1908 article in United States Geological Survey Bulletin 340, pp. 286-294.
GENERAL DESCRIPTION OF THE DEPOSIT.
Baringer Hill is located about 100 miles northwest of Austin, Tex. on the west bank of Colorado River, near the western edge of the Burnet quadrangle as mapped by the United States Geological Survey. It is 12 miles north of Kingsland, the nearest railroad point, 16 miles west of Burnet, and 22 miles northeast of the town of Llano. It is a low mound rising above the flood plain of the Colorado, and formed by the resistance to erosion of a pegmatite dike intruded in a porphyritic granite.
Few if any other deposits in the world, and certainly no other in America, outside of the monazite localities, have yielded such amounts of the rare-earth metal minerals as Baringer Hill.
The writer visited this region in the latter part of February, 1907 fortunately at a time when Mr. William E. Hidden, who has been largely instrumental in making this locality famous through his contributions to mineralogical literature on the rare minerals found here, was conducting mining operations.
The hill is named for John Baringer, who discovered in it large amounts of gadolinite about 1887. No one in the neighborhood knew what the mineral was and specimens were sent to a number of places before it was identified. A piece fell into the hands of Mr. Hidden, who at once looked up the deposit and afterwards obtained possession of the property. Meanwhile Mr. Baringer had taken out a quantity of gadolinite estimated at 800 to 1,200 pounds, which was largely picked up and carried off by persons in the neighborhood as curiosities. Some of the choicer pieces, showing crystal form, found their way into various museums. The property is now controlled by the Nernst Lamp Company, of Pittsburg, Pa., and is worked by that concern for yttria minerals. Since its acquirement by this company a considerable amount of work has been done on the deposit, consisting mostly of open cuts around the edge of the pegmatite,reaching a depth of 30 or 40 feet. A large block, 30 feet in height and more in diameter, consisting mostly of quartz, is left standing in the middle.
In general the " Llano region," in the heart of which Baringer Hill is located, is an island of pre-Cambrian rocks intruded by plutonics and surrounded by an irregular zone of Cambrian and other Paleozoics, including some that are possibly Devonian and some Carboniferous rocks. The inner portion includes parts of Burnet, Llano, and Mason counties, and is situated at almost the geographic center of Texas. The history of this island has been considerably discussed, and views differ as to whether it was an island during the deposition of the Cretaceous, by which the area is almost entirely surrounded, or whether it has been exposed by denudation of the later rocks. The coal measures extend to the north from the region, giving some evidence of an area of high land previous to the deposition of the Cretaceous. Personally, the writer is inclined to agree with the view that the region has been denuded, although his investigations have been but superficial. The plutonics are granitoid rocks of many textures, and differ considerably in composition. Large areas are composed of the rather coarse red granite, the principal outcrops of which occur near Marble Falls and from which the State capitol of Texas was built. Peculiar dikes of a chocolate-brown granite near Llano contain blue quartz.1
Other dikes containing this blue quartz are of a reddish color. In many localities the granite is very porphyritic, containing feldspars from 1 inch to 2 inches in longer diarneter. There are also gray and fine grained red granites, and in some places they have taken a gneissoid form. The granites are, at least in part, intrusive in crystalline schists and gneisses of uncertain origin, which are here and there graphitic and contain interbedded strata of crystalline limestone. There are some later dikes of diabasic character, which are comparatively fresh. Southwest of Llano are areas of serpentine and other basic rocks.
In many places the granites are cut by pegmatite dikes, ranging in width from a few inches to 60 feet, which show a much greater percentage of quartz than of feldspar and other constituents, and afford beautiful illustrations of the most acidic phase of pegmatites. In a 6-inch dike there may be but a few feldspar crystals from 1 inch to 3 inches long fringing the edges of the dike; in other dikes, or in other portions of the same dike, gradations from pure quartz to almost pure feldspar may be observed.
Baringer Hill is formed by such a dike on a huge scale. It is a small mound which, before mining was begun, rose perhaps 40 feet above a surrounding flat, was about 100 feet wide, and from 200 to 250 feet long. The longer axis runs east and west and is nearly at right angles to the course of the Colorado River at this point. The country tock is a coarse porphyritic granite with feldspar phenocrysts about 1 inch long. This granite seems to weather and erode rather easily, and the river has cut a flood plain perhaps one-fourth of a mile wide at this point, while the dike, owing to its greater hardness and freshness, has better withstood the erosion. The pegmatite, an unsymmetrical body with irregular walls, is intruded into the granite in what seems to be a pipe or short dike.
At the edges of the intrusion is a graphic granite of peculiar beauty and definite structure, being more like the text-book illustrations than the usual graphic granite found in the field. The altered band is from 1 foot to 5 or 6 feet thick, and apparently surrounds tha pegmatite. No segregation of the feldspar or quartz in particular parts of the dike can be noted, except that the feldspar may possibly be more inclined to occupy the sides of the intrusion. As far as shown it occupies most of the western and southern sides, and the quartz occupies the center and much of the eastern side.
One quartz mass is more than 40 feet across. The quartz has distinct white bands, from one-eighth to one-half inch wide, which seem to be due to a movement akin to flowage and are similar to those found in many pegmatitic masses in other portions of the country. The white banding is due to small liquid inclusions, many of them containing bubbles which either do not move from change of inclination of the fragment containing them, or do so but slowly. The cavities are minute, largely of irregular, angular shapes, suggesting at first glance particles of broken minerals, and occur in straight or broken lines that probably follow fine cracks which were later cemented. Groups of these cracks, with their inclusions, form the bands, which seem to lie approximately parallel to the walls of the dike or at such angles with them as might easily be formed by the flowage of the material into the space it occupied in the granite. The condition of the quartz seems to show that the pegmatite, after being forced into the granite, partly cooled and solidified and then made another small movement, or a series of slight movements, at which time the minute fractures were formed in the quartz and the magmatic fluids were forced into them, but as the mass was not yet totally solidified the cracks were effectually healed and the fluid was inclosed. Such movements may be supposed to have been consequent on the readjustment of the mass on cooling. Between the fracture bands the quartz is glassy and clear. At one place a vug was found large enough for a man to enter, lined with "smoky" quartz crystals reaching 1,000 pounds or more in weight. This would seem to indicate that the pegmatite had been intruded in a pasty or semifluid condition and that the vugs represent the spaces occupied by segregated water that was squeezed from the magma as the minerals took their final solidified form.
The feldspar is an intergrowth of microcline and albite, of a brownish flesh color, beautifully fresh, and occurs (1) in large masses reaching over 30 feet in diameter, and (2) as huge crystals, many of which, though they rarely show terminal planes, have one or more sharply defined edges, especially where partially surrounded by quartz. An edge 34 inches long was measured on one crystal thus embedded. A smaller crystal was seen which was about a foot long, weighed perhaps 20 pounds, and showed fine terminations and twinning planes.
A large amount of feldspar has been mined and thrown on the dump, and it is possible that in time the dump material may be utilized, either for its potassium content, as a fertilizer, or for pottery making.
Large crystals of fluorspar, measuring a foot along the edge, occur in the quartz, but this mineral does not form any considerable percentage of the mass. The fluorspar ranges from almost colorless to violet so dark that it is practically opaque. Where found alone in the quartz it was, so far as observed, of lighter color than where found with dark-colored minerals. Mr. Hidden informed the writer that it sometimes becomes luminous at the temperature of a living room.
Ilmenite occurs in radiating bunches of sheets or blades ranging From 1 inch to 10 or 11 inches in width and from one-sixteenth to one-fourth of an inch in thickness. In cross section the ilmenite looks like the ribs of a fan, with the outer ends from one-fourth to three-fourths of an inch apart. Similar aggregations take different angles, and numbers of such groups are found lying close together. With them occurs biotite mica in like bunches, the sheets of which are said to reach 3 feet in width by an inch in thickness. The mica is reported by Mr. Hidden to contain coesium and rubidium, and to be close to lepidomelane in constitution. Small flakes of lithia mica reaching half an inch in diameter are found, generally along cracks in the quartz. No muscovite was seen, but it is said to be found occasionally. Compared with the mass the total amount of mica is very small.
THE RARE-EARTH MINERALS.
The greatest interest in the dike centers in the accessory minerals, particularly in the occurrence of the rare-earth metal minerals, which, as stated, probably have never been found at any other place in such large masses and in such quantities as in this locality. So far the excavations are comparatively shallow, and such minerals as are found are more or less weathered. Many show their crystalline form, but owing to alteration the crystals are now imperfect.
Allanite, a variable silicate of calcium, iron, aluminum, and the cerium metals (cerium, praseodymium, neodymium, and lanthanum), and in smaller amount those of the yttrium group, occurs in large masses, one of which weighed 300 pounds and was embedded in purple fluorspar. It is a dense black mineral with a fine luster, and a hardness of about 6. Around the edges and along cracks it shows alteration to a brown substance having a hardness of about 5.5. The percentage of yttria ordinarily occurring in allanite is small and tsrely exceeds 2 1/2 per cent.
Cyrtolite is rather common in the dike in peculiarly fine, polysynthetic groupings with curved faces. It is brown on the surface, with a darker or nearly black interior, and is evidently a mixture of substances. It carries a considerable amount of zirconia and some yttria, and is supposed by Mr. Hidden to be an alteration product of zircon. If it is such a derivative, the original mineral was probably much more complicated than ordinary zircon. It makes a fair radiograph, which also gives evidence of its nonhomogeneity.
Fergusonite, a variable columbate of the yttrium group and other of the rare-earth metals, occurs in four varieties, so different as to be almost distinct minerals. The difference between them is due to oxidation and hydration. No anhydrous varieties are found. It is found in crystalline form surrounded by decomposition zones. Bunches of irregular crystals have been broken out, weighing over 65 pounds. It is generally a mixture of minerals, as may be easily seen on a smooth surface, from the different colors. The difference in composition is strikingly shown in a radiograph, the variations being marked by difference in radiation. According to the two analyses by Hidden and Mackintosh,2 the fergusonite obtained here carries from 31.36 to 42.33 per cent of yttria and accompanying rare earth metals, and 42.79 to 46.27 per cent of columbium dioxide. Tho two analyses give 1.54 per cent and 7.05 per cent of uranium oxides. These are probably very irregularly distributed through the material, as shown both by the mineral itself and especially by its radiographs, which are of striking beauty.
Gadolinite, a silicate of beryllium, iron, and yttrium, is the most important of the minerals found here. It contains about 42 per cent of the yttrium oxides, with a molecular weight of 260, and occurs in crystals and masses of irregular shape up to 200 pounds in weight. The outer portion of the mineral and that adjacent to the cracks is altered to dense brick-red material, but the mineral itself is of a fine, glassy black, with a smooth conchoidal fracture. Thin splinters are bottle-green in color. It has a specific gravity of a little over 4.2, and a hardness of 6.5 to 7. A specimen collected makes no impression on a photographic plate with fifty hours' exposure.
Polycrase, a columbate and titanate of yttrium, erbium, cerium, and uranium, occurs in grains, small masses, and plates, the last associated with ilmenite in such a manner as to suggest the probability of replacement. It normally contains between 20 and 30 per cent of yttrium oxide, but is in too small amount to be commercially important. It is very radioactive, and quickly affects a photographic plate.
Other rare-earth metal minerals found in the dike are yttrialite, rowlandite, nivenite, gummite of several varieties, thorogummite, mackintoshite, and tengerite. These minerals are apt to occur in any part of the dike, either in the quartz or the feldspar, but have so far been found mostly along the outer portions. A peculiarity of their occurrence is that they are found in bunches from which, if in quartz, radial cracks extend in every direction, and by following such cracks the minerals are found. An illustration of such an occurrence was published by William E. Hidden in 1905.3 The cause of these "stars," as they have been called by Mr. Hidden, is not clear, but the thought suggests itself that the rare-earth metal minerals may have crystallized first from the magma, and the solidifying quartz, being unable otherwise to accommodate itself to the incompressible nucleus, cracked in this manner.
Mr. Hidden stated that in mining ore of the largest pockets the faces and hands of himself and his assistant were affected as if by sunburn, and, as in sunburn, the covered flesh was not irritated. He suggested radioactivity as the cause, and inasmuch as the minerals under consideration are radioactive, the explanation seems plausible.4 The following was given by Mr. Hidden in a personal communication as a complete list of the minerals found in Baringer Hill:
Minerals found in Baringer Hill, Llano County, Tex.
SILICATES.
Albite; Microcline; occur as intergrowths making up the mass of the feldspar.
Allanite; a variable silicate of calcium, iron, the cerium metals, and less amounts of the yttrium group, in masses weighing up to 300 pounds, embedded in purple fluor spar.
Biotite; close to lepidomelane.
Cyrtolite; hydrated silicate of zirconium, yttrium, and cerium. Radioactive, abundant.
Gadolinite; a silicate of beryllium, iron, and yttrium in masses weighing up to 200 pounds.
Lithia mica; apparently a later deposition in cracks in quartz. Small flakes one-half inch or less across.
Orthoclase; not abundant.
Yttrialite; an anhydrous silicate of thoria, yttrium, and cerium earths. Contains about 30 per cent silica, 46 per cent yttria, 10 to 12 per cent thoria, and 5 to 6 per cent ceria. Does not occur in large quantity.
Rowlandite; practically a hydrated yttrium silicate. Contains 6 per cent fluorine.
COLUMBATES.
Fergusonite; four varieties, due to oxidation and hydration. Neither is anhydrous. Purest, 5.65 specefic gravity. So different as to be almost ditinct minerals. Crystals surrounded by decomposition zones.
Polycrase; columbate and titanate of yttrium, erbium, cerium, and uranium. Contains about 25 per cent of yttria.
OXIDES.
Hematite; specular, small quantity.
Magnetite; without metallic acids or rare earths.
Ilmenite; iron-titanium oxide in beautlful crystals, as well as plates up to 8 or 9 inches broad.
Rutile; titanium oxide, in prismatic and reticulated forms one-fourth inch thick:
Quartz; large masses and crystals of white quartz and "smoky" crystals up to 1,000 pounds in weight. Amethysts of gem quality reach 1 inch by one-half inch.
URANATES.
Mackintoshlte; 3 parts thorite to 1 part uraninite; contalus 13 per cent sllica and a small amount of yttria. Radioactive; several times more so than its alteratlon product.
Thorogummlte; formed from mackintoshite by additlon of H2O and alteration of UO2, to UO3.
Nivenite; a uranate of uranium, thorium, yttrium, and lead. Contains 10 per cent of lead. The most soluble uranate yet discovered; soluble in 5 per cent solution of SO3. Prints well and gives great detail. Occurs in cubes and masses. (See Dana's System of Mineralogy, p. 889, for two analyses.) Alters to gummite.
Gummite; several varieties.
PHOSPHATE.
Autunite; hydrous phosphate of uranium and calcium; secondary, not analyzed.
CARBONATES.
Tengerite; carbonate of yttrium and beryllium. Generally globular, but occurs also as crystals up to one-sixteenth inch in length singly and as little nests. May be a mixture of beryllium and yttrium carbonates.
Lanthanite; carbonate of lanthanum, containing also cerium, praseodymium, an, calcium. In incrustations on allanite.
SULPHIDES.
Chalcopyrite; iron-copper sulphide, massive, in small amount.
Pyrite; iron sulphide, cubic and octahedral.
Sphalerite; zinc sulphide; the purest fergusonite contains some zinc.
Molybdenite; molybdenum sulphide in scales 5 inches wide, which form masses weighing up to 10 1/2 pounds. Alters to powellite.
MOLYBDATE.
Powellite; calcium molybdate, in white crusts lining cavities where MbS has been. Sugary white radiating or plumose crystals, one-fourth to three-fourths inch long. Locally greenish.
It is interesting to note that among the numerous minerals in this dike no tourmaline, zircon, beryl, monazite, cassiterite, garnet, or tungsten minerals have been found. Cassiterite has been reported from the neighborhood, but its occurrence is extremely doubtful.
With the exception of the alteration products and probably of the lithia mica, which, as noted, occurs along cracks in the quartz, all the minerals are believed to be original constituents of the dike.
The possibility of finding dikes having a like variety of minerals it once suggests itself, and much prospecting has been done for them. A few specimens of the rare-earth metal minerals have been found at other places in the neighborhood, but only a few, and in small quantity. However, similar dikes occur, as already stated, and these have not all been thoroughly investigated. It is to be remembered that these minerals form but a small fraction of 1 per cent of the mass, and it might easily happen that comparatively large amounts could exist in a dike and not be exposed at the outcrop. They are minerals which are altered to softer products by exposure, and would thus be easily removed by erosion and weathering. The cracks surrounding nuclei of the minerals should be useful in prospecting.
ECONOMIC VALUE.
The economic interest in the rare-earth metal minerals centers in their incandescence on being heated, and owing to this property they have been much sought. Thoria, beryllia, yttria, and zirconia show it in the greatest degree. It was found, however, that thoria and beryllia, which form the bulk of the incandescent oxides used in gas mantles are too easily volatilized to be used in an electric glower, such as that of the Nernst lamp. Yttria and zirconia, however, will stand the necessary high temperature. Up to the discovery of this deposit it was practically impossible to get sufficient yttria-bearing minerals to manufacture the lamps, but fergusonite and gadolinite, with lesser amounts of cyrtolite, are found here in large enough quantity to meet the requirements. The zirconia is obtained from zircon brought from other localities.
In the manufacture of the glowers for the Nernst lamp, a paste consisting of 25 per cent of yttria and 75 per cent of zirconia is squirted into strips of the proper thickness, baked, and cut into the required lengths. When cold the mixture is nonconducting, but after being heated it becomes a conductor and gives a brilliant light.
The needs of the Nernst Lamp Company, which owns the deposit, require only the occasional working of the mine. After enough yttria minerals are obtained to supply its wants for a few months ahead the mine is closed. But a few hundred pounds per year are extracted.
Bibliography