Mineral stains have been devised for the rapid identification of many common minerals; Reid (1969) provides an extensive compendium. The identification of minerals, however, can now be achieved with much greater certainty using modern analytical techniques (SEM, microprobe and so on). Stains have been devised that display intracrystalline variations in chemical composition although techniques such as cathodoluminescence, fluorescence and backscatter electron imaging can display this type of information more comprehensively and reliably than staining. Some stains, however, are very versatile, they can be used in the field, on rock slabs or thin sections; they are inexpensive, easy to use and rapid to apply.
Staining Carbonate Minerals with Alizarin red S and potassium ferricyanide
The staining of carbonate minerals has a long history (Lemberg 1887) and involves a plethora of different methods (Freidman 1959, 1971; Reid 1969). The most widely used stain for carbonates employs a mixture of Alizarin red S and potassium ferricyanide dissolved in a dilute hydrochloric acid solution. Set procedures are published for this dual stain (Dickson 1965, 1966; Evamy 1963, 1969) but the stain can, within certain limits, be modified to suit the needs of the observer and the type of material being stained. The organic dye Alizarin red S (ARS) will produce a pink to red stain on any carbonate that will react with dilute acid. The reaction between carbonates and acid is usually controlled (1 to 2 minutes at 25oC for thin sections) so that the more reactive minerals, such as calcite and aragonite, stain red but the less reactive ones, such as dolomite and siderite, remain unstained. Calcite is however an anisotropic mineral that is more soluble in a direction parallel to its c-axis than normal to the c-axis. The ARS stain (at HCl concentrations between 0.2 and 1.0%, vol. /vol.) can differentiate calcite sections that are normal to the c-axis (stained pink) from those that are parallel to the c-axis (stained red). This appears counterintuitive - the c-axis normal section reacts more vigorously with the acid yet stains less intensely, and visa versa. This is due to greater evolution of CO2 bubbles from the c-axis normal surface preventing the stain from settling. The c-axis parallel sections produce fewer bubbles and stain precipitation is unimpeded. This differentiation is only effective at acid concentrations between 0.2 and 1.0% (vol./vol.) given stain times of between 1 and 2 minutes.
The intensity of the ARS stain is affected by HCl concentration. At 1.5% the stain is faint, vigorous evolution of CO2 bubbles occurs, and the section is etched thinner, from 30 to ~ 15 μm, after being treated for 1 minute. At 0.1% HCl concentration and 1 minute staining time the calcite c-axis orientation is weakly differentiated, and the intensity of the stain masks the underlying fabric. Staining for more than 2 minutes at 0.1% HCl causes the layer of stain to crack and peel off the stained surface. Reaction between calcite and acid in the 1 to 2 minutes recommended for staining is very sensitive; the acid strength can be adjusted to develop the desired stain intensity.
Dolomite does not stain using these acid concentrations but as iron is substituted into the dolomite lattice it becomes more reactive. Ferroan dolomite and ankerite react with dilute HCl, effervescing and staining with ARS. The ARS stain colour of ankerite (mauve) can be distinguished from calcite (red) but in the dual stain the mauve colour is disguised by the intense blue stain produced by potassium ferricyanide
Potassium ferricyanide (PF) produces a precipitate of Turnbull’s blue when ferrous iron is released to the staining solution. It might be expected that siderite (ferrous carbonate) would react with this stain but siderite does not react with dilute cold HCl, iron is not released to the staining solution and consequently no stain is precipitated. The rate of reaction of the PF stain, as with ARS, is controlled by reaction rate between carbonate and HCl. Calcites and dolomites containing ferrous iron do react with the stain but dolomite reacts less vigorously than calcite so the intensity of PF stain is not proportional to the iron concentration of the two minerals. The PF stain is very sensitive to ferrous iron concentrations in calcite and will distinguish differences in concentration of a few 100 ppm. Lindholm and Finkelman (1972) correlated iron concentration of calcite with stain colour, but to match their colour range their staining procedure must be reproduced exactly. The relationship between iron concentration and stain colour is non-linear as the reactivity of carbonate minerals increases with rising iron concentration (Reeder 1983). Ferroan calcite stains blue and ferroan dolomite stains green to turquoise. As iron concentration increases in these two minerals the colours converge. The difficulty in distinguishing such iron-rich carbonates can be overcome by staining with ARS alone.
When iron concentrations are low the dual staining procedure can be modified to increase iron differentiation. Extending the staining time in the dual stain beyond 90 seconds is inadvisable as the stain cracks and thick deposits of ARS obscure the section. Material with low iron concentrations can be stained first in a PF/HCl solution for 60 seconds before staining with the dual (ARS+PF) stain. The distribution of ferrous iron (resolvable to 1μm) in thin section or rock slab can be differentiated in a few minutes using this stain.
Distinguishing aragonite from calcite
Feigl’s stain (Feigl 1937), a solution of silver and manganese sulphates to which sodium hydroxide is added (for preparation see Friedman 1959), stains aragonite black. Other orthorhombic carbonates also react positively with Feigl’s stain. The black colour is due to a precipitate of manganese oxide and metallic silver. Calcite remains unstained during limited exposure to Feigl’s solution. The stain differentiates aragonite from calcite due to their different solubilities and also causes differential etching between the two minerals. Schneidermann and Sandberg (1971) have used this selective etching (imaged by SEM) as an addition feature of the stain to help distinguish between the two minerals. The latter authors also recommend using MnSO4• H2O, rather than MnSO4• 7H2O, to prepare the stain, diluting the original recipe with water, and warming the solution. Variations in crystal size and orientation have led to differing results using Feigl’s stain so that many regard the stain as unreliable.
Distinguishing Mg calcite from calcite
The Titan-yellow (Clayton yellow) stain for assessing the Mg content of calcite was described by Friedman (1959) modified by Winland (1971) and again improved by Choquette and Trusell (1978). Choquette and Trusell give the staining and fixing procedure for this stain which barely stains calcite with up to 3% MgCO3, stains Mg calcite with 5 to 8% MgCO3 pink to pale red, and Mg calcite with > 8% MgCO3 a deep red colour. This indication of Mg content is qualitative because crystal orientation and size affect the stain in addition to Mg content. The stain is cheap and simple to use but has not gained wide acceptance.Fig. 1 Response of common rock forming carbonate minerals to staining with alizarin red s / potassium ferricyanide stain (J.A.D.D.).
Stains have been used to determine mineralogy and intracrystalline chemical variations although both these properties can now be more reliably and quantitatively determined using analytical techniques. Stains can however be applied to outcrop, rock slabs or thin sections in a few minutes at negligible expense. Their application requires little training other than some basic safety training in the use of chemical substances and an understanding of how the stain operates.
Preparation of Alizarin red S and potassium ferricyanide stain
1) To 300ml of 0.5% HCl add 0.6g Alizarin red S and filter
2) To 200ml of 0.5% HCl add 4g potassium ferricyanide
3) mix the two solutions.
1) Sections can be etched before staining in 1% HCl for 5 seconds
2) Stain sections for 30 to 60 seconds in combined stain until ferroan and non ferroan varieties are differentiated. CO2 bubbles evolve from the reacting surface. Agitate the section and withdraw from staining solution briefly to break any CO2 bubbles that stick to the surface and prevent the stain from reaching with that part of the section. If the bubbles are not released the section it will have many small unstained circles.
3) Rinse the section gently with deionised water. The stain is delicate and is easily removed - don’t jet water at the surface or touch it. Once dry the stain is more durable but if you wish to archive the stained section it is best to cover slip.
4) Allow the stained section to dry by stacking vertically so the water runs off. The stain is water soluble so dry as quickly as possible without touching the stained surface.
The stain can be removed on an acetate peel from a thin section or a stained slab. This is best done while the stain is still damp after staining and before it dries out completely; once dry the stain will stick to the rock. The stained surface should be held horizontally and flooded with acetone. The acetate sheeting should be rolled onto the surface squeezing out the excess acetone. Leave the peel to dry completely - ideally 24 hours. If you remove the acetate shortly after application it is still soft and will distort and wrinkle. The water/acetone mix on the stained surface is critical - too much water and peel will be cloudy - not enough acetone and air bubbles are trapped beneath the peel! Successful peel are tricky to produce and require much practice.