Fillowite

Chemical formula: Na₃CaMn₂+₁₁(PO₄)₉

Fillowite is a rare mineral from the phosphate group, forming brownish, resinous masses and imperfect crystals, valued by collectors of rare minerals.

## Characteristics Fillowite is a complex sodium, calcium, and manganese phosphate, belonging to the alluaudite group. It typically occurs as granular or massive aggregates, rarely forming single, well-developed crystals. Crystals, if present, are thick-tabular or pseudo-rhombohedral, often with rounded faces. Most commonly, it is found as irregularly shaped masses embedded in rock. ## Physical Properties This mineral is characterized by a hardness of 4.5 on the Mohs scale. Its luster is most often described as resinous to greasy. It is a brittle mineral, with a density of approximately 3.43-3.53 g/cm³. It is usually translucent, and in thin fragments, it can be transparent. ## Colors and Varieties Fillowite ranges in color from yellowish, waxy, through shades of brown, reddish-brown, to almost black. The color change is often associated with the oxidation processes of manganese within its structure. No named varieties are distinguished. ## History and Name The mineral was first described in 1879 by George J. Brush and Edward S. Dana. Its name comes from A. N. Fillow, the owner of the Branchville mine in Connecticut, USA, where the discovery was made. This locality is recognized as the type locality for fillowite. ## Uses Fillowite has no industrial significance. However, it is a sought-after and valued mineral by collectors specializing in rare phosphates and minerals from granitic pegmatites.

Properties

Mohs hardness: 4.5
Luster: Vitreous to resinous
Streak: Pale yellowish brown
Density: 3.43-3.53
Cleavage: Imperfect on {0001}
Fracture: Uneven to sub-conchoidal
Transparency: Transparent to translucent
Crystal system: Trigonal

Diagnostic features

## Identification Fillowite is most often identified by its characteristic resinous or greasy luster, brownish color, and occurrence as massive aggregates in granitic pegmatites. Its hardness (around 4.5) and relatively high density are also helpful diagnostic features. It reacts with acids. ## Distinguishing from Similar Minerals It can be confused with other pegmatite phosphates, such as triplite, arrojadite, or dickinsonite. It is distinguished from triplite by its slightly lower hardness and often more resinous luster. Arrojadite can be very visually similar, and a definitive distinction often requires chemical analysis. Dickinsonite typically forms lamellar or radiating aggregates with a greenish color. ## Crystal Forms Fillowite crystals are rare. If they do occur, they take the form of thick tablets or pseudo-rhombohedra, often with poorly developed, rounded, or rough faces. It is much more commonly found as granular aggregates and solid masses, sometimes reaching considerable sizes.

Geological environment

## Genesis Fillowite is a primary phosphate mineral, crystallizing in the late stages of the formation of complex granitic pegmatites, rich in lithium and phosphorus. It forms in the intermediate zones and cores of these pegmatites. ## Mineral Associations It most commonly co-occurs with other phosphates, such as lithiophilite, triplite, dickinsonite, reddingite, as well as with quartz, microcline, albite, muscovite, and beryl. ## Localities The most important and classic locality, from which large masses of material originate, is the Branchville mine in Connecticut, USA. Other known localities include the Palermo No. 1 mine in New Hampshire (USA), Custer in South Dakota (USA), as well as pegmatites in the Karibib region of Namibia and in Hagendorf in Bavaria (Germany).

Rarity

Rare

For collectors

## Quality Criteria The most prized by collectors are specimens with well-formed, even if partially, crystals, which is a great rarity for this mineral. The intensity and attractiveness of the color are also important – specimens with a vibrant, reddish or honey-colored shade of brown are sought after. Large, homogeneous masses of fillowite from the classic Branchville locality also have historical and collectible value. ## Popular Localities Classic and most desirable specimens come from the discovery site – the Branchville mine in Connecticut, USA. Material from the Palermo mine in New Hampshire is also well-known to collectors. Specimens from Namibia and Germany are less commonly found on the market.

Care and storage

## Cleaning Fillowite specimens should be cleaned very carefully, using a soft brush to remove dust. Brief rinsing in distilled water is permissible, followed by thorough drying. Ultrasonic cleaners should be avoided, as they can cause fractures. ## What to Avoid Fillowite is sensitive to acids and strong chemicals. It should be protected from sudden temperature changes and prolonged exposure to direct sunlight, which can affect its color. As a relatively soft and brittle mineral, it is susceptible to scratches and mechanical damage. ## Storage It is recommended to store specimens in separate, padded boxes or on a soft surface in display cabinets, away from harder minerals. Stable conditions should be ensured, without high humidity and temperature fluctuations.

Sources

IMA Mineral List (CNMNC)