Lorence G. Collins



In the above published article Armitage and Back reported the results of taking biotite samples containing Po halos and heating them in several experimental tests between one and five hours at controlled temperatures of 250 to 700 degrees Centigrade. The purpose of their studies was to determine whether structural defects would occur in biotite under these conditions and whether Po halos would, therefore, be erased by the heat treatment. Because their experiments showed that structural damage and erasure of Po halos occurred in a very short time, they concluded that the host biotite and its contained P halos could not have formed at high temperatures as a result of crystallization of granite magma that would solidify at supposedly still higher temperatures and over millions of years of cooling time. Therefore, these investigators opined that the granite host rocks must have formed very rapidly and possibly even under cold conditions.

Their experiments, however, are seriously flawed for a number of reasons. These reasons include problems with their sample preparation, conditions of the experimental tests, and the composition of the biotite. In addition, their initial assumptions about the origin of granite were false. In the following sections, each of these items are examined.

1. Sample preparation

The investigators indicated that they used large biotite crystals, containing Po halos, from samples provided by Robert Gentry. Each sample was cleaved into thin "leaves" so that the Po halos could be found. These leaves were then cut into 1 inch by 1 inch chips and further trimmed to 1 cm by 1 cm around areas containing the Po halos.

It is apparent from their published photographs that the samples, prior to heating, were damaged by the trimming during the sample preparation. That is, pressures applied during the cutting, created wedges of microscopic air spaces between cleavage planes in the samples. This damage is apparent from the optical patterns of rainbow colors in the photographs. Such colors result from refracted light moving through air spaces of different thicknesses.

2. Conditions of the experiments

The damaged samples were then placed as loose fragments in crucibles in a heating oven at one atmosphere pressure in the presence of oxygen. These conditions in NO WAY duplicate the environment in which the granite and the biotite crystals were formed. The granite, pegmatite, or calcite vein in which the biotite crystals grew would have been under high pressures and in an oxygen-free environment.

3. Biotite composition

Biotite is a hydrated iron-aluminum silicate and commonly contains interstitial water in the crystal lattice. Therefore, the occurrence of this water would have caused the results observed in the experiments but would have had nothing to do with the origin of the Po halos or the stability of the biotite in the granite.

4. Experimental results

Because of the investigators' sample preparation, the conditions in which the samples were heated, and the occurrence of microscopic interstitial water in the biotite lattice, definitive conclusions are not possible. The tiny wedges in the damaged biotite samples allowed oxygen to enter the biotite chips (leaves) and to convert iron in the biotite lattice to magnetite (an iron oxide) or limonite (a hydrated iron oxide). These conversions would cause the greenish biotite crystals to turn either black or brown and would have created an opacity that would make any Po halos invisible. At the temperatures of 250 to 700 degrees Centigrade, interstitial water in the biotite lattice would have become steam that clouded the crystals on condensation or caused the crystals to flake because of expansion. The flaking would also destroy the optical clarity. In any case, the Po halos could still have been present but not visible.

The significant point is that under the proper conditions in which the biotite and Po halos were originally formed, the biotite crystals would have been confined between other crystals in solid rocks at high pressures and in the absence of free oxygen. Pressures of steam in and outside the biotite crystals would have been equal so that no expansion and flaking of crystals would occur. Therefore, none of the aforesaid changes, observed by Armitage and Back, would have taken place.

5. Initial assumptions

The assumptions that the granite was formed by crystallization from a melt at temperatures, which Armitage and Back suggested could range from 1,200 to 2,000 degrees Centigrade, is not necessarily true or in accord with recent work on melts of granites and other igneous rocks. Some melts of rocks rich in calcium, aluminum, magnesium, and iron, which do not have the composition of granite, might start to crystallize at these high temperatures, but these melts would have been completely dry. Some "dry" granite melts start crystallizing at temperatures above 800 degrees Centigrade, but after complete solidification, they do not have any biotite in them because this mineral requires water in its structure. Therefore, lacking biotite, these granites also could not have contained Po halos.

Admitted, a biotite granite, which crystallizes completely from "wet magma" near 600 degrees Centigrade, cannot have Po halos in its biotite. The short half lives of the Po isotopes and long time necessary for a large body of granite magma to crystallize would result in insufficient polonium remaining to precipitate in the biotite crystals. If, however, the host granite and Po-halo-bearing biotite were formed by replacement processes of an older igneous rock, already completely solidified from a former magma, then no problem exists. The granite and Po-halo-bearing biotite can be formed by completely natural processes, requiring no miracles. See discussion of these processes in presentation number 4 in this web site.

For more information contact Lorence Collins at:

Dr. Lorence G. Collins
California State University Northridge
Department of Geological Sciences
18111 Nordhoff Street
Northridge, CA 91330-8266
FAX 818-677-2820