Montgomeryite

Chemical formula: Ca₄MgAl₄(PO₄)₆(OH)₄·₁₂H₂O

A rare phosphate mineral forming small, colorless or white crystals and aggregates, valued by micromineral collectors.

## Characteristics Montgomeryite is a hydrated calcium, magnesium, and aluminum phosphate, typically occurring as small, tabular or bladed crystals. It most often forms rosetted aggregates, radial clusters, or coatings on other minerals. Individual crystals rarely exceed a few millimeters in length, making it primarily of interest in the field of micromineralogy. It is a transparent to translucent mineral, with a vitreous or pearly luster, particularly noticeable on cleavage planes. ## Physical Properties Montgomeryite is characterized by a hardness of 4 on the Mohs scale, classifying it as a relatively soft mineral. Its density is approximately 2.53 g/cm³. It exhibits perfect cleavage in one direction, which is visible on well-formed crystals. The luster is most often vitreous, and on cleavage surfaces, it can be pearly. ## Colors and Varieties This mineral is most often colorless or white. Rarer specimens exhibit a delicate, greenish tint. No named varieties of montgomeryite are distinguished. ## History and Name Montgomeryite was first described in 1940 by Esper S. Larsen III. The mineral is named in honor of Arthur Montgomery (1909–1999), an American mineralogist and collector, who provided the first samples for study. These specimens originated from the Variscit mine in Utah County, USA, which is the type locality for this mineral. ## Uses Montgomeryite has no industrial applications. Its significance is limited to its scientific and collectible value, especially for collectors specializing in rare phosphates and microminerals.

Properties

Mohs hardness: 4
Color: Dark green to light green, colourless, red, yellow
Luster: Vitreous, Pearly
Streak: White
Density: 2.53
Cleavage: On {010}, perfect; on {100}, poor.
Fracture: Uneven
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Montgomeryite can be identified by its characteristic, small, tabular crystals, often forming radial or rosetted aggregates. Key features include perfect cleavage in one direction, pearly luster on cleavage planes, and occurrence in paragenesis with other rare phosphates in pegmatites or guano. ## Distinguishing from Similar Minerals Montgomeryite is sometimes confused with other secondary phosphate minerals of similar appearance, such as wardite, millisite, or crandallite. It is distinguished from wardite by its crystal habit (wardite forms tetragonal, often pyramidal crystals) and crystallographic system. Identification often requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Crystal Forms Montgomeryite crystals are typically very small, tabular or bladed in habit, elongated along one axis. They most often occur as radial aggregates, rosettes, or spherulitic clusters. Less commonly, they form druses of small, colorless crystals lining rock fissures.

Geological environment

## Genesis Montgomeryite is a secondary phosphate mineral. It forms as a result of hydrothermal processes or weathering in the oxidation zones of phosphate deposits. It is most commonly found in guano deposits (e.g., in caves) and as an alteration product of primary phosphates (e.g., triplite) in granitic pegmatites. ## Mineral Associations This mineral often co-occurs with other phosphates. In pegmatites, its typical associated minerals include wardite, eosphorite, strengite, rockbridgeite, and triplite. In guano-type deposits, it occurs with hydroxyapatite, whitlockite, and other rare phosphates. ## Localities The most important and classic montgomeryite localities are in the USA. The type locality is the Variscit mine in Lucin, Box Elder County, Utah. It is also known from pegmatites in the Tip Top mine in Custer County, South Dakota, and from the Palermo No. 1 mine in North Groton, New Hampshire. Outside the USA, occurrences have been reported in the Sapucaia mine in Minas Gerais, Brazil, and in pegmatites in Hagendorf, Bavaria, Germany.

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with sharp, well-formed, and transparent crystals, forming aesthetic, rosetted or radial aggregates. The contrast with the matrix on which they occur also enhances their appeal. Due to the small size of the crystals, their perfect form and lack of damage are crucial. Specimens from classic localities, such as the Tip Top mine or Variscit Mine, are particularly sought after. ## Popular Localities For collectors specializing in microminerals, the most desirable specimens come from the Tip Top mine in South Dakota (USA), known for beautiful, colorless, rosetted aggregates. Classic specimens from the type locality in Utah are also valued, as are those from the Palermo mine in New Hampshire.

Care and storage

## Cleaning Montgomeryite specimens should be cleaned with great care due to their softness and perfect cleavage. The safest method is to use compressed air to remove dust. If wet cleaning is necessary, use only distilled water and a very soft brush, avoiding strong pressure. After cleaning, the specimen should be left to air dry completely at room temperature. ## What to Avoid Avoid contact with acids and other chemicals that can damage the mineral. Montgomeryite is sensitive to high temperatures, which can lead to dehydration and structural damage. Do not use ultrasonic cleaners or steam clean specimens. Protect it from mechanical impact and scratching. ## Storage Montgomeryite specimens, especially those with well-formed crystals, should be stored in separate, padded boxes or display cases to protect them from mechanical damage and dust. Due to the small size of the crystals, "micromount" boxes are popular. Avoid storing in areas of high humidity or those exposed to rapid temperature changes.