Hydroxylclinohumite

Chemical formula: Mg₉(SiO₄)₄(OH)₂

Hydroxylclinohumite is a rare magnesium silicate from the humite group, valued by collectors for its intensely orange or brown, well-formed crystals.

## Characteristics Hydroxylclinohumite is a mineral belonging to the humite group, chemically classified as a magnesium silicate. It forms tabular, short prismatic, or granular crystals. It often occurs as embedded, single crystals or small aggregates within the host rock, which is typically marble or carbonate-silicate rocks. Its crystals, though usually small, can reach several centimeters and are characterized by good crystal faces and a distinctive luster. ## Physical Properties This mineral has a hardness of 6 on the Mohs scale, making it relatively scratch-resistant. The luster is typically vitreous, and on fracture surfaces, it can be slightly resinous. It is a brittle mineral. Its density ranges from 3.21 to 3.35 g/cm³. Crystals can be transparent, but are more often translucent. ## Colors and Varieties Hydroxylclinohumite most commonly ranges in color from pale yellow, through intense orange, to reddish-brown and brown. Color is one of its main collector's assets. There are no named varieties, and classification is based primarily on the intensity and hue of the coloration. ## History and Name The mineral's name refers to its chemical composition (presence of the hydroxyl group -OH), monoclinic crystallographic system (clino-), and its relation to another mineral from the same group - humite. It was formally described and recognized as a distinct mineral species in 1965. It is a member of the overarching humite group. ## Uses Due to its rarity and small crystal size, hydroxylclinohumite has no industrial applications. The purest and most transparent specimens with intense color are occasionally faceted for advanced gemstone collectors, but their primary value is scientific and collectible.

Properties

Mohs hardness: 6.5
Color: Pale yellow to orange-yellow, almost colourless.
Luster: Vitreous to dull
Streak: White
Density: 3.13
Cleavage: None
Fracture: Irregular/Uneven,Conchoidal
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Key diagnostic features include a combination of intense, orange or brown color, vitreous luster, relatively high hardness (approx. 6), and occurrence in a characteristic geological environment – most often in white marbles. Crystals often show striations on their faces. ## Distinguishing from Similar Minerals Hydroxylclinohumite can be confused with other minerals from the humite group, such as clinohumite or chondrodite. Distinguishing them without advanced analysis (e.g., XRD) is often impossible. It differs from grossular (hessonite variety) by crystal habit (grossular crystallizes in the isometric system) and occurrence environment. Unlike sphalerite, hydroxylclinohumite is much harder and has a lighter streak. ## Crystal Forms It crystallizes in the monoclinic system, most often forming flattened, tabular crystals. Short prismatic forms are also found, as well as irregular grains embedded in the rock. Crystals can occur individually or form small, granular aggregates.

Geological environment

## Genesis It is a metamorphic mineral, formed under conditions of contact metamorphism. It forms as a result of the interaction of magnesium- and fluorine-rich hydrothermal solutions with carbonate rocks (limestones, dolomites) in contact zones with magmatic intrusions, such as syenites. It occurs mainly in carbonate-silicate rocks (skarns) and marbles. ## Mineral Associations It most commonly co-occurs with calcite, dolomite, forsterite, diopside, spinel, phlogopite, chondrodite, clinohumite, and other minerals from the humite group. The presence of these minerals is a strong indicator for identification. ## Localities The most important and well-known locality, which yields the world's best collector specimens, is in the Kuh-i-Lal region of Tajikistan. Other known localities include deposits in the Pamir Mountains, as well as isolated occurrences in Vietnam, Tanzania, and Burma (Myanmar).

Rarity

Rare

For collectors

## Quality Criteria The most highly prized hydroxylclinohumite specimens are characterized by an intense, saturated orange or reddish-brown color. Transparency (the higher, the better) and the completeness and sharpness of the crystal edges are also crucial. Specimens in which a well-formed, undamaged crystal is aesthetically set on a contrasting, white marble matrix fetch the highest prices. Crystal size is also of great importance. ## Popular Localities By far the most sought-after specimens by collectors are those from Kuh-i-Lal in Tajikistan. This locality provides crystals of unparalleled color, luster, and often significant size, set on white marble, creating exceptionally aesthetic compositions.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water and a delicate brush can be used, followed by thorough drying of the specimen. Ultrasonic cleaners should be avoided, as they can cause fractures. ## What to Avoid The mineral is sensitive to strong acids, which can damage it. Avoid sudden temperature changes and prolonged exposure to direct sunlight, which theoretically might affect its color, although this is not well documented. Protect from impacts due to its brittleness. ## Storage Hydroxylclinohumite specimens, especially those embedded in a delicate matrix (e.g., marble), are best stored in separate, padded boxes or display cases to prevent scratches and mechanical damage. They should be protected from dust and moisture.

External references

https://www.mindat.org/min-7729.html