Struvite-(K)

Chemical formula: KMgPO<sub>4</sub>·6H<sub>2</sub>O

Struvite-(K) is a rare potassium magnesium phosphate, forming colorless or white crystals, being the potassium analog of struvite.

## Characteristics Struvite-(K) is a hydrated potassium magnesium phosphate, belonging to the struvite group. This mineral forms tabular or short prismatic crystals, often with a square or rectangular cross-section. They are typically colorless to white and transparent. It is the potassium analog of the much more common struvite, in which ammonia (NH₄) is replaced by potassium (K). ## Physical Properties Struvite-(K) crystals exhibit a hardness of approximately 2 on the Mohs scale, making them very soft. They have a vitreous luster and are transparent. The mineral's density is about 2.12 g/cm³. ## History and Name The mineral's name refers to its chemical composition – it is the potassium (K) member of the struvite group. It was formally described as a new mineral by Anthony R. Kampf, Paul M. Adams, Joseph J. Pluth, and Ian M. Steele in 2000, and approved by the International Mineralogical Association (IMA) under number IMA1999-031. The type locality is B-7 Cave in the Cango Caves Nature Reserve in the Republic of South Africa.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
White
Density
2.12
Cleavage
Perfect on {001}
Fracture
Conchoidal
Transparency
Transparent
Crystal system
Orthorhombic

Diagnostic features

## Identification Struvite-(K) is identified by its characteristic, tabular or short prismatic crystals with a rhombic outline, very low hardness (it can be scratched with a fingernail), and genesis. It is transparent and has a vitreous luster. It dissolves in acids. ## Distinguishing from similar minerals It can be confused with other minerals from the struvite group, mainly common struvite (NH₄MgPO₄·6H₂O). Differentiation requires advanced chemical analysis (EDS, WDS) to confirm the dominance of potassium over the ammonium group. Visually, it is practically indistinguishable from struvite. Gypsum can also form similar colorless crystals, but it has a different form and is slightly harder (hardness 2). ## Crystal forms Crystals are typically tabular, flattened parallel to {001}, or short prismatic. They occur as single, well-formed crystals or in small aggregates.

Geological environment

## Genesis Struvite-(K) is a mineral of organic origin. It forms from the decomposition of potassium-rich organic matter under specific chemical conditions. In the type locality (Cango Cave), it formed from the interaction of fluids seeping through phosphorus- and potassium-rich bat guano with magnesium-rich drip waters. ## Mineral associations In the type locality, it co-occurs with newberyite, struvite, syngenite, monetite, whitlockite, and apatite. ## Localities The only confirmed and thoroughly described occurrence of struvite-(K) in the world is its type locality – B-7 Cave in the Cango Caves complex, near Oudtshoorn in the Western Cape Province, Republic of South Africa.

Rarity

Extremely rare

For collectors

## Quality criteria As an extremely rare mineral, any well-formed and undamaged crystal of struvite-(K) is considered a specimen of high scientific and collector value. The most desirable are transparent, sharp, tabular crystals on a rock matrix or in association with other rare phosphates. Crystal size is a key factor – specimens exceeding a few millimeters are exceptional. ## Popular localities The only source of specimens is the type locality in the Cango Caves, South Africa. Material from this locality is practically unavailable on the commercial market and is mainly found in institutional and research collections.

Care and storage

## Cleaning Specimens should be cleaned only very carefully, using a soft brush to remove dust. Contact with water, especially warm water, can cause dissolution or damage to the crystals. If liquid is necessary, absolute alcohol (ethyl or isopropyl) should be used and dried immediately. ## What to avoid Water, acids, bases, and all cleaning solutions must be strictly avoided. The mineral is sensitive to elevated temperatures, which can lead to its dehydration and destruction. It is very soft and brittle, susceptible to scratches and mechanical damage. ## Storage Struvite-(K) specimens require storage under stable conditions, away from moisture and heat sources. It is best to place them in a sealed, closable display box away from direct sunlight to prevent potential changes.

Sources

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