Hydrowoodwardite
Chemical formula: (Cu<sup>2+</sup><sub>1-x</sub>Al<sub>x</sub>)(S<sup>6+</sup>O<sub>4</sub>)<sub>x/2</sub>(OH)<sub>2</sub>·nH<sub>2</sub>O (x < 0.5, n > 3x/2)
A blue-green, hydrated copper and aluminum sulfate, typically forming spherical aggregates and crusts in the oxidation zones of ore deposits.
Description
## Characteristics Hydrowoodwardite is a secondary mineral of the woodwardite group, belonging to the hydrotalcite supergroup. It is a hydrated copper and aluminum sulfate. It most commonly occurs as spherical or botryoidal aggregates, as well as thin crusts and coatings. Less frequently, it forms fibrous or earthy aggregates. Its structure consists of microscopic, hexagonal plates, which are not visible to the naked eye. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 1. It exhibits a waxy to vitreous luster, and on fracture surfaces, it can also be pearly. It is translucent to opaque. Its density, calculated based on the formula and cell parameters, is 2.49 g/cm³. ## Colors and Varieties Hydrowoodwardite ranges in color from light blue and sky blue to blue-green. The intensity and hue depend on the copper to aluminum ratio in the structure. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral was formally described in 1999 by Thomas Witzke, Gunnar G. Raade, and Jochen Schlüter. Its name refers to the related woodwardite and its higher water content in the structure (the "hydro" prefix). The type locality (locus typicus) is the St. Briccius mine in Königswalde, Saxony (Germany). ## Applications Due to its rarity and small aggregate sizes, hydrowoodwardite has no industrial applications. It is solely an object of interest for collectors of rare minerals and micromounts.
Diagnostic features
## Identification Characteristic features of hydrowoodwardite include its light blue or blue-green color, botryoidal or spherical habit, and very low hardness. It occurs as thin crusts on the host rock, often in association with other secondary copper minerals. The luster is typically waxy. ## Distinguishing from Similar Minerals - **Woodwardite**: It is practically impossible to distinguish visually. Identification requires advanced studies, such as X-ray diffraction (XRD), to determine water content and unit cell parameters. - **Chrysocolla**: It can be very similar in color and aggregate form, but it is significantly harder (2.5-3.5 on the Mohs scale). - **Aurichalcite**: It forms acicular crystals in radiating aggregates and reacts with hydrochloric acid, unlike hydrowoodwardite. - **Rosasite**: It has a similar color and form but is harder (hardness 4) and also reacts with acids. ## Crystal Forms It crystallizes in the trigonal system, forming microscopic hexagonal plates. Macroscopically, it occurs almost exclusively as botryoidal, spherical, reniform aggregates, as well as coatings and earthy masses.
Geological environment
## Genesis Hydrowoodwardite is a secondary mineral, formed in the oxidation (weathering) zones of copper ore deposits. It forms as a result of precipitation from solutions rich in sulfates, copper, and aluminum. It often forms in post-mining environments, on the walls of old mine workings, where aluminum originates from weathering clay rocks or aluminosilicates of the gangue. ## Mineral Associations It co-occurs with other secondary minerals such as woodwardite, chalcoalumite, cyanotrichite, brochantite, malachite, azurite, gypsum, and iron oxides, e.g., goethite. ## Localities Key occurrences include: - **Greece**: Lavrion mining district (Kamariza, Sounion), known for excellent quality specimens. - **Germany**: Type locality in the St. Briccius mine (Saxony) and other localities in the Harz Mountains. - **United Kingdom**: Cornwall and Cumbria. - **Italy**: Mines in Sardinia. - **USA**: Arizona and Nevada.
Rarity
Rare
Collector aspects
## Quality Criteria The most prized hydrowoodwardite specimens are characterized by an intense, uniform, sky-blue color. Well-formed botryoidal or spherical habits, forming a continuous layer on a small, contrasting rock matrix, enhance their appeal. Specimens from classic localities, such as Lavrion, are particularly sought after. Due to the microscopic nature of its crystals, it is also a popular mineral among micromount collectors. ## Popular Localities The most famous specimens, valued for their aesthetic qualities, come from the historic Lavrion mining district in Greece. Specimens from the type locality in Germany hold significant value for systematic collectors.
Care and storage
## Cleaning Hydrowoodwardite is extremely soft (Mohs hardness 1) and brittle. It should only be cleaned dry, using a very soft brush to remove dust. Any contact with water is risky, as the mineral may partially dissolve or undergo structural changes. ## What to Avoid Water, ultrasonic cleaners, and all chemical agents, including acids and bases, must be strictly avoided. The mineral is sensitive to heat, which can cause dehydration and destruction. It should be protected from impacts and contact with harder minerals that could scratch it. ## Storage Hydrowoodwardite specimens are best stored in closed, padded boxes or display cases to protect them from mechanical damage and dust. A stable, dry environment, away from direct sunlight and heat sources, is recommended.