Leucite

Leucite is a mineral made of potassium, aluminium, silicon and oxygen (KAlSi₂O₆). It belongs to the feldspathoids, feldspar-like minerals that are poorer in silica. It forms white or grey, twenty-four-faced crystals that resemble cubes, and is most often found in potassium-rich, silica-poor lavas. Its name comes from the Greek for “white”. It used to be called “white garnet” as well.

Dentistry: Some glass-ceramics used for dental prostheses contain leucite crystals made artificially. In this “leucite-reinforced glass-ceramic” the crystals make up 35–45% of the volume and measure 1–5 microns; it is mainly used for front-tooth crowns, inlays and onlays (Ritzberger et al., 2010). Geology: Leucite is a sign that the rock came from a magma very rich in potassium and poor in silica; geologists recognise such magmas by leucite-bearing lavas (leucitites). History of chemistry: Leucite played a part in the discovery that potassium is present in minerals.

It has a hardness of 5.5–6 on the Mohs scale. It can scratch glass; a steel knife scratches it only with difficulty. Its specific gravity is 2.45–2.5 g/cm³.

Its best-known localities: Mount Vesuvius and Monte Somma, Campania, Italy; Alban Hills (Ariccia, Frascati, Villa Senni), Lazio, Italy; Roccamonfina, Campania, Italy; Vulcano, Aeolian (Lipari) Islands, Italy; Laacher See area, Eifel, Germany.

Known localities in Türkiye: Haydarlı area, Dinar, Afyonkarahisar, Türkiye.

In the rock it appears as white or grey, round, twenty-four-faced (trapezohedral) crystals; these look like cubes but are not really cubic (pseudocubic, tetragonal). Crystals can reach 9 cm. Its hardness is 5.5–6; a steel knife scratches it only with difficulty. Its cleavage is very poor and it breaks along curved, shell-like surfaces. Crystals are usually covered in very fine twin lines, which can be seen with a microscope or hand lens. Leucite is most often confused with analcime, nepheline and garnet; its crystal form is very like some of these. A microscope or chemical analysis is needed for a certain identification.

On the “street to diamond” value ladder it sits on step 3 of nine: Building and industrial stone. Leucite is mainly a rock-forming mineral and has no price of its own. Collectors look for large, sharp, well-formed crystals, specimens where the whole crystal shows clearly in the rock, and those from well-known places such as the Alban Hills in Italy or Vesuvius. Leucite in rock is often altered and dull; clean, lustrous crystals are rare.

Crystal-therapy websites claim that leucite helps with insomnia, headaches and immune deficiency, and speeds recovery from illness or injury. The same sites say leucite strengthens confidence and relationships, stops stress from clouding judgement, and is a suitable stone for the “crown chakra”. No study supports these claims. Leucite is an ordinary mineral of potassium-rich lavas, and no effect of a rock mineral on the body or on relationships is known. Lasting sleeplessness, headaches or immune problems call for a doctor. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.