Undergraduate research by Myles D. McMonigle, College of Marin, Kentfield, CA.
    Faculty Advisors: Kenneth Miller, Geology; Dr. Manus Monroe, Chemistry

The identification and chemical analysis of igneous rocks depends measurably on the nature, content and percentage of key feldspar minerals. There is a relatively accurate method of obtaining these percentages that eliminates much of the doubt concerning the feldspar make-up of the rock. It involves preparing a polished slab of the rock and then staining the polished surface with prepared chemicals. The K-feldspar minerals will stain yellow in color and the plagioclase feldspar will stain red making an easy estimate of the composition of these feldspars and a more accurate identification of the rocks chemistry.


The identification of an igneous plutonic rock depends essentially on a reliable method for determining its mineral chemistry. This is usually accomplished by a visual inspection of the component minerals. An estimate of the relative percentages of K-feldspar compared to the percentage of plagioclase feldspar in the rock is vital. When the potassium feldspar is dominant and quartz is significant the rock might be identified as granite, but if potassium feldspar and plagioclase feldspar were nearly equal then the rock should properly be referred to as Quartz Monzonite.

This paper suggests a relatively inexpensive and easy method for the identification of the chemistry of the feldspars and a more exact identification of the rock. It involves a method for differentially staining K-feldspar yellow, and plagioclase feldspar red on a flat surface of the rock. The method was described by Bailey and Stevens in 1960, and by Laniz in 1964 2. With certain alterations and modifications to their work this paper presents a very positive and useful method for the identification of the chemistry of the feldspars and a more specific identification of the rock.


To test this method a collection of igneous plutonic intrusions located in the coast ranges of California within easy access of the College of Marin campus known as the Salinian Terrane was selected5. The name comes from the Salinas valley, which forms a major alluvial depression. The Salinian Terrane is a wedge shaped crustal block of west-central California, the boundaries of which are the San Andreas fault on the east, the Nacimiento fault on the west, and the Big Pine fault on the south. The coastline of California from Point Sur in the south and northward to Point Arena delineates the side of this terrane exposed to the Pacific Ocean. All the major plutonic bodies of the coast ranges of California are located within this terrane. A simplified map of the area is shown here. Some of the most beautiful coastal and mountainous scenery of California is located here. The basement rocks consist mostly of Cretaceous granitoid plutons,

The geologic map of California reveals that there are nine major plutonic bodies. Each forms a separate salient geomorphic relief feature. From north to south the nine selected sites referred to in this report are: 1) Bodega Head, 2) Point Reyes, 3) Montara Mountain, 4) Ben Lomond Range, 5) Santa Lucia Range, 6) Sierra De Salinas Range, 7) Gabilan Range, 8) La Panza Range, 9) Mt.Pinos. Great care was taken to make sure that the samples taken from each site were a good representation of the entire outcrop and were least weathered. Each field sample ranged in size from 12 to 18 inches in diameter. Each was tagged with a location designation and indicated with a symbol on the geologic map sheet used for that specific locality. There were six California geologic map sheets, scale of 1:250,000, used for this study6.


The samples were brought back to the laboratory where they were cut and polished before staining. We choose to cut each of them into a rectangular, 4 in.x 5 in.x 2 in. shape. Size is a personal preference, the only restriction being the ability to fit it within the containers being used for staining. Our field samples, all over twelve inches in diameter, had to be sliced first into large slabs approximately 2 in.thick on a thirty-six inch diamond blade rock saw, and then cut to our desired dimensions on a smaller trim saw. If the saw uses an oil coolant, as ours did, wash off the excess oil from the slab with water. After this, the surface to be stained needs to be polished only enough to remove any grooves made by the saw blade. We used 220 grit carborundum on a rotary lap, wheel to do this. After this wash off the slab thoroughly in tap water, no detergents, and allow it to dry. Now you should prepare a laboratory site where the staining can take place and where you can mix the staining solutions.


It is recommended that the staining be accomplished beneath a fume hood because the acids used are caustic. Good ventilation is necessary. We were able to use the college chemistry laboratory for this. The staining process involves the use of three chemicals. First, Hydrofluoric Acid (HF) is used to etch the polished surface. After that Amaranth is used to stain the plagioclase on that surface a red color. Then Cobaltinitrite is used to stain any K-feldspar on that surface a yellow color. 

The ideal arrangement is to use two adjoining fume hoods. Beneath one fume hood place an open top non-corrosive container large enough to easily accept the flat polished surface to be stained. Fill it, to a depth of about one-inch, with a 46-52 percent solution of Hydrofluoric Acid. Do not fill it too high. You do not want the acid to spill over the sides when you dip the rock face into the acid. Be careful, this acid is very caustic. Use a (HF) resistant apron and gloves and ware eye protection goggles when handling Hydrofluoric Acid. Next to the acid container place a large container of distilled water to serve as a rinsing bath. Under the second fume hood place a similar container filled to an equal depth with a seven percent solution of Amaranth. Amaranth (C2oHllN2Na3OlOS3) comes in powder form.

Mix about 33 grams of it per 500ml. of distilled water. Also under the same fume hood, and in another same size container, place the same volume of a 20 percent solution of Sodium Cobaltinitrite (Na3CO(NO2)6). It also comes in powder form. Mix about 20 grams of it per.100ml. of distilled water. Next to each of these acids place a separate container of distilled water to serve as a rinsing bath for each acid. Nearby this hood there should be a separate area for drying the stained slabs. A blow dryer can be used or they can be left to dry overnight.

With the chemicals mixed and placed in the working areas, the next step is to begin the process of staining.


The staining is accomplished by following these steps:

Step 1: Dip the polished face, about 1/4 inch submerged, into the Hydrofluoric Acid for approximately one minute. 

Step 2: Rinse the wet slab face in the distilled water rinsing bath, next to the (HF), for 5-10 seconds. This requires either gently moving the face around under the surface of the water, or gently dipping the face in and out of the water several times.

Step 3: Dip the same face into the Amaranth solution for approximately 2-5 seconds.

Step 4: Now rinse off the Amaranth in its rinsing bath for approximately 5-10 seconds. Follow the same rinsing method described in step 2 above.

Step 5: While holding the slab in a tilted position, allow any excess Amaranth to drain off until the stained face is no longer "wet looking". A blow dryer can be used for this, but be very careful not to blow off any of the red colored stain which has formed on the plagioclase feldspar. 

Step 6: Now you are ready to stain the K-feldspar. Dip the same face of the slab in the same manner as above, into the solution of Sodium Cobaltnitrite for approximately one minute.

Step 7: Rinse off the Sodium Cobaltinitrite in the rinsing bath provided for this purpose. Follow the same rinsing method as described is step 2. The K-feldspar should now be stained a yellow color. 

Step 8: Dry off the slab as in step 5 being very careful not to blow away any of the thin film of colored stain.

Step 9: Place the stained slab, face up, on a bed of paper towels and let dry under a heat lamp for several minutes or leave to dry overnight. The staining is now completed. Any plagioclase has taken on a red color and if there is any K-feldspar present it will be stained yellow. Quartz and most ferromagnesian minerals will remain unstained and in their original color. 

Step 10:This step is optional. After the stained surface is completely dry you may want to protect it by very gently spraying on a thin film of clear plastic. The type used to protect pastel artwork. Otherwise the stained colors will smudge or rub off if not carefully handled.


The silicate compositions of the majority of common feldspars can be expressed in terms of the three component triangular phase diagram shown here, in which KAlSi3O8 (orthoclase-or), NaAlSi3O8 (Albite-Ab) and CaAl2Si2O8 (Anorthite-An) occupy the corners of a triangle7. The members of the series between Or and Ab are known as the alkali feldspars and the members in the series between Ab and An as the plagioclase feldspars. All the feldspars are tectosilicates meaning that all the oxygen ions in the SiO4 tetrahedron are shared with neighboring tetrahedra. The fundamental unit on which all silicates is based consists of four 02+ ions at the apices of a regular tetrahedron surrounding and coordinated by one Si4+ ion at the centers This is referred to as the SiO4 tetrahedron. Aluminum also coordinates with four oxygens arranged at the apices of a regular tetrahedron and may link with SiO4 tetrahedra in polymerized groupings. The potassium and calcium ions are located within the interstitial space created by a cage of tetrahedra. 

During the first step of the "staining" process these minerals come into contact with Hydrofluoric Acid. The small HF molecule can attack the Si-O bonds on any surface configuration. The Si-O bonds and the H-F bond will break and bonds will occur between 0-H and Si-F. When this occurs the silicate cages which surrounds the potassium and calcium cations are broken, thereby exposing the K+ in the K-feldspar and the Ca2+ in the plagioclase. The layer of broken silicate cages is assumed to exist in the outer layer (first plane) of the mineral. It is assumed that the Ca2+ remains in the cage because if it leaves the slab surface, other cations must enter the broken cage in order to keep charge neutrality. It is also assumed that the Na+ ions can not enter the broken cage due to the size of these ions and their charge repulsion.

In step three of the "staining" process this surface is introduced to a solution of Amaranth (C20HllN2Na3OlOS3). In solution, the alcohol group of the Amaranth molecule looses itsí hydrogen and becomes a ketone. This gives the oxygen two sets of unpaired electrons. When the slab surface (the one recently introduced to the HF) comes into contact with the Amaranth solution, the unpaired electrons of the ketone and the unpaired electrons of one of the nitrogen atoms kelate "claw" the freshly exposed Ca ion of the plagioclase. Since the calcium ion is still "embedded" within the broken silicate cage, the amaranth molecule forms a thin precipitate on the face of the plagioclase mineral. This precipitate is recognized as a red "stain" on the plagioclase feldspar(see sample below)

After some experimentation it was recognized that the elements of Group-II on the periodic table will react with the Amaranth solution. Magnesium and calcium form a slight precipitate, while strontium and barium ions form a definite precipitate. Lithium, sodium, and potassium ions do not react with the Amaranth solution. Sodium rich Albite (NaAl2Si2O8) of the plagiociase solution series does not form a stain. A good example of this was seen when the mineral perthite was tested. Perthite is an exsolution intergrowth of thin albite lamellae with K-feldspar. The thin Na rich albite lamellae (about lmm wide) did not leave a "stain". But all the K-feldspar between the thin lamellae of albite did leave a vivid yellow "stain". This emphasized the thin lamellae of the perthitic structure and showed that this test is very selective. 

In step six of the "staining" process the rock slab is introduced to a solution of Sodium Cobaltinitrite (Na3CO(NO2)6). When the slab face makes contact with the solution, the freshly exposed potassium attracts a highly charged Co(NO2)6 forming a bond with potassium within the broken cage. The potassium cobaltinitrite forms a thin yellow precipitate on the face of the K-feldspar mineral. This precipitate forms the yellow "stain" in the image below.


Color images of the nine stained slabs are linked in the table below. Visual identification of the percentages of feldspar was made and the rocks were named by using a classification system patterned after Pirsson and Knopf, 196l and Grout, 1959.  A comparison was also made by using the classification scheme adopted by the International Union of Geological Sciences (IUGS) for plutonic rocks shown here.

Thin sections were made of each corresponding slab and analyzed under the petrographic microscope to provide additional inspection of each sample. The thin section description matched favorably with what was seen in the stained slabs. The results are as follows:
Location (small image) Detail of Stained Rock Thin Section
1. Bodega Head Granodiorite Granodiorite
2. Point Reyes Granodiorite Granodiorite
3. Montara Mountain Quartz Diorite Quartz Diorite
4. Ben Lomond Mountain Quartz Diorite Quartz Diorite
5. Santa Lucia Range Quartz Diorite  Quartz Diorite
6. Sierra De Salinas  Quartz Monzonite Granite
7.Gabilan Range  Porphyritic Granodiorite Granodiorite
8. La Panza Range Porphyritic Granodiorite Porphyritic Granodiorite
9. Mount Pinos  Granodiorite Granodiorite

Side by side comparison of the stained and unstained samples.


Bailey, E.H; Stevens, R.E., 1960
Selective staining of K-feldspar and plagioclase on rock slabs and thin sections:
The American Mineralogist, vol. 45 (Sept-Oct)

Laniz, R.V.; Stevens,R.E.; Norman, M.B., 1964
Staining of plagiociase feldspars and other minerals with F.D. and C red no. 2:
U. S. Geological Survey, Prof. Paper 501-B

Ernst, W.G., 1981, 
The Geotectonic Development of California:
Rubey, Vol.11, Prentice Hall

Sullivan, R., Galehouse J., 1990, 
Geological Excursions in the Salinian Terrane 
American Assn. of Petroleum.Geologists

Jennings, C. W., 1977, 
Geologic Map of California, Scale, 1:750,000
California Division of Mines & Geology

California Division of Mines & Geology ' 
Geologic map sheets (scale 1:250,000) each: Santa Rosa, San Francisco, San Jose, Santa Cruz, San Luis Obispo, Los Angeles

Deer,W.A., Howie, R.A., Zussman, J, 1963, 
Rock Forming Minerals
John Wiley & Sons, New York

Klein, C., Hurlbut, C.S., 1993 
Manual of Mineralogy, 21st.ed.
John Wiley & Sons, New York

Pirsson, L.V., Knopf, A,, 1961 
Rocks and Rock Minerals, 3rd.ed.
John Wiley & Sons, New York

Grout, F.F., 1959, 
Kemp's Handbook of Rocks
D.Van Nostrand Co., New York

Blatt, H, Tracy, R.J., 1995, 
Petrology, 2nd-ed., 
W.H. Freeman & Co, New York

Converted to the web by Jim Locke, Geology, College of Marin