Gillespite
Chemical formula: BaFe<sup>2+</sup>Si<sub>4</sub>O<sub>10</sub>
Gillespite is a rare barium iron silicate, prized for its intense red color and characteristic tabular crystals.
Description
## Characteristics Gillespite is a silicate mineral distinguished by its vivid red color. It forms thin, tabular or platy crystals with square or octagonal outlines, often occurring as micaceous or scaly aggregates. Its appearance is sometimes compared to mica due to its perfect, unidirectional cleavage, which allows for easy separation into thin plates. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 4. It exhibits a vitreous luster, and a slightly adamantine luster on fracture surfaces. It is transparent to translucent. Gillespite shows strong pleochroism, changing color from carmine red to pale pink depending on the viewing direction. Its density ranges from 3.33 to 3.40 g/cm³. ## Colors and Varieties The dominant and essentially only color of gillespite is an intense red, sometimes transitioning to reddish-brown. There are no distinct color varieties or commercial names. ## History and Name The mineral is named in honor of Frank Gillispie, an Alaskan collector who first found its specimens. It was described as a new mineral species in 1922 by Waldemar T. Schaller. ## Uses Due to its rarity and attractive appearance, gillespite is a valued and sought-after collector's mineral. It has no industrial applications.
Diagnostic features
## Identification The most important diagnostic features of gillespite are its intense red color, tabular crystal habit, and perfect cleavage in one direction. Strong pleochroism (color change from carmine to pink) is very characteristic. This mineral also exhibits red fluorescence under ultraviolet light (shortwave). ## Distinguishing from Similar Minerals It can be confused with some red micas (e.g., lepidolite), but gillespite is significantly denser, crystallizes in the tetragonal system, and has a different chemical composition. Eudialyte, although sometimes similar in color, forms crystals of a different habit and occurs in a different geological environment (nepheline pegmatites). ## Crystal Forms Crystals are typically thin, tabular, with a square cross-section. It occurs as single crystals grown on the host rock or as lamellar, scaly, and rosette aggregates.
Geological environment
## Genesis Gillespite is a contact metamorphic mineral. It forms in contact zones between magmatic intrusions and silica-rich sedimentary rocks, such as sandstones or quartzites. Its presence is associated with a barium-rich environment. ## Mineral Associations A characteristic mineral associated with gillespite is sanbornite. It also co-occurs with minerals such as celsian, diopside, quartz, tourmaline, and witherite. ## Localities The most important occurrences of this mineral are in the USA. The type locality is the Dry Delta area in Alaska. However, the most well-known and prized specimens come from the vicinity of Incline in Mariposa County, California. Occurrences have also been reported in Yukon (Canada) and Baja California (Mexico).
Rarity
Rare
Collector aspects
## Quality Criteria The most highly valued gillespite specimens are characterized by an intense, saturated red color, high transparency, and well-formed, sharp crystals of significant size. Crystals set on a contrasting, light matrix, such as white sanbornite or quartz, are particularly sought after, as this highlights their color and form. ## Popular Localities Specimens from Mariposa County, California, are considered the best in the world. They are characterized by exceptional color and often form striking compositions with other rare barium minerals.
Care and storage
## Cleaning Gillespite specimens should be cleaned very carefully, using a soft brush and distilled water. Due to its perfect cleavage, ultrasonic cleaners are absolutely not recommended, as they can cause the crystals to break apart into thin lamellae. ## What to Avoid Avoid contact with harder minerals to prevent scratches. The mineral is sensitive to impact and pressure, which can cause delamination. Protect it from contact with strong acids. ## Storage It is recommended to store gillespite specimens in separate, padded boxes or display cases to minimize the risk of mechanical damage. It does not require special conditions regarding light or humidity.