Images
Formula: CaAl2(CO3)2(OH)4.4H2O
Hydrated carbonate containing hydroxyl, dresserite group
Crystal System: Triclinic
Specific gravity: 2.21 to 2.24 measured, 2.213 calculated
Hardness: 2½
Streak: White
Colour: Chalky white to pale blue, pale yellow, cream, grey; pale rose or brownish pink to dark violet in
chromium-bearing varieties; colourless in transmitted light
Solubility: Readily soluble in acids. Decomposed by boiling water with separation of calcium carbonate and hydrous
aluminium oxide
Common impurities: Cr
Environments
Aluminohydrocalcite formed at low temperature by the action of carbonated waters on
allophane or dickite in
dolostone and limestone;
chromian varieties are typically formed near altering chromite from
serpentinite. Associated minerals include
allophane, dickite,
gibbsite, gypsum,
aragonite, baryte and
iron-manganese oxides
(HOM).
Localities
In the Chitral District, Khyber Pakhtunkhwa Province, Pakistan, aluminohydrocalcite is associated with
aragonite, quartz and
dickite
(Dana).
At the type locality, Potekhina Village, Sorsk, Khakassia, Russia, aluminohydrocalcite is associated with
allophane, volborthite,
malachite, cuprite, native
copper, limonite,
calcite and "wad" (a mixture of black
manganese oxides). It occurs as radiated
spherulitic groups or small needle shaped crystals, formed by the action of calcium carbonate solutions on the
allophane
(AM 13.569).
At Woodleaze Quarry, Tytherington, South Gloucestershire, England, UK, aluminohydrocalcite is recorded from cavities
within a sandstone filling in a sink hole in the Carboniferous
Limestone, associated with
gibbsite, allophane, and a
carbonate-containing aluminous phase of variable composition. Alumohydrocakite appears to have formed through the
action of CaHCO3-rich groundwater on allophane and the
carbonate-containing phase.
At Woodleaze Quarry, alumohydrocalcite occurs in several forms:
(1) aggregates of minute white spheroids with smooth surface and pearly lustre, composed of tightly-packed radiating
aggregates of acicular or compound prismatic crystals, encrusting both allophane
and a white carbonate-bearing aluminous phase
(2) mats of easily separable fibrous crystals up to 100 mm in length, which en masse have a silky lustre
(3) light pink crusts otherwise identical to the botryoidal cavity linings
(JRS 2.2.5-10).
Back to Minerals