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Major Element Technique

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(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 anhydrous rock powder that results from the LOI technique. An initial mixing is done by hand, gently rolling the bottle in your hands to gently mix the two powders. After 20 to 30 seconds of hand mixing the bottle is transferred to a Spex Mixer Mill for 10 minutes. The homogenous powder is transferred into a 25 cc. 95% Pt-5% Au crucible, 3-4 drops of a 2% solution of Lithium Iodide 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 ring stand. During the heating, the crucible is covered with a 95% Pt-5% Au lid that will also act a 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 usually 12-15 minutes in duration with the sample being vigorously stirred while holding the crucible with a pair of Nichrome tongs at both the 5-minute and 10 minute marks. After sufficient heating, so that the sample is fully convecting, the Pt lid is removed from the crucible with a pair of tongs and is heated over a second meeker burner until it is red hot. With a second pair of tongs the crucible is removed rapidly from suspension on the ring stand over the first burner and emptied onto the hot Pt lid. Withsome 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 lid, which has been held quite level, is now set down on a flat surface, in our lab this is a flat slab of polished 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 that was exposed to the air. The sample can be stored indefinitely in a dessicator. The major element oxides, SiO2 , Al2O3 , Fe2O3T (where T = total iron), MnO, K2O, Na2O, 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 made by Wheaton) 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 the plastic caps up on a paper towel for the purposes of drying.
  2. Weigh out 3.6000 +/- 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) or M (major).
  3. Weigh out 0.4000 +/- 0.0001 of anhydrous whole rock powder onto a piece of weighing paper (previously weighed and tared) and transfer to the same bottle as in step 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 labeled the bottle!!!
  6. Mix sample for 10 minutes in the Spex mixer mill.
  7. If you stop at this step without going onto Part B, store bottle in a dessicator.

Platinum crucible with lid being heated with a meeker burner; second meeker burner available for additional heat.

Platinum crucible with lid being heated with a meeker burner; second meeker burner available for additional heat.

(B) Fuse the sample to make the 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 5-minute mark again at 10-12
minutes, and the third 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 the entire sample will be used in casting the
glass disc. You must mix three times to insure sample homogeneity.

(C) Do not forget the following items:

  1. When fusing the sample, always wear safety goggles.
  2. Add 2-3 drops of Lithium Iodide (LiI) solution to powder after it has been transferred to Pt crucible, from small, labeled plastic squeeze bottle.
  3. Be extremely careful in cleaning Pt lid! DO NOT SCRATCH!!!
  4. With a magic marker, label sample number on concave side of the glass disc, which is the side that cools in contact with air.
  5. Place ½ 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 a scrub brush. Rinse in distilled water and then rinse with 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.

Last Revised: June 2018