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A modern, evaporitic occurrence of teruggite, Ca₄MgB₁₂As₂O₂₈.18H₂O, and nobleite, CaB₆O₁₀.4H₂O, from the El Tatio geothermal field, Antofagasta Province, Chile

K. A. Rodgers^1,, R. Greatrex¹, M. Hyland², S. F. Simmons³ and P. R. L. Browne^3,*

¹ Department of Geology, University of Auckland, Private Bag 92019 Auckland, New Zealand
² Department of Chemical and Materials Engineering, University of Auckland, Private Bag 92019 Auckland, New Zealand
³ Geothermal Institute, University of Auckland, Private Bag 92019 Auckland, New Zealand

^* E-mail: prl.browne{at}auckland.ac.nz

Teruggite is the dominant phase in a soft, off-white, poorly-layered and weakly-cemented surface crust, 10–15 mm thick, occurring in the high-temperature El Tatio geothermal field of Chile. Other minerals present include halite, which is present throughout but also forms a thin (<0.5 mm), brittle, cratered surface to the deposit, nobleite, ulexite and opal-A, with possible traces of illite-smectite and at least one unidentified phase. With the exception of ulexite, none of the minerals associated with teruggite at El Tatio has been reported from other occurrences of this mineral, nor do they occur with nobleite in its sole other known occurrence in Death Valley. EDS and XPS analyses of the main mass of the deposit show the presence of Ca, As, B, Na, and Cl, consistent with the identified mineral assemblage, but with elevated concentrations in Ca and Cl that are presumably associated with a further phase. Little Mg is present and the El Tatio teruggite appears deficient in this element, with Ca presumably replacing Mg in the structure. Unlike earlier documented occurrences of teruggite, that at El Tatio is evaporitic, modern and surficial. It is located some 50 m from the nearest hot (~50°C) pool and there is no evidence of association with fluid discharge. As such, the deposit has presumably derived from a fluid moving in the uppermost levels of the El Tatio field; perhaps a heavily modified version of the brines found in the deep wells.

KEYWORDS: teruggite, nobleite, ulexite, halite, evaporite, geothermal field, El Tatio, Chile

This article has been cited by other articles:

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