Detailed chemical composition of tequesquite, a pre-Hispanic and traditional mineral resource used in Mexico for culinary purposes

Abstract

Tequesquite is an alkaline mineral complex used in Mexico with culinary purposes since pre-Hispanic times. It is produced through evaporation and crystallization of remnant former Lake Texcoco brackish waters. Despite pre-Hispanic background use of this mineral complex, it was only until the first half of the XX century that analyses were performed to determine its gross chemical composition. However, more detailed information is unavailable in spite of recent technological advances in analytical procedures. In this study samples from different sources were obtained and analyzed to obtain more information about its chemical and elemental composition and to determine possible variations in their composition. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDXRA) and X-ray (XRD) diffraction were employed to determine composition of raw and depurated samples (in which clays and sands were removed through solubilization in deionized water). SiO 2 , NaCl, Na 2 CO 3 , and trona were identified in raw samples, whereas NaCl, trona, K 2 SO 4 , and KCl were detected in samples free of insoluble material. SEM images showed diverse crystal morphology types with different elemental composition, whereas XRD patterns revealed that tequesquite depicts the same diversity of chemical species despite different origin. Presence of toxic compounds was discarded, thus confirming its safe use for human consumption. This is the first report of detailed composition of an ancient mineral resource mainly used for culinary purposes that could be of potential use in other applications (e. gr. Spirulina cultivation). RESUMENEl tequesquite es un complejo mineral alcalino usado en México desde tiempos prehispáni-cos para la preparación de alimentos. Se produce a través de la evaporación y cristalización de las aguas salobres y alcalinas del lecho del antiguo Lago de Texcoco. No obstante los antecedentes históricos de uso de este recurso mineral, no fue sino hasta la primera mitad del siglo XX cuando se realizaron análisis químicos para determinar su composición general. En la actualidad, su composición detallada es desconocida pese a que se dispone de recursos analíticos muy avanzados. En el presente estudio se obtuvieron y analizaron muestras de tequesquite en diferentes localidades, a fin de establecer su composición elemental, así como las posibles variaciones. Para esto, se usó microcopía electrónica de barrido con espectroscopia de rayos X de energía dispersiva (SEM-EDXRA, por sus siglas en inglés) y difracción de rayos X (XRD) para analizar muestras brutas y depuradas (en estas últimas se eliminaron arcillas y arena mediante solubilización en agua desionizada). En las muestras sin procesar se identificaron SiO 2 , NaCl, Na 2 CO 3 y trona, mientras que en las muestras libres de material insoluble se detectaron NaCl, trona, K 2 SO 4 y KCl. Las imágenes de SEM mostraron diferentes tipos de morfología cristalina con distinta composición elemental, mientras que los patrones XRD demuestran que el tequesquite...

A study of dust airborne particles collected by vehicular traffic from the atmosphere of southern megalopolis Mexico City

“…The city was founded on an islet in the lake which has been artificially drained since then whereas the urban area has grown. The water in the basin is mostly salty and the lake's bed was exploited for artisanal salt production by pre-hispanic societies (mainly NaCl and Na 2 CO 3 with traces of Mg and K) (Williams, 2010;Flores-Hernández and Martínez-Jerónimo 2016) and industrially from 1948 to 1993 (Alcocer and Williams 1996). Part of the lake remains mostly as a salt marsh in the eastern part of Mexico City.…”