Yukonite

Chemical formula: Ca₇Fe₃+₁₅(As₅+O₄)₉O₁₆·₂₅H₂O

Yukonite is a rare, hydrated calcium iron arsenate, forming amorphous, resinous masses of reddish-brown to black color.

## Characteristics Yukonite is an amorphous (non-crystalline) mineral from the arsenate group, meaning it does not form regular crystals. It occurs as masses, crusts, and aggregates with a resinous or glassy appearance. Its surface is often uneven, botryoidal, or earthy. Due to its amorphous nature, its properties can be variable. ## Physical Properties As an amorphous mineral, yukonite does not exhibit cleavage. Its fracture is conchoidal to uneven. The hardness on the Mohs scale is low, ranging from 2 to 3, which means it is a soft mineral. The luster is characteristic, vitreous to resinous, sometimes dull in earthy aggregates. It is a relatively light mineral. ## Colors and Varieties Yukonite ranges in color from reddish-brown, through dark brown, to almost black. Thin fragments of the mineral may be translucent red. No color or commercial varieties are distinguished. ## History and Name The mineral's name comes from its discovery locality – the Yukon district in Canada. It was first described by geologist and chemist George Christian Hoffmann in 1906, although it was originally collected by Robert G. McConnell in 1891 at the Daulton mine near Windy Arm, Tagish Lake. For many years, its status as a distinct mineral species was questioned, but the International Mineralogical Association (IMA) confirmed its status in 2022, simultaneously redefining its chemical formula.

Properties

Mohs hardness: 2-3
Luster: Vitreous to resinous
Streak: Yellow-brown
Cleavage: None
Fracture: Conchoidal to uneven
Transparency: Translucent to opaque
Crystal system: Amorphous

Diagnostic features

## Identification Yukonite is identified by its amorphous, resinous or glassy form, reddish-brown to black color, and low hardness. Its conchoidal fracture and vitreous-resinous luster are also characteristic. It occurs in the oxidation zones of ore deposits rich in arsenic. ## Distinguishing from Similar Minerals It can be confused with other amorphous iron arsenates, such as pitticite or pharmacosiderite (in amorphous forms), as well as some iron oxides (e.g., goethite in botryoidal forms) or silicates. Final differentiation often requires chemical analysis (EDS) to confirm the presence of calcium, iron, and arsenic in appropriate proportions. It differs from goethite by its lower hardness and often more resinous appearance. ## Crystal Forms As an amorphous mineral, yukonite does not form crystals. It occurs exclusively as amorphous masses, reniform or botryoidal aggregates, crusts, veins, or as earthy material.

Geological environment

## Genesis Yukonite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits rich in arsenic, especially arsenopyrite. It forms as a result of the reaction of arsenic-rich solutions with iron- and calcium-bearing minerals under low-temperature conditions. ## Mineral Associations It often co-occurs with the minerals from which it forms, i.e., arsenopyrite, as well as other secondary arsenates such as scorodite, pharmacosiderite, and symplesite. Goethite, jarosite, and quartz may also accompany it. ## Localities The most important and classic yukonite localities are in Canada, in the Yukon district (Daulton and Venus mines, Tagish Lake). It is also known from historical silver and cobalt mines in the Jáchymov and Cínovec regions in the Czech Republic, and from Saxony in Germany (e.g., Schneeberg). It also occurs in mines in the Lavrion region in Greece.

Rarity

Rare

For collectors

## Quality Criteria The collector's appeal of yukonite primarily depends on the form and aesthetics of its aggregates. Specimens with a distinct, botryoidal or reniform structure and an intense, resinous luster are most valued. The size of the specimen and a contrasting association with other minerals, such as quartz or scorodite, are also important. Clarity is not a key criterion, as this mineral is inherently opaque. ## Popular Localities Classic and most sought-after specimens by collectors come from the type locality in Yukon, Canada. Historical specimens from European localities, such as Jáchymov (Czech Republic) or Schneeberg (Germany), are also highly prized due to their scientific and historical significance.

Care and storage

## Cleaning Yukonite is a very delicate mineral and potentially sensitive to water. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to dry-dust. Water, especially hot water, should be avoided as it can damage the specimen. ## What to Avoid Contact with chemicals, acids, and detergents must be strictly avoided. The mineral is soft and brittle, so it must be protected from impacts, scratching, and vibrations. As a hydrated mineral, it is sensitive to changes in temperature and humidity – high temperatures can cause water loss and its disintegration. ## Storage Yukonite specimens are best stored in stable conditions, in a closed display box, away from direct sunlight, heat sources, and sudden changes in humidity. It is advisable to isolate it from harder minerals to avoid scratches.

Sources

IMA Mineral List (CNMNC)