Abstract:
Metastable, amorphous aluminosilicates such as allophane have long been suspected to control the
solubility of Al and SiO2 in groundwater and soil pore-fluids. However, few efforts to determine the
trigger mechanisms and rate of these important and ubiquitous geochemical reactions have previously
been attempted. Meanwhile, at much higher temperatures, amorphous aluminosilicate scales are
precipitated from geothermal fluid within pipelines and geothermal reinjection wells, causing costly
formation damage, and pipe and valve blockages. However, despite >40 years of research, a trigger
mechanism for the formation of these aluminosilicate scales has yet to be conclusively identified.
Allophane deposited from spring water at Silica Rapids, Mt Ruapehu, New Zealand, provides the lowtemperature
example of aluminosilicate precipitation for this study, while geothermal aluminosilicate
scales deposited in pipelines at the San Jacinto-Tizate power project, Nicaragua, supply the high
temperature case. Solid samples from both locations are characterised in terms of composition via Xray
fluorescence (XRF) and field emission scanning electron microscopy (FESEM), and structure via
X-ray diffraction (XRD). These materials are then compared to those produced in solution
experiments, conducted at both low and high-temperature, under pH and compositionally-controlled
conditions. A hypothesis, wherein the supersaturation of gibbsite/boehmite may be the trigger for both
low and high-temperature reactions was tested, and a qualitative understanding of the rates of both
aluminosilicate precipitation reactions obtained.
Geothermal aluminosilicate scales are X-ray-amorphous and enriched with K, Ca, and Al; with
(K/Na)[scale]/(K/Na)[brine], (Ca/Na)[scale]/(Ca/Na)[brine], and (Al/Si)[scale]/(Al/Si)[brine] ratios of ~6, 40, and
200-800, respectively (mass basis), while [K+Na+2Ca]/Al and Si/Al mole ratios are ~1.0, and as low
as 5.1, respectively. Synthetic aluminosilicates produced in K-spiked geothermal experiments attain
identical (K/Na)[solid]/(K/Na)[brine] ratios to geothermal scales, while [K+Na+2Ca]/Al and Si/Al mole
ratios are ~1.0, and as high as 4.1, respectively. Silica Rapids allophane contains ≤0.5 wt.%
K2O+Na2O+CaO, with Si/Al ratios of ~0.65. Synthetically-produced allophane similarly contains
<0.5 oxide wt.% alkali elements, and is Al-rich; with Si/Al mole ratios of 0.65-0.97. The onset of both
synthetic Silica Rapids and geothermal aluminosilicate formation is found to coincide with gibbsite
saturation, while the rate of both reactions is rapid, in accordance with field observations. Based on
these results, some possible natural hydrothermal analogues to geothermal aluminosilicate scales are
proposed, and potential geothermal scale mitigation strategies examined.