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Formula: K3Al5(PO3OH)6(PO4)2.18H2O
Hydrated acid phosphate
Crystal System: Trigonal
Specific gravity: 2.12 to 2.15 measured, 2.12 calculated
Hardness: Very soft
Streak: White
Colour: Yellow, grey, white, colourless in transmitted light
Solubility: Readily soluble in acids
Environments
Taranakite is a secondary mineral formed from phosphatic
solutions derived from bird or bat guano reacting with clay or aluminous
rocks under perenially damp conditions; it is the most common phosphate mineral in caves. Associated minerals include
vashegyite, leucophosphite,
minyulite, francoanellite,
brushite, ardealite,
strengite, variscite and
vivianite
(HOM).
Localities
At the Parwan lava caves, Bacchus Marsh, Moorabool Shire, Victoria, Australia, the substrate of
tarakanite/kaolinite is likely to have formed by a chemical reaction
between clay minerals and phosphatic solutions derived from guano. This would
have occurred at low pH (strongly acid), as tarakanite precipitates and is stable in highly acidic environments.
There are included fragments of brown montmorillonite within the
tarakanite
(AJM 13.1.28-29).
The Apulian caves in Southern Italy are karst caves where brushite,
hydroxylapatite and tarakanite occur, probably formed by
interaction between phosphatic solutions derived from bat guano and the substratum
calcite and clay minerals. The data
also suggest precipitation from solution for brushite and
hydroxylapatite. The caves are very humid and contain a large amount
damp bat guano. Taranakite forms irregular aggregates within the insoluble residue of
limestone, and also occurs as fine layers within the guano. In some caves,
other phosphates such as ardealite,
francoanellite, strengite,
variscite and vivianite are present
(AM 76.1722–1727).
At the Oni-no-Iwaya cave, Hiroshima, Japan, tarakanite occurs in lenses associated with a minor amount of
apatite aggregates of white powdery crystals intercalated in
clay sediments which consist of mica,
chlorite, halloysite,
montmorillonite and kaolinite.
It is suggested that a chemical reaction between phosphoric acid in bat guano and the potassium and aluminum ions dissolved
from surrounding clay may play an important role in the formation of
taranakite
(AM 60.331–334).
At the type locality, the Sugar Loaf Islands, New Plymouth, New Plymouth District, Taranaki Region, New Zealand,
tarakanite occurs as pale cream, fine-grained material in seams, formed by a weathering reaction at the interface
between trachyte and seabird guano
(AJM 19.2.32), and associated with wavellite
(Mindat).
The Pig Hole Cave, Giles county, Virginia, USA, is in a cherty
limestone and contains large deposits of bat guano. The temperature
in the cave is between 8oC and 10oC. A few quartz crystals
occur attached to the roofs and walls of some passageways in the cave. Notable alluvial deposits constituted of cobbles and
boulders of sandstone and
quartzite plus silt and
clay also occur within the cave. Clusters of transparent
selenite crystals occur in the clay in
a few places. A pit was dug to investigate the
guano deposit and it was in this pit that the
brushite and taranakite were found first. The upper part of the pit is
composed of a felt of brown bat hair interlayered with bat excreta (now mostly gypsum).
The material of the bottom of the pit is brown and contains masses of black carbonaceous material,
brushite and taranakite. The taranakite occurs as masses of a
flour-like powder restricted to a narrow zone adjacent to the clay bottom, and
decreasing in abundance upward away from the clay.
It is suggested that the taranakite was formed by interactions between the bat guano (and possibly the bat hair)
and the clay. If this is so, most of the aluminium would have been derived from
the clay and the potassium and phosphorus from the guano and/or hair
(AM 41.616-626).
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