Perrierite-(La)
Chemical formula: (La,Ce<sup>3+</sup>,Ca)<sub>4</sub>(Fe<sup>2+</sup>,Mn<sup>2+</sup>)(Ti<sup>4+</sup>,Fe<sup>3+</sup>,Al)<sub>4</sub>(Si<sub>2</sub>O<sub>7</sub>)<sub>2</sub>O<sub>8</sub>
A rare titanium and lanthanide silicate from the chevkinite group, forming small, black, prismatic crystals in volcanic rocks.
Description
## Characteristics Perrierite-(La) is a mineral belonging to the sorosilicates of the chevkinite group. It is the lanthanum analogue of perrierite-(Ce). It typically forms small, prismatic crystals with a slender or stout habit, often embedded in the host rock. It usually occurs as single, isolated grains or small aggregates. Its color is black, sometimes with a reddish-brown tint. ## Physical Properties This mineral is characterized by a hardness of 5.5-6 on the Mohs scale. It has a vitreous to resinous luster. It is opaque, and in very thin fragments, it can be translucent. Its density is relatively high, ranging from 4.4 to 4.6 g/cm³. ## Colors and Varieties Perrierite-(La) occurs in uniform colors: black, reddish-black, or reddish-brown. No color varieties or commercial varieties are distinguished. The suffix "-(La)" indicates the dominance of lanthanum among the rare earth elements in its chemical composition. ## History and Name The mineral's name refers to its relationship with the previously known perrierite. The name "perrierite" was given in honor of the Italian mineralogist and chemist Carlo Perrier (1886–1948). Perrierite-(La) was formally recognized by the International Mineralogical Association (IMA) as a new mineral species in 1999, after the identification of specimens in which lanthanum is the dominant element.
Diagnostic features
## Identification Identification of perrierite-(La) under amateur conditions is very difficult. Preliminary identification is based on its black color, prismatic crystal habit, high density, and specific occurrence environment (alkaline volcanic rocks). Definitive identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS), to confirm the chemical composition with lanthanum dominance. ## Distinguishing from Similar Minerals Perrierite-(La) is visually indistinguishable from perrierite-(Ce) and chevkinite-(Ce), which occur in similar environments and have a similar appearance. Differentiation requires chemical analysis. It can also be confused with other black, heavy minerals, such as aegirine (different crystal habit, lower density) or ilmenite (usually more metallic luster). ## Crystal Forms Perrierite-(La) crystals are prismatic, with a cross-section ranging from rectangular to almost square. They occur as single, well-formed individuals or as granular aggregates within the rock.
Geological environment
## Genesis Perrierite-(La) is a mineral of magmatic origin. It forms as a primary component in alkali-rich, silica-saturated volcanic rocks. It is most commonly found in tuffs and products of volcanic eruptions, where it crystallizes from magma in the late stages of its cooling. ## Mineral Associations This mineral co-occurs with other minerals typical of alkaline environments, such as sanidine, aegirine, arfvedsonite, titanite, apatite, and zircon. ## Localities It is a very rare mineral, known from a few localities worldwide. The type locality (locus typicus) is the Sacrofano caldera in Lazio, Italy. It is also found in the nearby Tolfa volcanic complex. These are the main and best-documented occurrences of this mineral.
Rarity
Very rare
Collector aspects
## Quality Criteria As a rare and usually microscopic mineral, perrierite-(La) is valued primarily by specialized collectors of rare minerals and systematists. Specimens with well-formed, sharp, and undamaged crystals are most highly prized, especially if they are embedded in a contrasting rock matrix accompanied by other rare minerals. Crystal size is crucial – any specimen with crystals exceeding a few millimeters is considered exceptional. ## Popular Localities The most prized specimens, which are also the standard for this mineral, come from Italian localities in the Lazio region, especially from the Sacrofano caldera.
Care and storage
## Cleaning Perrierite-(La) specimens are usually small and brittle, so they should be cleaned with great care. The safest method is to use a soft brush to remove dust. If necessary, the specimen can be rinsed with distilled water. Ultrasonic cleaners are not recommended. ## What to Avoid Avoid contact of the mineral with strong acids and bases. As a mineral containing rare earth elements, it may be weakly radioactive, so it is recommended to wash hands after contact with the specimen. It should not be heated or exposed to sudden temperature changes. ## Storage Specimens, especially those on a rock matrix, are best stored in sealed collector boxes to protect them from dust and mechanical damage. Due to potential radioactivity, it should not be stored in direct proximity to radiation-sensitive minerals.