A new massive deposit of allophane raw material in Ecuador

Abstract

In Ecuador, DINAGE (known today as the Servicio Geológico Nacional) and the German Federal Institute for Geosciences and Natural Resources have discovered a huge allophane deposit covering an area of >4000 km2. This study presents the results from an investigation of a 16-m thick vertical sequence from this deposit, supposedly the weathering product of two different volcanic ash deposits. In particular, the distribution of alkali metals within the uppermost layer indicates that the weathering process is still ongoing.According to the mineralogical composition, an allophane-rich layer (allophane facies) could be distinguished from the underlying halloysite-rich layer (halloysite facies). A 2-m thick transition zone is characterized by the presence of gibbsite and intermediate specific surface area values. Only a few imogolite fibers could be identified (by scanning electron microscopy), indicating the dominance of allophane over imogolite in the allophane facies. Single allophane particles were investigated by atomic force microscopy, though this method was less accurate than transmission electron microscopy with respect to the determination of the primary particle diameter. Carbon isotope analysis (14C) suggested an age of ∼20,000 y for the allophane layer.Within the allophane facies, a 4-m thick layer occurs containing 70–80 wt.% allophane with an N2-BET specific surface area of >300 m2/g. Based on infrared and energy-dispersive X-ray diffraction measurements, an Al/Si ratio of 1.3–1.4 was established for this allophane, which is between Al-rich and Si-rich allophane. The allophane layer may be of economic value due to the large allophane content, the small amount of organic matter, and the significant thickness of the deposit.

Removal of barium, cobalt, strontium and zinc from solution by natural and synthetic allophane adsorbents

“…TEM-EDX analysis of cloud-like particle aggregates of NatAllo yielded an averaged atomic Al/Si ratio of 1.3 ± 0.2 ( Fig. 3d), corroborating the atomic Al/Si ratio of 1.3-1.4 for Ecuadorian allophane previously reported in Kaufhold et al [24]. Such allophanes are very often referred to as "stream deposit allophanes" or "silica springs allophane", according to Parfitt [19].…”

“…XRD analysis confirmed the dominance of allophane in NatAllo (~78 wt.%; [24,29]) which also contains minor quantities (each < 5 wt.%) of quartz, cristobalite, feldspar (albite and orthoclase), hornblende, halloysite, gibbsite and goethite (Fig. 1).…”

“…NatAllo was used as received without further purification (sample 4-7; [24]). SynAllo-1 and SynAllo-2 were precipitated by modifying the method of Wada et al [25].…”

Removal of Barium, Cobalt, Strontium, and Zinc from Solution by Natural and Synthetic Allophane Adsorbents

“…TEM-EDX analysis of cloud-like particle aggregates of NatAllo yielded an averaged atomic Al/Si ratio of 1.3 ± 0.2 (Figure 3d), corroborating the atomic Al/Si ratio of 1.3-1.4 for Ecuadorian allophane previously reported in Kaufhold et al [24]. Such allophanes are very often referred to as "stream deposit allophanes" or "silica springs allophane", according to Parfitt [19].…”

“…XRD analysis confirmed the dominance of allophane in NatAllo (~78 wt.%; [24,29]), which also contains minor quantities (each <5 wt.%) of quartz, cristobalite, feldspar (albite and orthoclase), hornblende, halloysite, gibbsite, and goethite ( Figure 1). The XRD patterns of SynAllo-1 and SynAllo-2 exhibited two broad peaks centered at ~3.4 Å and ~2.3 Å , which is typical for short-range ordered aluminosilicates such as allophane.…”