Influences of Heat, Acid and Laser Ablation on Zircon’s Structure

Yanling Wang

OPT 407: SEM Practicum

Department of Earth and Environmental Sciences, University of Rochester

 (Mud Tank zircons, Australia)*

Introduction

With its high crystallization temperature and resistance to changes, zircon, with a chemical formula of ZrSiO4, becomes the favorite mineral of geochronologists and geochemists in the past two decades. Experimental geochemists study its elemental and isotopic distribution by growing zircons in silicate melt under appropriate temperature and pressure. The zircon crystals in such experiments are usually very tiny (several to hundreds microns) due to the limitation of growing time. Thus separating these zircon crystals from the glass is a tricky work. Hot hydrofluoric acid is the most commonly used chemical to dissolve the glass and separate zircons. However, whether heat and acid treatments change zircon’s structure or not is unknown. One of the goals of this project is to observe the influence of heat and acid on zircon’s structure.

Also, geochronologists obtain the age information and chemical composition of natural zircon. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is a powerful technique to perform ultra-high sensitive chemical analysis down to ppb (parts per billion) level on solid samples-without any sample preparation. Although previous work about the depth profiling gives a general idea about how the pits look like after laser ablation (“top hat” profile, Woodhead et al., 2004), how the laser ablation change the zircon’s topography is rarely explored. This project is aiming to investigate the topography of zircon left by laser ablation with the assistance of microscopic analysis.

Sample and preparation

Zircons studied in this project are from Mud Tank carbonatite, Austrilia. To study its structural characteristics, two groups are divided. One is zircon slabs with grain size of larger than 3 mm, the other group is small zircon grains powdered to <72 µm. Both groups contain both pristine zircons and acid-treated zircons. Laser ablation was applied on two zircon slabs, with spot size ranging from 5 µm to 85 µm. Carbon sputter coating was carried out to make the sample conductive.

Methodologies

  Seven methods are involved in this project:

1.         1. Light Optical Microscopy: observing the samples sizes, optical properties

2.         2. Sputter coating with carbon: making zircons conductive for SEM analysis

3.         3. Scanning Electron Microscopy (SEM): obtaining the morphology of sample

4.         4. Backscattered Electron (BSE) Imaging: showing more details of morphology

5.         5. Energy Dispersive Spectroscopy (EDS): X-ray Microanalysis of elements

6.         6. Atomic Force Microscopy: obtaining the topography of the laser pits

7.         7. Colorization: using Photoshop to colorize SEM images

Images and results

    1.       Pristine zircons

    (1) Zircon slabs with grain size larger than 3mm.

 

 

(2) Zircon powders with grain size less than 72 µm.

(3) X-ray analysis of zircon

    2.       HF acid treated zircons

(1      (1) Cold HF treated

(2      (2) 100ºC HF treated

(3       (3)  200ºC HF treated

3.       Heat ± cold acid treated zircons

(1)  500ºC in silver capsule

(2) 500ºC in silver capsule + cold HF treated

4.       Laser ablated zircons

5. AFM 3D images of laser ablated zircons

Conclusion

Pristine zircons show its intact structure under SEM, including tabular or prismatic crystal shapes, irregular but sharp edges. Heat treatment changes the surface structure of zircon in large extent. Acid treatment can also change zircon’s structure but only at a high temperature. Cold and low temperature (100 ºC) HF treated zircons show no obvious differences while hot acid (200 ºC) brings slight damage to zircons.

Acknowledgement

I would like to thank Brian McIntyre for giving such a wonderful course of Opt407 and all his help with this project. I would also like to thank our TA, Rohit for the great help during this semester. I am grateful to my advisor, Dustin Trail, for offering the zircon samples and his inspiring discussion and advice.

Reference

J Woodhead, J Hergt, M Shelley et at., 2004. Zircon Hf-isotope analysis with an excimer laser, depth profiling, ablation of complex geometries, and concomitant age estimation. Chemical Geology, 209: 121– 135

Mud Tank zircon pictures are from the following websites:

1. http://www.exceptionalminerals.com/tucson2013-3.htm

2. https://www.mineralauctions.com/auctions/special-consignment-auction-392/zircon-large-crystal-498-grams-18896.html

3. http://www.bespoke-gems.com/SacredGeometrics_Gemstones_Rough_Zircon.php