Definition
Triploidite is a rare arsenate–phosphate mineral that consists principally of manganese, iron, arsenic, oxygen, and hydrogen. Its idealized chemical formula is Mn₂(Fe)AsO₄(OH).
Overview
Triploidite belongs to the triplite group of minerals and crystallises in the orthorhombic system. It is typically found as brown to reddish‑brown, subhedral to euhedral crystals or massive aggregates. The mineral is a secondary phase that forms in oxidised zones of arsenic‑bearing, manganese‑rich ore deposits, often in association with minerals such as triplite, arsenopyrite, and rhodonite. First described in the late 19th century, type localities include the Bingham Canyon district of Utah, USA, and several occurrences in the Carlin Trend, Nevada. Because of its arsenic content, triploidite does not have commercial applications and is of interest mainly to mineral collectors and researchers.
Etymology/Origin
The name “triploidite” derives from the Greek roots tri‑ (“three”) and ‑oid (“resembling”), reflecting its structural relationship to the mineral triplite, which contains three distinct cations in its crystal lattice. The suffix “‑ite” is a standard mineralogical ending denoting a mineral species.
Characteristics
| Property | Description |
|---|---|
| Chemical formula | Mn₂(Fe)AsO₄(OH) |
| Crystal system | Orthorhombic (space group Pnma) |
| Unit‑cell parameters | a ≈ 9.90 Å, b ≈ 11.55 Å, c ≈ 5.80 Å (values may vary with Fe/Mn substitution) |
| Color | Brown, reddish‑brown, sometimes orange‑brown |
| Habit | Prismatic to tabular crystals; massive aggregates |
| Mohs hardness | 5–5.5 |
| Specific gravity | 4.3–4.5 |
| Luster | Sub‑vitreous to pearly |
| Transparency | Translucent to opaque |
| Optical properties | Biaxial (+); refractive indices nα ≈ 1.80, nβ ≈ 1.81, nγ ≈ 1.84; birefringence δ ≈ 0.04 |
| Cleavage | Poor to indistinct on {010} and {100} |
| Streak | Light brown to yellowish brown |
Chemical analyses indicate that iron may substitute for manganese to varying degrees, producing a solid‑solution series between pure Mn₂(Fe)AsO₄(OH) and Fe‑rich compositions. The hydroxyl group (OH) in the formula distinguishes triploidite from the phosphate‑dominant triplite (Mn₂(Fe)PO₄F).
Related Topics
- Triplite – a closely related phosphate mineral (Mn₂(Fe)PO₄F) sharing the same structural group.
- Arsenate minerals – a class of minerals containing the AsO₄³⁻ anion, including scorodite, adamite, and olivenite.
- Manganese minerals – minerals in which Mn is a principal cation, such as rhodochrosite and braunite.
- Oxidation zones of ore deposits – geological environments where primary sulfides alter to secondary oxides, carbonates, and arsenates, producing minerals like triploidite.
Note: The above description is based on established mineralogical literature and databases.