Reprinted from: Gems & Precious Stones. Mineral Resources U.S., 1916, Part II - Nonmetals. Department of Interior, US Geological Survey, Government Printing Office, Washington; pages 896-898; 1919.
EXHIBIT OF TOURMALINE AND OTHER GEM MINERALS IN THE PEGMATITES OF SOUTHERN CALIFORNIA
By Waldemar T. Schaller
The gem-bearing pegmatite region of southern California has been somewhat intensively studied by the writer under the joint auspices of the United States National Museum and the United States Geological Survey, and an exhibit representing the region has been deposited in the National Museum, Washington, D. C.
The gems are contained in one case which has three horizontal shelves and two sloping shelves. The exhibit consists of specimens of the country rocks (granite and gabbro) and of their associated pegmatite dikes, in which are found the gem
tourmalines and other gem minerals. The collection is shown on the first floor, main hall, east end, of the new National Museum Building, Washington, D. C.
The nearly flat-lying pegmatite dikes, from which most of the specimens shown were obtained, crop out on the hills north and east of
Pala, San Diego County, Cal., and are of the compound, unsym-metrical type whose different parts are thought to be due to differentiation processes rather than to multiple injections of material into reopened fissures. The upper portion of the dike is locally known as the "top rock" and is a mixture of a coarse, granular aggregate of
quartz and
feldspar and of a graphic pegmatite. No gem stones are found in this "top rock." The lower portion of the dikes, locally called the "bottom rock," is a much finer grained granular quartz-
albite rock characterized by numerous wavy bands of brownish-red
garnets. These bands lie nearly horizontal in their general trend, being parallel to the slight dip of the dike. The "bottom rock" is likewise free from gem stones, but both it and the "top rock" are of great interest from the scientific point of view. Between the "top rock" and the "bottom rock" is the middle portion, called the "pay streak" by the miners, in which the gem minerals of value are found. Here also occur the cavities or pockets which often yield an abundance of the well-crystallized minerals shown in the exhibit.
The top horizontal shelf on the south side of the exhibit case contains the granitic and gabbro country rock, and also the partly altered gabbro—a loose, friable rock—and the completely altered gabbro, which as a brown iron-stained
clay has been washed into the cracks and seams of the pegmatite rock. Where such a crack extends into a pocket the clay has coated the gems and associated minerals found therein. There are shown, for example, white feldspars of the pegmatite coated with the brown clay derived from the gabbro country rock. The origin of the clay of the gem pockets is thus explained.
The second horizontal shelf on the south side contains different varieties of the "top rock," consisting of graphic pegmatite and granular pegmatite. The third horizontal shelf shows the mineral aggregates of the middle part or "pay streak," which yields on decomposition the loose, friable material forming the gem pockets. This same shelf also shows examples of the banded "bottom rock."
The sloping shelf on the south side contains large specimens of the different varieties of the pegmatite rock, including granular, graphic, and banded pegmatites. Several of these larger specimens have been sawed and polished and are well adapted for use as an ornamental stone, especially when cut obliquely so as to form wavy lines and circular effects resembling bird's-eye wood. One specimen in particular consists of a large section of the entire pegmatite dike and shows the aggregate of lithium minerals in the upper portion or "top rock," the granular pegmatite of the middle portion, and the banded "bottom rock."
The sloping shelf on the north side of the case illustrates the mineral contents of the gem pockets. A sample of the gem-bearing clay or pocket material is first shown, below which is an equal amount of similar gem clay separated into its constituent minerals; thus the relative proportions of the gem tourmaline, the clay washed into the pocket from the decomposed gabbro country rock, and the various minerals associated with the gem tourmaline are exhibited. For example, the pocket material from the
Tourmaline King mine, at Pala, shows much
lepidolite,
orthoclase, clay, and gem tourmaline (
pink and
green), and smaller amounts of
muscovite and quartz. Similar gem-bearing clay from the
Tourmaline Queen mine, at Pala, shows in addition to much pink tourmaline, large amounts of clay, quartz, albite, and
cookeite, but practically no orthoclase. A gem pocket from the
Ed Fletcher Jr. mine, at Pala, shows in addition to much pink tourmaline, clay, and albite, a considerable amount of lepidolite, with only a little orthoclase. A gem pocket from the Pala Chief mine, at Pala, shows considerable gem
kunzite, with lepidolite, quartz, clay, cookeite, and orthoclase, and smaller amounts of albite and muscovite. A similar pocket from the Caterina mine at Pala, shows only
spodumene, pink clay, and quartz. The exhibit also includes a pocket containing an abundance of small
blue tourmalines; a small pocket from the Tourmaline King mine, very rich in gem tourmalines; and several pockets free from any gem stones. Such pockets are known by the miners as "dead ones."
On this same sloping shelf are shown also several large specimens of the minerals associated with the gem pockets. Among these minerals may be noted a fine example of orbicular muscovite, an altered
perthite (feldspar) crystal, a large
amblygonite crystal, and several specimens of spodumene (kunzite) in the matrix. These kunzite specimens are very difficult to collect, as in general the matrix of the kunzite is so friable that it breaks to pieces when it is taken out of the mine.
The three horizontal shelves above the sloping shelf on the north side of the case contain well-developed and well-crystallized specimens of the different minerals found in the gem-pocket zone of the pegmatite dikes. Among these minerals may be noted in particular a good series of the various forms of lepidolite (including several well-crystallized specimens), crystals of muscovite, fine tourmaline crystals, albite and orthoclase in well-developed crystals,
pink beryl,
stilbite,
cassiterite, a large crystal of
lithiophilite,
bismuth,
bismuthite,
bismuthosphaerite,
purpurite,
hematite and
pyrite,
apatite,
pucherite,
topaz,
manganotantalite, a fine example of clear pink kunzite in the matrix, and the phosphate minerals first found in this locality, namely,
palaite,
salmonsite, and
sicklerite.
