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Interaction of phosphate-bearing solutions with gypsum; epitaxy and induced twinning of brushite (CaHPO (sub 4) .2H (sub 2) O) on the gypsum cleavage surface

Andre Jorge Pinto, Amalia Jimenez and Manuel Prieto
Interaction of phosphate-bearing solutions with gypsum; epitaxy and induced twinning of brushite (CaHPO (sub 4) .2H (sub 2) O) on the gypsum cleavage surface
American Mineralogist (February 2009) 94 (2-3): 313-322

Abstract

Under slightly acidic conditions and 25 degrees C, the interaction between phosphate-rich aqueous solutions and gypsum cleavage fragments results in the surface precipitation of brushite (CaHPO (sub 4) .2H (sub 2) O) crystals, which grow epitaxially on the (010) surface of gypsum. Using an A-centered unit-cell setting for both brushite (Aa) and gypsum (A2/a), the epitaxial relationship implies matching of the planes (010) of both structures and correspondence between equivalent crystallographic directions within these planes. The crystal habit of the overgrowing brushite is thin tabular to laminar on {010} with {111} and {111} as side faces and a clear elongation on [101]. There are two orientations of the brushite plates on the gypsum surface related to each other by a twofold axis on [010]. Thus, the overgrowth is an aggregate of parallel brushite crystals that may be twin-related, with the twofold axis as the twin law. During the interaction, gypsum dissolution is coupled with brushite growth until saturation with respect to both minerals is reached. A model of this thermodynamically driven dissolution-crystallization process is presented using the geochemical code PHREEQC. The epitaxial relationships are explained by comparing the bond system and the crystallographic properties of both minerals.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 94
Serial Issue: 2-3
Title: Interaction of phosphate-bearing solutions with gypsum; epitaxy and induced twinning of brushite (CaHPO (sub 4) .2H (sub 2) O) on the gypsum cleavage surface
Affiliation: Universidad de Oviedo, Department of Geology, Oviedo, Spain
Pages: 313-322
Published: 200902
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 38
Accession Number: 2009-031249
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200917
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