Lópezite

Chemical formula: K<sub>2</sub>Cr<sup>6+</sup><sub>2</sub>O<sub>7</sub>

Lópezite is a rare potassium dichromate, forming small, orange-red crystals, valued by collectors for its intense color and origin.

## Characteristics Lópezite is a very rare mineral from the chromate group, being the natural form of potassium dichromate. It occurs as small, tabular or prismatic crystals, rarely exceeding 1-2 mm in length. It usually forms small aggregates, coatings, or single, well-formed crystals grown on the host rock. Its most characteristic feature is its intense, orange-red to red color. ## Physical properties This mineral is relatively soft, with a hardness of about 2.5 on the Mohs scale. It is brittle and has a vitreous luster. Crystals are usually transparent to translucent. Its density is approximately 2.67 g/cm³. ## Colors and varieties Lópezite occurs in a characteristic, uniform color from orange-red to hyacinth-red. No color varieties or commercial varieties are distinguished. ## History and name The mineral was first described in 1937 by Mark C. Bandy. The name commemorates Emiliano López Saa (1871-1959), a Chilean mining engineer and mineralogist who studied nitrate deposits in Chile. The type locality is the nitrate mines in the Atacama Desert, Chile. ## Uses Due to its extreme rarity, small crystal size, and water solubility, lópezite has no industrial application. Its significance is purely scientific and as a collector's item.

Properties

Mohs hardness
2.5
Luster
Vitreous
Streak
Orange-yellow
Density
2.67
Cleavage
Perfect on {100}
Fracture
Conchoidal
Transparency
Transparent to Translucent
Crystal system
Triclinic

Diagnostic features

## Identification Lópezite is primarily recognized by its intense orange-red color, small, tabular crystals, and specific occurrence environment – in nitrate deposits (saltpeter). A key diagnostic feature is its water solubility. ## Distinguishing from similar minerals It can be confused with other minerals of similar color, such as crocoite or realgar. However, crocoite usually forms longer, prismatic crystals and is significantly harder (2.5-3 vs 2.5 for lópezite). Realgar is much softer (1.5-2) and tends to decompose under light. The water solubility test unequivocally distinguishes lópezite from these minerals. ## Crystal forms Crystals are usually small, tabular or short-prismatic in habit. They often occur as single, well-formed crystals grown on a substrate or as small, loose crystalline aggregates.

Geological environment

## Genesis Lópezite is a secondary mineral, formed under very specific, dry climatic conditions. It crystallizes from solutions rich in chromium, potassium, and oxygen in porous sedimentary rocks, mainly in nitrate deposits (Chilean saltpeter). ## Mineral associations It most often co-occurs with minerals typical of nitrate deposits, such as nitratine (soda niter), niter (potassium nitrate), halite, as well as dietzeite and tarapacáite, which are also chromates. ## Localities The most important and classic localities for this mineral are in Chile, in the Antofagasta and Tarapacá regions, in the Atacama Desert. These are mainly historical nitrate mines, such as Oficina Maria Elena, Oficina Pampa, and Oficina Paposo.

Rarity

Very rare

For collectors

## Quality criteria The most prized lópezite specimens are those with sharp, well-formed, and transparent crystals of intense, orange-red color. Rich aggregates of crystals on a contrasting, light rock matrix are also highly valued. Due to the small size of the crystals, even millimeter-sized specimens, if well-formed, are considered valuable. ## Popular localities Specimens from historical, classic localities in Chile, such as Oficina Maria Elena and other nitrate mines in the Atacama Desert, are by far the most sought after by collectors. Material from these sites is considered reference material for the species.

Care and storage

## Cleaning The mineral is water-soluble, so it must absolutely not be cleaned wet. To remove dust, a very soft brush or compressed air from a safe distance can be used very carefully. ## What to avoid Avoid all contact with water, water vapor, and other liquids. The mineral is sensitive to moisture, which can damage or completely dissolve it. It should be protected from high temperatures and direct sunlight, which can cause fading. ## Storage Lópezite specimens must be stored in a dry environment. It is best to place them in a sealed, airtight box (such as a "membrane box" or a box with a desiccant), away from light sources and temperature changes. Avoid storage in damp rooms, such as basements.

External references

Sources

Read more