page_title: Taranakite
section_title: Structure
hierarchical_section_title: Taranakite / Structure
caption_attribution_description: English: Atomic environment within a layer of the taranakite unit cell, showing three crystallographically distinct aluminum centers linked by HPO42- units. Potassium cations are not shown. Aluminum is shown in grey, phosphorous purple, and oxygen red. Structural parameters taken from http://rruff.geo.arizona.edu/AMS/minerals/Taranakite (based on S. Dick, U. Goßner, A. Weiß, C. Robl, G. Großmann, G. Ohms, T. Zeiske (1998). "Taranakite — the mineral with the longest crystallographic axis". Inorg. Chim. Acta 269 (1): 47–57) doi:10.1016/S0020-1693(97)05781-2
context_page_description: Taranakite is a hydrated alkali iron-aluminium phosphate mineral with chemical formula (K, Na)₃(Al, Fe³⁺)₅(PO₄)₂(HPO₄)₆·18H₂O. It forms from the reaction of clay minerals or aluminous rocks with solutions enriched in phosphate derived from bat or bird guano or, less commonly, from bones or other organic matter. Taranakite is most commonly found in humid, bat inhabited caves near the boundary of guano layers with the cave surface. It is also found in perennially wet coastal locations that have been occupied by bird colonies. The type location, and its namesake, the Sugar Loaf Islands off Taranaki, New Zealand, is an example of a coastal occurrence.
  Taranakite forms small white, pale yellow, or gray crystals, which are typically found in pulverulent nodular aggregates, or crusts. Taranakite crystallizes in the hexagonal system, and is noted as having the longest crystallographic axis of any known mineral: the c-axis of the taranakite unit cell is 9.505 nanometers long.
context_section_description: Taranakite crystallizes in the hexagonal crystal system (hexagonal scalenohedral, 32/m) with the space group R3c. The unit cell dimensions are a = 870.25 pm and c = 9505 pm, enclosing a volume of 6.234 nm³. The c-axis is the longest of any known mineral.
  The unit cell of taranakite contains six layers of composition K₃Al₅(HPO₄)₆(PO₄)₂(H₂O)₁₂, each 13.78 Å thick and separated by layers of water. The rigid structure of each layer is built around HPO₄²⁻ groups coordinating three crystallographically distinct aluminium centres, each of which has coordination number six. Near the middle of each layer, an aluminium ion octahedrally coordinates six HPO₄²⁻. Two other oxygens in each hydrogen phosphate group coordinate the other distinct aluminium centres, which in turn are coordinated octahedral to three hydrogen phosphate groups and three water molecules. This structure forms an Al'–P–Al''–P–Al' linkage nearly parallel to the c-axis, with the other distinct aluminium atom offset, and nearly vertically below a PO₄³⁻ ion.
  Taranakite readily loses water when heated. Thermal gravimetric analysis shows two endothermic water loss events occurring in the ranges 80–140 °C and 140–300 °C corresponding to the sequential loss of five and thirteen water molecules to form francoanellite and a noncrystalline material. Heating to 500 °C results in complete dehydration to form K₃Al₅P₈O₂₉. In the range 562–595 °C crystalline AlPO₄ and KAlP₂O₇ form.
caption: Atomic environment within a layer of the taranakite unit cell, showing three crystallographically distinct aluminium centres linked by HPO42− units (potassium ions not shown).
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