Franklin & Marshall College Franklin & Marshall College

    • u-h-51cad908a909-jpg
    • u-h-4987f6ddaf7a-jpg
    • u-h-fd8c9b4b32b0-jpg

Major Element Technique

  • images-departments-earth-crucible1-jpg

A. Lithium tetraborate - whole rock powder preparation - Introduction

3.6000 grams +/- 0.0002 of lithium tetraborate is weighed out into a clean glass bottle followed by 0.4000 grams +/- 0.0001 of the rock powder and are mixed for 10 minutes in a Spex Mixer Mill. The homogeneous powder is transferred into a 25 cc. 95% Pt-5% Au crucible, 3 drops of a 2% solution of Lil is added to the powder to reduce the viscosity of the mixture as it is heated over a Meeker burner while mounted on a standard ringstand. During the heating the crucible is covered with a 95% Pt-5% Au lid which will also act as the mold into which the molten sample will be poured and cast into a disc shape. The bottom of the Pt lid is flat and highly polished, thus, the side of the disc in contact with the Pt lid is the one exposed to the primary X-ray beam.

The heating period is normally 10 minutes in duration with the sample being vigorously stirred while holding the crucible with a pair of Nichrome tongs at both the 3 minute and 6 minute marks. After sufficient heating so that the molten sample is thoroughly convecting, the Pt-lid is removed from the crucible with a pair of tongs and heated over a second bunsen burner until it is red-hot. With a second pair of tongs the crucible is removed rapidly from suspension on the ringstand over the first burner and emptied onto the hot Pt-lid. With some practice virtually all of the crucible's contents are transferred to the lid. Immediately upon completing the pouring event the still hot crucible is dropped into a warm beaker containing sufficient 4N HCl to cover the crucible. With the other hand, the crucible which has been held quite level is now carefully sat down on a flat surface; in our lab this is a flat polished piece of granite. The sample will cool in 3 to 5 minutes so that the glass disc can be labeled with a magic marker, labeled on the side of the disc exposed to the air. The disc can be stored indefinitely in a desiccator. The major element oxides, SiO2, Al2O3, Fe2O3, MnO, K2O, Na20, TiO2, P2O5, CaO, MgO, are determined using this technique together with Sr, Zr, Cr and V.

Safety Warning: Lithium tetraborate poses a hazard to skin, eyes, and internal organs. For more information see www.fishersci.com/msds
Acetone poses health and fire hazards. For more information see www.fishersci.com/msds
Lithium iodide poses a hazard to skin, eyes, and respiratory system. For more information see www.fishersci.com/msds

Instructions: 

1. Thoroughly clean and dry several glass sample bottles (24 ml glass bottles from Thomas Scientific) and lids if clean bottles are not already available. The final cleaning step is always to rinse in acetone and lay bottle on its side and turn the plastic caps up on a paper towel for the purposes of drying.

2. Weigh out 3.6000 grams +/- 0.0002 of Li2B4O7 onto a piece of weighing paper (previously weighed and tared) and transfer to a clean glass bottle. Label glass bottle with magic marker (not lid) with sample number followed by WR (whole rock).

3. Weigh out 0.4000 grams +/- 0.0001 of whole rock powder onto a piece of weighing paper (previously weighed and tared) and transfer to the same bottle as in 2.

4. Carefully rotate sample bottle in your hands so as to mix the powders without letting them touch the lid.

5. Make sure you have labelled the bottle!

6. Mix sample for 10 minutes in the mixer-mill.

7. If you stop at this step without going onto Part B, store bottle in a dessicator.

B. Fuse sample to make glass disc per demonstration given by your instructor. Key factor to always remember-- mix your liquid sample three times in the Pt crucible; once at the 3 minute mark, again at 5 to 6, and the 3rd time just before pouring into the Pt lid. Also, be sure to incline the Pt crucible in the flame before pouring to break the surface tension between liquid and metal so that most or all the sample will be used in casting the glass disk. You must mix three times to insure sample homogeneity.

Do not forget the following items:

1. When fusing sample always wear safety glasses.

2. Add two or three drops of LiI solution to powder after it has been transferred to Pt crucible, from small labelled plastic squeeze bottle .

3. Be extremely careful in cleaning Pt lid! DO NOT SCRATCH!!!

4. With a magic marker, label sample number on concave slide of glass disc - side which cools in contact with air.

5. Place 1/2 of a small kimwipe in the bottom of a pillbox and put glass disc, label side up, into the container and cover with lid. Do not label the pillbox itself.

6. Store pillbox in a dessicator.

7. Clean the glass bottles and caps you used in distilled water, using scrub brush. Rinse in distilled water and then rinse in acetone and let dry in air on a paper towel for an hour or so. Lay the bottles on their sides to dry. Recap and put back into the storage trays.

8. The waste from this procedure is considered hazardous and must be collected and disposed of properly.

 

« back to Cutting and Crushing on to Trace Element Technique »
^back to top  back to X-ray Laboratory home