Gmelinite-K

Chemical formula: K₄(Si₈Al₄)O₂₄·₁₁H₂O

Gmelinite-K is a rare mineral from the zeolite group, the potassium analog of gmelinite-Na, forming colorless or brownish, vitreous crystals.

## Characteristics Gmelinite-K is a hydrated potassium aluminosilicate, belonging to the zeolite group. It is the potassium analogue in the gmelinite series, where potassium is the dominant cation. It forms well-developed, hexagonal crystals, often in the form of pseudo-rhombic dodecahedra or hexagonal pyramids. Crystals can be horizontally striated. It usually occurs as small, individual crystals or their aggregated groups in rock cavities. ## Physical Properties This mineral has a hardness of 4.5 on the Mohs scale. Its density is approximately 2.00 g/cm³, which is a relatively low value, typical for zeolites. The crystals have a vitreous luster, which can be dull on some surfaces. They are transparent, and their fracture is conchoidal. ## Colors and Varieties Gmelinite-K is most often colorless, but it can take on brownish hues. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral's name refers to its chemical composition – it is the potassium (K) member of the gmelinite series. The name of the entire group comes from the German chemist and mineralogist Christian Gottlob Gmelin (1792-1860). Gmelinite-K was formally recognized as a distinct mineral species relatively recently, compared to other members of the group. ## Uses Due to its rarity, gmelinite-K has no industrial applications. It is solely an object of interest for collectors specializing in mineral systematics or the zeolite group.

Properties

Mohs hardness: 4.5
Luster: Vitreous , Dull
Streak: White
Density: 2.00
Fracture: Conchoidal
Transparency: Transparent
Crystal system: Hexagonal

Diagnostic features

## Identification A characteristic feature of gmelinite-K is the form of its hexagonal crystals, often resembling pseudo-rhombic dodecahedra. Low hardness (4.5), vitreous luster, and occurrence in gas vesicles in volcanic rocks are key indicators. It reacts with hydrochloric acid, forming silica gel. ## Distinguishing from Similar Minerals Distinguishing gmelinite-K from other members of the gmelinite series (gmelinite-Na, gmelinite-Ca) is impossible without advanced chemical analyses (e.g., EDS). It can be confused with chabazite, which also forms similar crystals, but chabazite usually has a pseudo-cubic form, not hexagonal pyramids. Quartz is much harder (7 on the Mohs scale). ## Crystal Forms Gmelinite-K crystallizes in the hexagonal system. Tabular or bipyramidal crystals are most commonly found. It often forms characteristic, intergrown crystals or small druses filling cavities in the host rock.

Geological environment

## Genesis Gmelinite-K is a hydrothermal mineral. It forms at low temperatures, crystallizing in gas vesicles and fissures in volcanic rocks, such as basalts and phonolites. It is one of the last minerals to crystallize in the zeolite formation sequence. ## Mineral Associations It often co-occurs with other zeolites, such as chabazite, heulandite, thomsonite, analcime, as well as with calcite and quartz. ## Localities The most important confirmed localities for gmelinite-K include the type localities: Mount Alluaiv in the Lovozero Massif (Kola Peninsula, Russia) and San Giorgio di Perlena in the province of Vicenza (Italy). This mineral is also found in other places around the world where suitable volcanic rocks occur, but it remains a rare find.

Rarity

Rare

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp, and undamaged crystals of high transparency. Contrast with the rock matrix and the presence of associated minerals are also important. Crystal size is a significant factor – larger and well-formed specimens are much rarer and more desirable. ## Popular Localities Specimens from the type localities, i.e., the Kola Peninsula in Russia and the Vicenza region in Italy, are the most classic and sought after by collectors specializing in zeolites.

Care and storage

## Cleaning Gmelinite-K specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, avoiding prolonged soaking. The specimen should be allowed to dry completely at room temperature. ## What to Avoid As a zeolite, gmelinite-K is sensitive to high temperatures, which can cause loss of water from its crystal structure and permanent damage. Contact with acids and strong chemicals should be avoided. It should not be cleaned in ultrasonic cleaners. ## Storage Specimens are best stored in closed, stable containers or display cases to protect them from dust and mechanical damage. Avoid areas with high humidity or sudden temperature changes.