Orthojoaquinite-(La)

Chemical formula: NaBa<sub>2</sub>Fe<sup>2+</sup>La<sub>2</sub>Ti<sup>4+</sup><sub>2</sub>(SiO<sub>3</sub>)<sub>8</sub>O<sub>2</sub>(OH,O,F)·H<sub>2</sub>O

Orthoclasyclite-(La) is an extremely rare mineral from the cyclosilicate group, found as tiny, brown crystals in nepheline syenites.

## Characteristics Orthoclasyclite-(La) is a very rare mineral belonging to the joaquinite group. It forms small, tabular or prismatic crystals, reaching up to 1 mm in size. It usually occurs as single, well-formed crystals or small aggregates. Its name refers to its orthorhombic symmetry and the dominance of lanthanum (La) in its chemical composition. ## Physical Properties The crystals are characterized by a vitreous luster. The Mohs hardness is 5.5, and the density ranges from 3.96 to 4.04 g/cm³. The mineral is brittle and exhibits an uneven fracture. ## Colors and Varieties Orthoclasyclite-(La) has a characteristic brown to reddish-brown color. It is translucent. No commercial or color varieties are distinguished. ## History and Name The mineral was discovered in nepheline syenite at the Poudrette quarry on Mont Saint-Hilaire in Quebec, Canada. It was described in 1997 by George Y. Chao and Robert A. Gault. The name "orthoclasyclite" refers to its orthorhombic crystallographic system and its kinship with joaquinite, and the suffix "-(La)" indicates lanthanum as the dominant rare earth element in its structure.

Properties

Mohs hardness
5.5
Luster
Vitreous
Streak
Light brown
Density
3.96-4.04
Cleavage
Good on {100} and {010}
Fracture
Uneven
Transparency
Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Practical identification of orthoclasyclite-(La) relies on advanced analytical methods, such as X-ray spectroscopy (EDS/WDS), due to its small size and co-occurrence with other rare minerals. In collector conditions, identification is possible based on its characteristic brown color, tabular crystal habit, and most importantly, its confirmed locality (Mont Saint-Hilaire). ## Distinguishing from Similar Minerals It can be confused with other minerals from the joaquinite group, such as joaquinite-(Ce) or strontiojoaquinite. Differentiation requires chemical analysis to determine the dominant rare earth element (lanthanum) and crystal symmetry. Visually, it may resemble some brown titanites or eudialytes, but it differs from them in crystal form and environment of occurrence. ## Crystal Forms It forms small, tabular crystals with a prismatic habit, often terminated by pyramids. Crystals are usually well-formed but do not exceed 1 mm in length.

Geological environment

## Genesis Orthoclasyclite-(La) is a mineral of magmatic origin. It forms in the late stages of crystallization in pegmatites and miarolitic cavities within alkaline igneous rocks, such as nepheline syenites. ## Mineral Associations This mineral occurs in association with other rare minerals typical of Mont Saint-Hilaire syenites. It co-occurs with aegirine, microcline, nepheline, sodalite, eudialyte, catapleiite, sérandite, and other minerals from the joaquinite group. ## Localities The only confirmed locality for orthoclasyclite-(La) in the world is its type locality – the Poudrette quarry on Mont Saint-Hilaire, La Vallée-du-Richelieu Regional County Municipality, Montérégie, Quebec, Canada.

Rarity

Extremely rare

For collectors

## Quality Criteria Due to its extreme rarity, every analytically confirmed specimen of orthoclasyclite-(La) is valuable to specialized collectors of rare minerals (so-called "micromounters"). The most highly prized specimens are those with well-formed, sharp crystals of intense brown color, set on a contrasting rock matrix, such as white analcime or nepheline. ## Popular Localities The only source of specimens is the Poudrette quarry in Quebec, which is world-renowned for hundreds of rare and unique mineral species.

Care and storage

## Cleaning Due to the extreme rarity and small size of the crystals, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used to remove dust. Avoid contact with water and chemicals. ## What to Avoid Avoid ultrasonics, steam cleaners, acids, and all chemical solvents. The mineral is brittle, so protect it from impacts and scratches. Store away from moisture. ## Storage Orthoclasyclite-(La) specimens are usually very small and often left on their original rock matrix. The safest way to store them is to place the specimen in a sealed "micromount" box, protecting it from dust and mechanical damage.

Sources

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