Zunyite

Zunyite is a rare chlorine-bearing aluminosilicate with the formula Al13Si5O20(OH,F)18Cl, built from linked silicon tetrahedra and aluminum-oxygen octahedra in an unusual, tightly bonded framework. It crystallizes in the isometric system, most often forming sharp, glassy tetrahedral crystals that may be modified by the cube, occasionally showing contact or penetration twinning on {111}. Colors range from colorless and grayish-white to pale flesh-red, with inclusions of hematite sometimes tinting crystals brown; the mineral typically forms where feldspar is hydrothermally altered in aluminous shales and volcanic rocks, often alongside kaolinite and quartz.

The species takes its name from the type locality, the Zuni Mine on Anvil Mountain near Silverton in the San Juan Mountains of Colorado, where chemist W. F. Hillebrand first described it in 1884 alongside the rare mineral guitermanite. Classic Colorado specimens show glassy, translucent, triangular crystals to a few millimeters richly covering kaolinite-rich matrix, and the Zuni Mine material remains the benchmark against which zunyite from other localities is judged. The species is also documented from Ouray and Saguache Counties, Colorado, and from Silver City, Utah, including the Ruby Mountain area of the Tintic District.

Today the most celebrated zunyite comes from the Dome Rock Mountains of La Paz County, Arizona, particularly the famous Electric Meatball boulder and the nearby Big Bertha Mine, where an erratic boulder discovered in a farmer's field in the early 1980s yielded some of the largest, sharpest, and most gemmy tetrahedral crystals known for the species, occasionally faceted for collectors. Other notable occurrences include Postmasburg in South Africa's Northern Cape, where pale pink crystals occur in aluminous shale; Kuni village in Japan, prized for transparent tetrahedral crystals; and Qalat-e Bala in Hormozgan Province, Iran, which has produced brown crystals intergrown with hematite. Collectors seek zunyite for its rarity, its crisp isometric crystal habit that is unusual among aluminosilicates, its vitreous luster, and the direct link its best specimens provide to storied localities like the Zuni Mine and the Electric Meatball find.