Quartz extracted from sediments and bedrock for trapped charge dating (TCD) (Ikeya, 1978; Aitken, 1985; Huntley
Frequently, a simplified manner of expression is used, that non quartz ingredients are dissolved (Bell and Zimmerman, 1978) and quartz grains are etched isotropically. Such simplification allows us to perform the dose rate calculation in a straightforward manner. However, important work performed by Bell and Zimmerman (1978) and later Porat
In this work we investigate quartz grain size loss due to chemical etching with independent methods of microscopy IA, laser diffraction (LD) and mass loss. It allows us to gain insights into how HF etching affects grain size and shape and how uniform this process is. With microscope IA we will be able to test assumptions made when the etch depth is assessed only from the mass loss.
The material used for measurements was collected from the excavations at Ludwików site during the fieldwork of the project. The sampling site (
Fig. 1
Location of the sampling site (A) and material collection from an active sand pit (B).

The material was divided into smaller portions and weighed. The sample material was dried at 70 °C and the drying process was monitored by measuring the water loss from the samples. Subsequently, the samples were treated with 20% HCl and 20% H2O2 each time for 24 h at room temperature to remove any carbonates and organic material. Subsequently, the samples were washed in distilled water to achieve a neutral pH value. The desired grain size range was isolated by sieving with mesh widths of 180 μm and 200 μm (ISO 3310-1:2000). Next, the material was distributed into phials (preferred sample mass of ca. 5 g) and separated using a heavy liquid – sodium polytungstate solutions of densities of 2.62 g · cm−3 and 2.75 g · cm−3. This removed 11% of light fraction and 0.5% of heavy minerals. To ensure quartz purity, a sample selected from the material was checked with a Motic BA 310Met polarising mineralogical-petrographic microscope equipped with reflected-transmitted light sources.
To prepare quartz for TCD, the grains were routinely immersed in 40% HF acid for 45–60 min, depending on the grain size of the material (Fuchs and Lomax, 2019; Bartyik
Initial and final mass loss of etched quartz grains and estimated etched depth of the grains.
0* | 3.5739 | 3.5661 | 3.5033 | 3.4977 | 0 ± N/A |
30 | 3.5513 | 2.9595 | 3.5981 | 3.0251 | 5.1 ± 0.3 |
45 | 3.5161 | 2.7271 | 3.5524 | 2.7978 | 7.0 ± 0.4 |
60 | 3.5400 | 2.5144 | 3.5099 | 2.5116 | 9.5 ± 0.5 |
90 | 3.6021 | 2.2142 | 3.4692 | 2.0754 | 13.8 ± 0.8 |
120 | 3.5805 | 1.8717 | 3.4540 | 1.7820 | 17.6 ± 1.0 |
150 | 3.5299 | 1.5243 | 3.4676 | 1.5269 | 21.7 ± 1.2 |
180 | 3.4273 | 1.2808 | 3.5608 | 1.2963 | 25.2 ± 1.4 |
Measurement to assess material loss during sample preparation and weighting that is not related to etching.
Fig. 2
Random grains before (A) and after 40% HF etching for 60 min (B) and 180 min (C). Note smaller grains ranging from ca. 5 μm up to 100 μm detected after etching (B and C), marked with an arrow.

Fig. 3
Random grains before (A) and after 40% HF etching for 60 min (B) and 180 min (C). Note smaller grains ranging from ca. 5 μm up to 100 μm detected after etching (B and C), marked with an arrow. The microscope IA algorithm marks different grains with different colours and draws rectangles for visual validation.

To measure the etching depth, the weight of all portions of quartz grains before and after 40% HF etching was measured. It was done with the various etching durations described in the previous section. Weighting was performed with a WAA 100/C/1 balance (manufactured by RADWAG). This balance has the verification scale interval of
Samples for laser analysis of grain size distribution were mixed with distilled water. Overnight solutions were mixed using a rotary shaker and sodium hexametaphosphate was added to this mixture (Poręba
Fig. 4
Laser volumetric grain size distributions (A) and grain size distributions (B). Microscope IA with volumetric grain size distributions (C) and grain size distributions (D). IA, Image analysis.

To assess how grains are etched, we roughly estimate etching depths as 0.5Δ
To obtain insights and statistically relevant data on how grains are etched during HF treatment, a Delta Optical microscope was used to obtain digital images. Before each measurement, the microscope used was calibrated using Lacey Carbon Coated 200 Mesh Copper Grids (from SPI Supplies, USA). This allowed us to convert the lengths in pixels to micrometers and obtain data for further analysis. Images with grains of sand were placed on a black background. We placed as many grains as possible without creating clusters. In other words, the grains did not overlap. Typical images are shown below in
The IA program was created in Python based on an OpenCV (Bradski, 2000) image processing library and according to the code provided by Bhattiprolu (2020). The software algorithm applies a binary threshold, removes noises and detects contours. Next, the software detects and removes grains that overlap each other and grains that are on the border of the image. The typical image passed through this process is shown in
Finally, the algorithm determines the projected area, major, minor axes, perimeter and effective diameter estimated from this area. To obtain statistically significant data on average, each measurement was obtained from ca. 50 images of different grains. We assume that volume (V) and area (A) are related i.e. V∼A3/2 (Califice
Analogically to LD measurements, we calculate values for microscope IA.
Due to instrumental limitations, the microscope IA set in this experiment is sensitive down to ca. 20 μm grains.
The three methods which were used to assess the effect of different durations of HF etching on the size and shape of quartz grains measure different physical properties (Li
In microscope IA, to estimate the volume, we use the area, and this area is used to assess volumetric distributions. In
When we use distributions based on the number of grains, we see that the etching process is even more complex (
In
Fig. 5
Roundness distribution of quartz grains etched with 40% HF acid for microscope IA. Roundness is defined as 4πA/P2, where A is the area and P is the perimeter. IA, Image analysis..

Fig. 6
Roundness and equivalent diameter of quartz grains etched with 40% HF acid for microscope IA. Roundness is defined as 4πA/P2, where A is the area and P is the perimeter. IA, Image analysis.

To assess the thickness removed, we used mass loss data (
The slopes plotted in
Fig. 7
Mass etching depth (δrm) vs. etching time (t) assessed from mass loss calculations. LD 1/2ΔDV, LD, 50 and microscope IA 1/2ΔDV, LD, 50 equivalent radius changes. Light blue and red areas correspond to LD 1/2ΔDV, LD, 10 – 1/2ΔDV, LD, 90 and microscope IA 1/2ΔD, IA, 10 – 1/2ΔDV, IA, 90 ranges. LD, Laser diffraction.

The depth estimates from mass loss estimates and from LD might be overestimated because roundness decreases during etching (
The results of our research show that the estimated etch depth from mass loss is in agreement with earlier work (Leko and Komarova, 1973; Porat
The estimated etching rates for the mass loss estimates correspond to 0.151 ± 0.003 μm · min−1. In case of LD, etch depth estimation from 0.5Δ
Microscope IA and LD grain size distributions (
The increasing span of the volumetric distributions assessed with LD (
Although microscope IA has its own limitations, which are caused by providing 2D projections of 3D grains, the results of the present study demonstrate that the etch depth assessed from mass loss and LD might be overestimated. Statistical data from microscope IA also revealed that etching decreases roundness with time (
Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Initial and final mass loss of etched quartz grains and estimated etched depth of the grains.
0 |
3.5739 | 3.5661 | 3.5033 | 3.4977 | 0 ± N/A |
30 | 3.5513 | 2.9595 | 3.5981 | 3.0251 | 5.1 ± 0.3 |
45 | 3.5161 | 2.7271 | 3.5524 | 2.7978 | 7.0 ± 0.4 |
60 | 3.5400 | 2.5144 | 3.5099 | 2.5116 | 9.5 ± 0.5 |
90 | 3.6021 | 2.2142 | 3.4692 | 2.0754 | 13.8 ± 0.8 |
120 | 3.5805 | 1.8717 | 3.4540 | 1.7820 | 17.6 ± 1.0 |
150 | 3.5299 | 1.5243 | 3.4676 | 1.5269 | 21.7 ± 1.2 |
180 | 3.4273 | 1.2808 | 3.5608 | 1.2963 | 25.2 ± 1.4 |