturation of its red color, and the richness and uniformity of its coverage on the host rock. Since this mineral does not form visible crystals, their form is not a criterion for evaluation. The most prized samples are those where macaulayite forms a thick, uniform layer on a light background of quartz or feldspar, providing good contrast. ## Popular Localities All macaulayite specimens available on the collector's market come from a single location in the world: Bennachie hill in Aberdeenshire, Scotland. This is the only known locality for this mineral.

Care and storage

## Cleaning Specimens containing macaulayite should not be cleaned mechanically or chemically. Any attempt to scrub, even with a soft brush, can permanently remove the delicate, powdery layer of the mineral. Only very careful blowing with compressed air from a distance to remove dust is permissible. ## What to Avoid Contact with water, acids, detergents, and any other chemicals should be strictly avoided. Specimens should not be exposed to vibrations, impacts, or friction. Store away from sources of moisture. ## Storage Specimens with macaulayite are best stored in closed, padded "micromount" boxes to protect them from dust, mechanical damage, and moisture. Display should only be in sealed display cases.