Clay minerals and major elements concentrations of Zhuanglang Miocene red clay in Longzhong Basin, China

This article presents clay minerals and major elements data of Miocene red clay from Zhuanglang core (ZL) from the eastern Longzhong Basin, China. The dataset including the contents of main clay minerals such as smectite, kaolinite, illite and chlorite and other important clay minerals parameters as illite crystallinity, Illite 5 Å/10 Å, kaolinite/(illite +chlorite) ratio and major elements and its ratio as SiO2, Al2O3, MgO, Na2O, K2O, K2O/Al2O3 and SiO2/Al2O3 in the Mid-Miocene climatic optimum (MMCO). The X-ray diffraction (XRD) and X-ray Fluorescence (XRF) experiments were made at the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences. The data provide the evidence for understanding the MMCO and its driving factors.


Type of data
Samples dry completely in room temperature before measuring

Experimental features
The air-dry samples were treated with H 2 O 2 to remove organic matter and dilute HCl to remove the calcium carbonate; the clay fractions were separated by gravity separation using Stoke's Law. X-ray diffraction (XRD) analysis of the oriented mounts was carried out for each sample under natural (air-dried) conditions (N), ethylene-glycol solvation for 24 h in a desiccator (EG), and heating at 550°C for 2 h (H). XRD patterns were obtained using a PANalytical X'Pert Pro MPD diffractometer with CuKα radiation and Ni filter, under a voltage of 40 kV and a current of 40 mA. The scans were performed from 3°to 30°, with a step size of 0.0167°. And a high resolution scan from 24°to 26°2θ at a rate of 0.1°2θ per minute was carried out to isolate the contributions of kaolinite and chlorite in the 3.5 Å peak. The major elements of clay fractions were determined using a PANalytical PW4400 X-ray fluorescence (XRF) spectrometer. Data source location 35°13′N, 106°05′E, Zhuanglang drilling core, Zhuanglang County, Gansu Province, China Data accessibility Data are within this article and related references Related research article Song et al. [1] Value of the data Provide basic clay mineral data for both local and global comparisons. Data could be used to display the contents of main clay minerals and its paleoclimatic characteristics in the MMCO period.
Data provided a reasonable interpretation for the MMCO event. Data given here could motivate the studies on clay minerals in future.

Data
This article presents clay minerals and major elements data of Miocene red clay from Zhuanglang (ZL) core from the eastern Longzhong Basin, China (Fig. 1). The concentrations of main clay minerals includes mectite, kaolinite, illite and chlorite and other important clay minerals parameters as illite crystallinity, kaolinite/(illite þ chlorite) ratio. The main clay minerals types, contents, illite crystallinity, illite 5 Å/10 Å, kaolinite/(illite þchlorite) from 17 to 12 Ma are given in Table 1. The major elements include SiO 2 , Al 2 O 3 , MgO, Na 2 O, K 2 O. The major elements of clay fractions and their ratios from 17 to 12 Ma of the ZL red clay are showed in Table 2. Fig. 2 represents X-ray diffraction (XRD) patterns of clay fractions from the representative sample and Fig. 3 indicates the variations of clay minerals composition and characteristic indices of the ZL red clay in the MMCO period.

Materials
The ZL drilling site (35°13′N, 106°05′E) is located at the eastern margin of the Longzhong Basin, near the Liupan Mts, which divides the Chinese Loess Plateau into two parts (Fig. 1). The ZL drilling    core combing from two parallel drilling sites has a composite length of 654 m and detailed magnetostratigraphic correlations indicated that the ZL drilling core spanned from 25.6 to 4.8 Ma [2]. According to the magnetostratigraphy [2], grain size record [3], magnetic susceptibility variations [2,4] of the ZL core, we chose 80 samples at depths from 240 m (12 Ma) to 398 m (17 Ma) for clay minerals and geochemical analysis [1].

X-ray diffraction measurements
The isolation of o 2 μm clay fractions was followed by the Chinese oil and gas industry standard analysis SY/T 5163-2010 [5]. The air-dry samples were treated with H 2 O 2 to remove organic matter and dilute HCl to remove the calcium carbonate; the clay fractions were separated by gravity separation using Stoke's Law determinations and centrifugation in deionized water to remove free ions. In order to identify and quantify the clay minerals, XRD analysis of the oriented mounts was carried out for each sample under natural (air-dried) conditions (N), ethylene-glycol solvation for 24 h in a desiccator (EG), and heating at 550°C for 2 h (H) [6,7]. XRD patterns were obtained using a PANalytical X'Pert Pro MPD diffractometer with CuKα radiation and Ni filter, under a voltage of 40 kV and a current of 40 mA. The scans were performed from 3°to 30°, with a step size of 0.0167°. And a high resolution scan from 24°to 26°2θ at a rate of 0.1°2θ per minute was carried out to isolate the contributions of kaolinite and chlorite in the 3.5 Å peak. The ratio of kaolinite to chlorite was  Semi-quantitative analysis of clay minerals were determined from the basal reflection peak areas. Smectite (17 Å) (including random illite/smectite mixed-layers), illite (10 Å) and kaoliniteþchlorite (7 Å) were performed on the ethylene-glycol curve by the XRD diagram using MacDiff 4.2.6 software [8,9]. The relative abundances of clay minerals were determined using this formula: 4×I(illite 10 Å) þI (smectite 17 Å)þ2×I(kaolinite, chlorite 7 Å) ¼ 100%.

X-ray fluorescence measurements
Concentrations of major elements of clay fractions were measured by using a Philips PW4400 X-ray Fluorescence (XRF) spectrometer at the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences. All samples were dried at low temperature (43°C) for 72 h and ground to about o 75 μm (a 200 mesh size). About 0.6 g powder clay fraction samples were mixed with 6 g of Li 2 B 4 O7-Li 2 CO 3 fusion reagent in Platinum crucibles; the mixed samples were placed in a Claisse Fluxy melting furnace and fused for 5 min at high temperature (about 1000°C) before being formed into a glass sheet [10,11]. The calibration curve was established using 16 Chinese National Standard soil reference samples (GSS-1 to GSS-16) [12,13]. The reproducibility of elemental measurements were evaluated by repeat analysis using the National Standard soil reference sample GSS-8, with analytical uncertainties o 3% for major elements.