Gilmarite
Chemical formula: Cu<sup>2+</sup><sub>3</sub>(As<sup>5+</sup>O<sub>4</sub>)(OH)<sub>3</sub>
Gilmarite is a rare copper arsenate, forming characteristic acicular crystals of an intense green color, aggregated into radial clusters.
Description
## Characteristics Gilmarite is a hydrated copper arsenate, valued by collectors for its distinct emerald-green or bluish-green color. It most commonly occurs as very fine, acicular or hair-like crystals. These form characteristic radial-fibrous aggregates, rosettes, and spherical clusters, growing on rock surfaces. Due to the small size of the crystals, the best specimens are usually observed under magnification. ## Physical properties This mineral is relatively soft, with a Mohs hardness of about 3. It has a vitreous luster and is transparent to translucent. Its density is significant, approximately 4.2-4.3 g/cm³, which is typical for arsenates containing heavy metals. ## Colors and varieties The color palette of gilmarite is narrow, ranging from emerald to bluish-green. The intensity of the color depends on impurities and crystallization conditions. No named color varieties or commercial forms are distinguished. ## History and name The mineral was named in honor of Gilmar Wendt (born 1933), a German mining engineer and mineral collector, who specialized in minerals from the Greek Laurion (Lavrion) district. Gilmarite was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 1995. The type locality is the Hilarion mine in Kamariza (Laurion, Greece). ## Applications Gilmarite has no industrial applications. It is solely an object of interest for collectors of rare minerals.
Diagnostic features
## Identification Characteristic features of gilmarite include its intense green color, acicular crystal habit, and radial aggregates. Its occurrence environment – oxidation zones of copper and arsenic deposits, in association with other secondary arsenates – is also of key importance. ## Distinguishing from similar minerals Gilmarite is sometimes confused with other green, acicular copper minerals. - **Malachite**: has a similar fibrous habit but reacts vigorously with hydrochloric acid, unlike gilmarite. - **Olivenite**: usually has an olive-green hue and forms more prismatic, thicker crystals. - **Minerals from the agardite group**: are visually very similar (bluish-green, acicular), and their definitive distinction from gilmarite often requires advanced chemical (EDS) or X-ray (XRD) analyses. ## Crystal forms Gilmarite crystallizes in the form of slender, acicular or hair-like crystals. They almost always occur as radial clusters, fans, rosettes, or spherical aggregates, whose diameter rarely exceeds a few millimeters.
Geological environment
## Genesis Gilmarite is a secondary mineral, formed in the oxidation zones (so-called iron caps) of hydrothermal ore deposits rich in copper and arsenic. It crystallizes as a result of the weathering of primary sulfides and arsenides under low temperature and pressure conditions. ## Mineral associations It most often co-occurs with other secondary copper and arsenic minerals, such as olivenite, conichalcite, cornwallite, clinoclase, cornubite, as well as with iron oxides (goethite, hematite) and quartz. ## Localities The most important and well-known gilmarite localities worldwide include: - Hilarion mine in the Laurion district, Greece – the type locality, yielding the best specimens. - Clara mine in the Black Forest, Germany. - Cap Garonne mine in France. - Majuba Hill mine in Pershing County, Nevada, USA.
Rarity
Very rare
Collector aspects
## Quality criteria The collector's appeal of gilmarite depends on several factors. Highly valued are specimens with well-formed, rich, spherical aggregates of an intense, emerald-green color. The aesthetics of the rock matrix and the presence of rare associated minerals, which enhance the scientific and visual value of the specimen, are also important. Due to the small size of the crystals, most specimens are microminerals (so-called "micromounts"). ## Popular localities Specimens from the historic mines in the Laurion district of Greece are by far the most sought after by collectors. They are characterized by classic development and intensity of color.
Care and storage
## Cleaning Gilmarite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Ultrasonic cleaners are absolutely not recommended, as they can damage delicate aggregates. ## What to avoid As a soft and brittle mineral, gilmarite is susceptible to mechanical damage. Contact with acids and other strong chemicals should be avoided. Due to its arsenic content, hands should be washed after each contact with the mineral, and inhalation of dust that might be generated if it is damaged should be avoided. ## Storage The best way to store gilmarite is to place it in a separate, sealed box (a "micromount" type) that protects it from dust, light, and mechanical damage. It should be kept under stable humidity and temperature conditions.