Earth Record
Mineralogy and formation
Llanite exists in one county on Earth. Llano County, Texas. Nowhere else. A Precambrian rhyolite porphyry, approximately 1.1 billion years old, featuring blue quartz crystals and pink feldspar phenocrysts in a fine-grained greenish matrix.
The blue in the quartz comes from inclusions of ilmenite or similar minerals. The magma cooled slowly underground, allowing the large crystals to form before the surrounding matrix solidified. Texas designated llanite as an official state rock type. The scarcity is real, single locality, single formation, single county.
Variable (Trigonal quartz + Monoclinic feldspar in igneous matrix) structure
Chemical FormulaComplex (SiO2 quartz + KAlSi3O8 feldspar in rhyolite matrix)Crystal SystemVariable (Trigonal quartz + Monoclinic feldspar in igneous matrix)Mohs Hardness6Specific Gravity2.60-2.70LusterVitreous to dullColorBlue-BrownIMA StatusrockType LocalityLlanite rhyolite occurrence, Llano County, Texas, USAIMA Numberpre-IMA USA (Llano CountyTexas)
Telling it apart
Llanite is a porphyritic rhyolite from the Llano region of Texas containing blue quartz phenocrysts and pink feldspar crystals in a dark matrix, and sellers sometimes present it as a rare crystal species. It is a rock, not a mineral. The blue quartz crystals within it are standard quartz whose blue color comes from microscopic mineral inclusions scattering light. The pink crystals are feldspar.
The dark matrix is the fine grained volcanic groundmass. No single hardness or specific gravity applies to the whole rock because it is a multi mineral aggregate. If the specimen shows blue round spots and pink spots in a dark gray to brown matrix and comes from Texas, llanite is a reasonable identification. Calling it a crystal species or a rare mineral inflates what is honestly an interesting locality rock.
Spotting the real thing
Llanite: a rhyolite with blue quartz and orange feldspar phenocrysts. Mohs 6-7 (rock hardness). Specific gravity 2.
60-2. 70. The blue quartz eyes in the matrix are distinctive.
Named after Llano County, Texas. Not commonly faked due to limited commercial value. If offered from a non-Texas locality, verify.
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