# Diagnosis of the Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, China
## 5 Discussion
### Types of Alkaline Lake Diagenesis and Their Causes
#### 5.1.2 Quartz Dissolution  
Quartz dissolution is a typical alkaline diagenetic phenomenon [51; 52]. Three different quartz dissolution phenomena were found in the core thin sections of the Fengcheng Formation: (1) the edge of quartz particles is dissolved, and it becomes serrated or bay-shaped (Figure 4f); (2) the quartz particles are corroded inside, and microscopic dense \"honeycomb\" corrosion pits can be seen on the mineral surface in SEM tests (Figure 4g); (3) quartz grains develop an irregular linear distribution of fractures, and some of the fractures are locally widened by further corrosion.  
The dissolution of quartz in the Fengcheng Formation was affected by Na\({}^{+}\), temperature, and alkalinity, and it occurred during the burial diagenetic period. Quartz dissolution is closely related to the alkaline diagenetic environment [1; 17; 51; 53], especially with the alkaline cation Na\({}^{+}\), which can form sodium silicate on the surface of quartz particles through hydration reactions, and then, they crack to cause quartz erosion [54; 55; 56]. The Fengcheng Formation rocks were formed in an alkaline lake environment [39; 40; 41; 56; 57]. The Fengcheng Formation volcanic rocks are sodium-rich volcanic rocks, and the content of Na\({}_{2}\)O in the volcanic rocks is much higher than that of K\({}_{2}\)O [58]. The Na element mainly exists in albite. The albite content in the volcanic rocks of the Fengcheng Formation is high [33], and the hydrolysis of albite increases the content of Na\({}^{+}\), which causes the cations in the alkaline lake water to be dominated by Na\({}^{+}\)[40]. The alkaline diagenetic environment rich in Na\({}^{+}\) is favorable for quartz dissolution because the high Na\({}^{+}\) content increases the probability of Na\({}^{+}\) forming complexes on the quartz surface.  
Under the same conditions, the dissolution rate of quartz at a high temperature (430 \({}^{\circ}\)C) is 11 orders of magnitude faster than that at a relatively low temperature (25 \({}^{\circ}\)C) [59], that is. High temperature is beneficial for the dissolution of quartz. In addition, the pH value also affects the dissolution of quartz. When the predecessors conducted hydrothermal experiments on the dissolution of quartz in an alkaline environment at a temperature of 130 \({}^{\circ}\)C and a pH value of 9.5, the quartz was significantly dissolved [52]. Therefore, the ground temperature was calculated according to the burial depth of the Fengcheng Formation, and it was concluded that such temperature conditions can be satisfied [59; 60]. The pH value of the alkaline lake during the deposition of the Fengcheng Formation was in the range of 9-11 [25]. During the diagenetic stage, the alkalinity decreased, but the temperature increased. The temperature change had a greater impact on the quartz dissolution than the pH change [52]. Therefore, the Fengcheng Formation has the condition of quartz dissolution in a burial alkaline environment. In addition, due to the fact that quartz dissolution is positively correlated with Na\({}^{+}\), the catalytic activity of the alkaline cation Na\({}^{+}\) increases with the increase of alkalinity [50; 58], and the increase of alkalinity can promote the dissolution of quartz. Regarding temperature and alkalinity, the dissolution of quartz is intensified with the rise of temperature in the middle- and high-temperature alkaline diagenetic stage. For example, the serrate dissolution of quartz edge is intensified and then becomes harbored, and the local dissolution of internal cracks in quartz minerals is intensified and widened.