Guanghai Shi's research while affiliated with China University of Geosciences (Beijing) and other places

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Publications (1)

Figure 1: (a) Simplified tectonic map of China [51]. (b) Geological map of Longhua chalcedony deposit and its adjacent areas (modified after RGHPMI 1996).
Figure 2: Field photographs showing (a) outcrop of irregular chalcedony. (b) Outcrop of chalcedony with many breccias, the lower fracture was fully filled by chalcedony, whereas chalcedony gradually decreased and finally vanished up to the upmost part. (c-d) Photographs of chalcedony-bearing rocks.
Figure 5: Photomicrographs showing a formation sequence of chalcedony.
Representative chemical compositions of the alkali feldspar in host rock.
Representative chemical compositions of the biotite in host rock.

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Low-Crystallinity Index Chalcedony from Longhua, China: Characteristics and Formation
  • Article
  • Full-text available

August 2023

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Lithosphere

Qiuli Yan

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Guanghai Shi

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

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