Mineral Electronic Probe Analysis

1. Principle of Mineral Electron Probe Analysis

Make the sample to be tested into electron probe light sheet or thin sheet.Under the bombardment of high voltage current (voltage 15kv-20kv, current 1 10-8-5 10-8A, spot diameter about 1-5μm\15-30μm), the minerals are excited to characteristic X-rays. Different elements have different wavelengths of characteristic X-rays. The intensity of X-ray is related to the concentration of elements. There are two ways of X-ray detection: spectrometer and energy dispersive spectrometer.

2. Equipment

2.1 Spectrometer

The key of the spectrometer is how to connect the unknown characteristic spectral line with the known element number Z. For this reason, it is assumed that there is a specific crystal (we call it spectroscopic crystal) with a plane spacing of D. when X-rays of different characteristic wavelengths are irradiated on it, if the Bragg condition (2dsin θ = λ) is satisfied, diffraction will occur. Obviously, for any given incident angle θ, only a certain wavelength λ satisfies the diffraction condition. In this way, we can establish a series of corresponding relations between the angle of θ and the corresponding elements in advance. When a certain X-ray excited by an electron beam irradiates the spectroscopic crystal, we can receive the X-ray signal of this wavelength in the corresponding direction of 2 θ angle with the incident direction, and at the same time, we can detect the corresponding chemical elements. As long as the detector scans continuously at 2 θ angle, the continuous measurement can be realized in the whole range of elements.

The single wavelength X-ray diffracted by the spectroscopic crystal is received by the X-ray detector. The commonly used detector is proportional counter. When a certain x-ray photon enters the counter tube, the gas in the tube ionizes and generates an electric pulse signal under the action of electric field. The electrical signal output from the counter is amplified into a voltage pulse signal of about 0-10V through the preamplifier and main amplifier, and then sent to the pulse height analyzer.

2.2  Energy dispersive spectrometer

The X-ray photon from the sample enters the Li drift Si solid state detector through the beryllium window. Each X-photon energy absorbed by silicon crystal will produce electron hole pairs in the crystal. X photons of different energies will produce different pairs of electron holes. For example, Kα radiation from Fe produces 1685 electron hole pairs, compared with 2110 for Cu. Knowing the hole logarithm can calculate the corresponding charge and the voltage pulse at the fixed capacitance (1μμF).

The digital to analog converter in the multi-channel pulse height analyzer first converts the pulse signal into digital signal, and establishes the corresponding relationship between the voltage pulse amplitude and the channel address (there is a corresponding relationship between the channel address number and the X-ray energy). The commonly used X-ray energy range is 0.2-20.48kev. If the total number of channel addresses is 1024, the corresponding energy range of each channel address is 20ev.Low X-ray energy corresponds to small channel number. High X-ray energy corresponds to large channel number. According to the number of X-rays recorded at different trace sites, the X-ray intensity of various elements can be determined. It is used to measure the relative content of each element in the sample. Then, the curve of pulse number and pulse height is displayed on X-Y recorder or cathode ray tube, which is the energy spectrum curve of x-photon.

3. Sample Preparation

3.1 Sampling

Take a block sample with a diameter of no more than 10mm from the rock sample.

3.2 Wash oil

Oil samples need to be extracted with chloroform below grade 3 fluorescence, see SY/T5188 for specific oil washing method

3.3 Select sample analysis surface

Use a representative, flat, fresh section as the analysis surface

3.4 Sample loading

The sample is glued to the sample pile with latex or conductive adhesive, and the analysis surface is kept parallel to the upper surface of the sample pile.

3.5 Drying

The samples were naturally dried at room temperature

3.6 Dust

Blow off the sample surface debris and dust with the ear ball, keep the sample fresh section clean

3.7 Coating

The rock sample is gold-plated or carbonized by vacuum coating instrument, and the coating sample is put into the dryer for analysis.

4. Analysis Steps

4.1 Scanning electron microscope observation

4.1.1 Observe and record the occurrence form of clay minerals at 300-500 times.

4.1.2 The aggregate morphological characteristics or single crystal characteristics of clay minerals were observed and photographed at 500-10000 times.

4.2 Energy spectrum analysis

4.2.1 Determine the position of clay mineral crystals to be analyzed, usually by selecting flatter aggregates.

4.2.2 X-ray energy spectrum of sample collected.

4.2.3 Identify spectral peaks of each element

4.2.4 Determine the list of component analysis elements

4.2.5 According to the quantitative analysis method of energy disperses spectrometer, the correction calculation is carried out, and the results are normalized to get the oxide analysis data of the clay mineral composition.

4.3 Analysis and judgment of results

The observed analysis results in 4.1 and 4.2 are compared with the typical characteristics in Appendix A and Appendix B, and the type of mineral measured is determined according to the identification characteristics in Chapter 9.

5. Composition Analysis

The main elements are silicon (Si), aluminum (Al), oxygen (O), potassium (K) and calcium (Ca). Its component characteristics are mainly reflected in the potassium oxide (K2O) content of 2.8%~9.9%[Figure B.5D].

6. Analysis Report

In addition to the requirements of GB/T27025, the analysis results should also include the following contents:

A) For petroleum geological samples, the corresponding region, well number, horizon and lithology of the samples should be explained.

B) Explain the occurrence state and morphological characteristics of clay minerals.

C) Determination results of clay mineral elements.

D) Identification names of clay minerals.

7. Implementation Standard

SY/T6027-2012 Quantitative analysis method of rock and mineral by electron probe microanalysis.