_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.
[Illustration: Fig. 16. Crystal faces on microcline specimen shown in
Plate III, A.]
Many of the pegmatite dikes near Lake Buchanan in Llano and Burnet
counties have produced some good red microcline specimens and cutting
material (Pl. III, A, and fig. 16). Many of these crystals are more
pinkish than those in Gillespie County, but this is commonly due to the
fact that the crystal faces of the Lake Buchanan area crystals are
somewhat more weathered than the fresh Gillespie County crystals.
Numerous other local areas in the counties mentioned, as well as some
localities in Hudspeth and Culberson counties, have also produced small
amounts of red and pink microcline of gem quality.