Getting started

---prepare uncovered thin sections leaving it a bit thicker (yellow quartz)

---polish with 600-grit SiC (may use finer powders, the finer the better)

---clean with dilute ethanol (to remove SiC rests, fat, dust etc.)

---read and sign security warning about HF usage (ask István for such)

---set up your staining facility near a sink; the beakers should have the appropriate size and shape to serve as chemical baths into which your thin sections will be dipped. Now prepare all the reagents except HF:

In a 100 ml beaker, solve 0.02 g K-rhodizonate in 60 ml distilled water.

In a 100 ml beaker, solve 4.4 g BaCl₂ in 80 ml distilled water.

In a 100 ml beaker, solve 36 g Na-cobaltinitrite in 60 ml distilled water.

---Fill a large (e.g. 500 or 1000 ml) beaker with distilled water and put it in the sink.  (Water is used for rinsing the thin sections between the individual staining steps.) Allow a gentle stream of water to flow out of the tap which gradually replaces the water in the beaker. If possible, use distilled water. 

---Set up lab wipes or paper towels with something to rest the sections against so that they dry on their edges rather than on flat sides.

---Finally, in the fume hood, prepare a bottle of HF (48 to 50%). Whenever working with HF, be very careful, calm and patient as it is extremely aggressive and toxic. Work only in the hood and keep all the security regulations.

---pour 48% HF into the etching vessel, with about 1 cm liquid height.

---close the vessel and place your section onto the aperture which has the appropriate size for your sections; take care that it fits precisely into the frame, leaving no leakage gap. (Fig. 1)

---never touch the section by hands; use a flat tweezers

---cover the dish with an upside-down plastic bowl to keep off air currents.

---etch for 45 sec; increase time if HF appears to attenuate (it remains strong for min. ~30 min.). The sections should never touch the HF liquid itself, only the vapor.

---complete etching of all sections, and keep them in the fume before proceeding to stain. The etched surfaces should not get in contact with any object.

---drop a section into Na-cobaltinitrite solution; remove after 45 sec

---rinse thoroughly in distilled water

---shake off excess water and blot section edge with many-times folded lab wipes or paper towels

---dip the section into BaCl₂ solution, agitate shortly, remove after 2 sec

---rinse in distilled water, for 10 sec

---check whether there is a fine, thin, even water film covering the whole surface of the section

---place several drops of K-rhodizonate on the slide, in a very rapid succession. Help the solution distributing on the surface by agitating or tilting it gently

---after 3 sec (may vary upon plag composition), rinse in distilled water

---shake off excess water and blot section edge and dry gently with compressed air

---examine the plags with a microscope to determine whether they are sufficiently stained. If they are light grey/very light pink then cover briefly again with K-rhodizonate and rinse again. Always work with wetted surface and use short contact times (1-3 sec)

---cover just as the ordinary slides. Be careful; the stained surface is very fragile and fingers, and even water, could rub off the stain.

---Quartz remains unstained.

---Plagioclase will be stained by K-rhodizonate, turning them into pink.

---K-feldspar will be stained by Na-cobaltinitrite, turning them into yellow.

---BaCl₂ is needed between these two staining steps, for fixing.

---K-rhodizonate: 1-2 hours max. Always prepare it freshly (pure crystalline material is hygroscopic, thus clumps tend to form. You may have to filter the solution first).

---Na-cobaltinitrite: about 6 months (after use, filter and keep in a brown bottle)

---BaCl₂: stable, but BaCO₃ may form and precipitate due to contact with CO₂.

---The K-rhodizonate stain may be too pale. If so, repeat staining as many times as desirable.

---If you have a fine-grained sample, (very fine sand, silt) the plagioclase stain tends to pervade the thin section and thus obscure some quartz grains.

---Staining intensity is proportional to An%; the more calcic the plag, the more stain it takes up. Pure Ab is not expected to take up the K-rhodizonate stain.

Tamás Mikes

This compilation is largely based on:

Houghton, H. F. 1980. Refined techniques for staining plagioclase and alkali feldspars in thin section. Journal of Sedimentary Petrology 50, 629-631.