Low-Crystallinity Index Chalcedony from Longhua, China: Characteristics and Formation

Abstract A low-crystallinity index chalcedony was found in the rhyolitic ignimbrite of Late Jurassic Zhangjiakou Formation, located Longhua County, Hebei Province, China. This occurs as fillings along fragile fractures host rock and is distinct from any other deposits, such known basalt carbonate-related types. The ignimbrite, comprising sanidine (50–70 vol.%), plagioclase (10–15 quartz (8–10 magnesian biotite (3–5 accessory minerals. appears long lenticular veins irregular-shaped bodies, occasionally containing small fragments surrounding at boundary. It colored yellow, red, and/or white/colorless, with physical properties specific gravity 2.55–2.56, reflection 1.54, Mohs hardness 6.07–6.34, weight loss 1.97%–2.32% by heating. From boundary to inner center, its growth structure changes comb-like macrocrystalline thin fiber crystallites then a relatively uniform cryptocrystalline phase, indicating precipitation crystalline sequence. Electron probe Raman spectroscopy analyses reveal that component minerals are α-quartz moganite red inclusions hematite. Quartz exhibits platelet shapes tiny pores, which cemented nanograins, closer opal. It’s crystallinity indexes (CIs) range ~1–3, indicated X-ray diffraction patterns. low CI structural features, together occurrence, suggest temperature 40°C–80°C during formation. All these show distinction those most reported chalcedonies. interpreted an intermediate transitional type normal opal, shedding new light on understanding microcrystalline silica mineral aggregate exploration for similar gem deposit.