Crystal Sizes and Pegmatite Dikes

"... primordial ²¹⁸po halos imply that Precambrian granites, pegmatitic micas, and other rocks which host such halos must be primordial rocks." (Gentry, 1979, p. 474).

In his book Gentry gives his criteria for identifying the created granites. In trying to rebut Dalrymple's testimony at the Arkansas trial, he explains the difference between Hawaiian lava and Precambrian granites. Dalrymple was comparing the igneous texture of solidifying basaltic lava and extrapolating to how a granite, which also has a crystalline texture, solidifies at depth. Gentry says, however, that the comparison of lava and granite is erroneous because they are different in grain size and mineralogy.

"In bulk Composition and mineralogy the lava specimens are olivine-rich basalt, grossly different from any granite.

"The Kilauea-lki samples are fine-grained... The Precambrian granites, on the other hand, are generally characterized as being coarse-grained.. This means the only similarity between granites and the lava specimens is the interlocking, intergranular arrangement of the crystals making up the rocks. This characteristic can be accounted for naturally by slow cooling of the lava in the case of the Kilauea-lki specimens - or by rapid or instantaneous cooling from a primordial liquid in the case of the granites.

"It is a fact that a hot fluid rock, such as that produced at Kilauea-lki, can cool over a period of a few years to form fine-grained volcanic rocks composed of microscopic-sized crystals. The same is true of rocks that form when granites deep in the earth are melted. The granite melt may extrude onto the surface and cool rapidly to form a glassy rock; or it may cool more slowly beneath the surface to become rhyolite, a fine grained rock… Both the glassy rock and the rhyolites are intrinsically different from the coarse-grained granites." (1986, p 130)

Unfortunately, Gentry botches this whole argument by not realizing that there are many types of igneous rocks. He says that rhyolite is a natural form of granite that forms at depth very slowly and that rapid cooling results in a glass (I suspect he was referring to obsidian). He said, "Both the glassy rock and the rhyolites are intrinsically different from the coarse-grained granites." To emphasize this point in his book he shows a drill-core sample of rhyolite taken from a depth of 1,683 feet at Inyo Domes, California, and compares it to a sample of medium-grained granite. His point is that crystal size determines the formation of the rock - coarse-grained granite is supernaturally and instantaneously created, whereas fine-grained rock is intrusive and glassy rock is extrusive. It was obvious to me in reading this part of his book that Gentry was having a hard time with this material. Note the contradiction on the rate of lava cooling?

As any undergraduate geology student knows, the two basic types of igneous rocks are extrusive (such as flows) and intrusive (such as dikes and plutons). Rhyolite is the extrusive equivalent of granite by definition. Their chemical compositions are the same. The sample Gentry shows as an "intrusive rhyolite," in fact, came from a conduit which fed a volcanic flow at the surface (Dalrymple, personal communication, 1987; Eichelberger and others, 1984). It is suspected that development of the texture of the glassy rock Gentry mentions, the obsidian, was dominated by lack of water, not cooling rate, during its formation (Eichelberger. personal communication, 1987). In fact, at that site it can be shown that the rhyolite cooled first. The water content of the rhyolite is higher, and it is suspected that, at considerable depth, the texture is coarser due to an even higher water content (Kasameyer and others, 1985, p 385).

Gentry further shows his lack of geological understanding in claiming that granite and basalt cannot be compared because they are mineralogically different. If it's composition Gentry wants to compare, he should compare rhyolite and granite. If it's grain size he wants to compare, he should compare gabbro and granite. Gabbro is a coarse-grained intrusive rock like granite, and has the same phaneritic texture, but its composition is the same as that of basalt.

Gentry has made numerous other errors simply because he does not understand enough geology. He says (1986, p. 131. emphasis mine)

".. the tiny crystals of which rhyolite is composed bear no comparison in size to the very large crystals found in certain regions within granites known as pegmatites. … (Most of the polonium halos in mica...were found in specimens of biotite taken from pegmatites)"

Well, there you have it, Gentry has just destroyed his own argument. At the three above sites, the pegmatites cut other rock units.

What Gentry is trying to say is that large crystals do not form in nature and so require a supernatural origin. Actually the intrusive rocks in the Shield show a wide variation in grain size. For example, the Addington Pluton, near Kaladar south of Bancroft, is fine-grained but contains stringers of coarse-grained quartz and some biotite (personal observation). Coarse-grained granitic rocks of all ages are found on all of the continents. For example, some of the youngest plutons in the Sierra Nevada contain feldspar crystals 5-10 centimeters long yet are only 80-90 million years old. Many of the dikes in the Shield are course-grained pegmatites, and it bears remembering that as Guilben and Park (1986, p. 488) pointed out:

Pegmatites represent the final water-rich, siliceous melts of intermediate to silicic igneous magmas, and can generally be thought of as final residual melts…

Although pegmatites can be found in almost any shape, they are moat commonly dikelike or lensoid. Most pegmatites are small, but dimensions can vary from a few meters to hundreds of meters in the longest dimension and from 1 cm to as much as 200 meters in width.... Since igneous pegmatites characteristically solidify late in igneous activity, they tend to be associated with plutonic or hypabyssal intrusions from which the volatile fractions could not readily escape. The great majority of pegmatites developed in deep-seated high-pressure environments.

If, as Gentry claims, the criterion for distinguishing a created rock from a naturally crystallized rock is grain size, then it is fair to inquire at what grain size that distinction is to be made and why. The grain sizes of dikes range from microscopic to several tens of centimeters. In one pegmatite dike near Madawaska, north of Bancroft, which was mined for feldspar, a single crystal measuring 7 meters long and weighing 300 tons was extracted. At the Silver Crater Mine, hornblende crystals 3 meters long have been observed [Photo right is a large biotite book at the surface exposure of the calcite vein-dike at Silver Crater. Photo by J.R. Wakefield, 1987]. I've seen crystals the size of one's fist on Turner’s island east of Bancroft. These are all found in cross-cutting dikes that are clearly intrusive.

Porphyries pose another serious problem for Gentry’s interpretation. Porphyries are found in volcanic flows or as dikes where large crystals up to centimeters long (called phenocrysts) of a single mineral are set in a very fine-grained matrix of the same and/or other minerals. How would Gentry explain such textures? Geologists can easily explain them. Different minerals melt at different temperatures. So, as a magma chamber cools the minerals with the highest melting point crystallize first. If at some time, the magma moves to a different location (while the first solid mineral is still suspended in the liquid) and is injected as dikes and sills or extruded as flows, then the remaining magma will cool too quickly to form large crystals and the phenocrysts will be trapped in the fine-grained matrix.

Igneous rocks come in a variety of grain sizes and the principle factor that controls grain size is the cooling rate - slow cooling results in large crystals, fast cooling results in small crystals, and very rapid cooling results in glass. This has been known to geologists and chemists for at least 150 years and can be readily demonstrated in the laboratory. Gentry is simply wrong in his conclusions about the importance of grain size.

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