Composition: NaAlSi₃O₈, 50% to 30%; CaAl₂Si₂O₈, 50% to 70%. Crystal system: triclinic. Hardness: 6.0 to 6.5. Specific gravity: about 2.60. Luster: vitreous to sometimes pearly. Color: straw yellow, white, greenish, gray, reddish, bluish, and green. Sometimes shows a play of colors on particular cleavage surfaces. Streak: uncolored. Cleavage: three directions. Fracture: uneven to conchoidal. Tenacity: brittle. Diaphaneity: transparent to translucent. Refractive index: about 1.56. Dispersion: low.
Very fine facet-quality labradorite has been found about 20 miles south of Alpine, Brewster County. The labradorite occurs loose in the soil as slightly weathered or frosted cleavage fragments, commonly showing one or more crystal faces ([Pl. II], B). The pale-yellow or straw-yellow color of these fragments, as well as their lack of internal imperfections, makes these stones excellent gem material. Individual pieces that exceed three-fourths inch in their longest dimensions are rare. Cut stones of more than 5 or 6 carats from this locality are scarce. The source of this material is uncertain, but it is probably weathering out of an underlying igneous rock.
Microcline
Composition: KAlSi₃O₈. Crystal system: triclinic. Hardness: 6.0 to 6.5. Specific gravity: 2.54 to 2.57. Luster: vitreous to pearly. Color: white, pale yellow, red, blue green, bluish. Streak: white. Cleavage: four directions, usually three of these distinct. Fracture: uneven. Tenacity: brittle Diaphaneity: transparent to translucent. Refractive index: about 1.52 to 1.53.
Very fine crystals of blue microcline have been found east of Packsaddle Mountain and near Kingsland in Llano County. Crystals exceeding 1 foot in length have been found, although most are only a few inches long. The color of the microcline is mostly pale blue, but some crystals are darker. Microcline crystals associated with milky or vein quartz, smoky quartz, some biotite, and rarely cassiterite occur in pegmatite dikes which vary in size from a few inches to several feet in thickness. The color of this microcline is pale in comparison to microcline from some other localities in the United States, but the Texas blue microcline does yield pleasing cabochons. Perfect crystals of this material are prized by collectors. Blue or greenish microcline is often called amazonite or amazon stone.
Bluish microcline associated with quartz and topaz has also been reported near Katemcy, Mason County.
Red microcline is common in several central Texas counties and is a primary constituent of many of the igneous rocks in those counties. Large crystals of perthitic red microcline occur in pegmatite dikes of Mason, Llano, Burnet, and Gillespie counties. Any feldspar quarry or other pegmatite mining operation in any of these counties is likely to contain large red microcline crystals and fragments. Unfortunately, the good crystals that may have been present are often shattered by blasting during quarrying operations.
Feldspar quarries in northeastern Gillespie County have yielded some good red cabochon material as well as good crystals. Here the microcline occurs with milky and smoky vein quartz, smoky quartz crystals, clear quartz crystals, greenish muscovite, and biotite. Many of the older quarries in Gillespie County have not been active for some time, and the dumps and quarry walls have been diligently searched by collectors.
Fig. 16. Crystal faces on microcline specimen shown in [Plate III], A.