Acanthite Pseudomorph after Pyrargyrite- La Valenciana Mine, Guanajuato, Mexico

Acanthite Pseudomorph after Pyrargyrite
Locality: La Valenciana Mine, Guanajuato, Mexico
Dimensions: 4.8 x 3 x 2.5 cm
Ex. Dr. Claude H. Yoder, Charles A. Dana Emeritus Professor of Chemistry at Franklin & Marshall College (see below)

What first drew me to this specimen was how perfectly the original crystal shapes have been preserved. Even after replacement by acanthite, the distinctive pyrargyrite habit remains immediately recognizable, creating a piece that is as educational as it is visually compelling. It is a wonderful illustration of pseudomorphism—one mineral replacing another while faithfully retaining the external form of the original crystals.

Intergrown crystal forms rise from the matrix in a complex, sculptural arrangement. Their dark steel-gray surfaces carry a fine metallic sparkle that emphasizes the ridges, edges, and terminations, while the open structure rewards examination from multiple angles. Each view reveals another relationship among the preserved forms and further demonstrates the remarkable completeness of the replacement process.

La Valenciana Mine is one of the historic silver-producing mines of the Guanajuato district. In a pseudomorph such as this, the original pyrargyrite was chemically replaced by acanthite while its outward morphology survived. The result preserves evidence of both the earlier crystal growth and the later alteration that transformed its mineral composition.

The specimen hails from noteworthy scientific provenance. It was formerly part of the collection of Dr. Claude H. Yoder, Charles A. Dana Emeritus Professor of Chemistry at Franklin & Marshall College in Lancaster, Pennsylvania. Yoder earned his B.A. from Franklin & Marshall in 1962 and his Ph.D. from Cornell University in 1966. His research included apatite structure and ionic substitution studied through techniques such as solid-state NMR, infrared spectroscopy, and X-ray diffraction. He also authored Ionic Compounds: Applications of Chemistry to Mineralogy, a work connecting chemical bonding and atomic structure with the symmetry and physical properties of minerals and crystals.

Condition: Remnants of old hot glue are present on the reverse from a previous display mount. The residue is confined to the back and does not detract from the specimen’s overall appearance or frontal presentation.