Limestone is a "chemical" sedimentary rock that is composed mostly of calcite (CaCO3) that precipitates from seawater. Limestone, the most common chemical sedimentary rock in Texas, is commonly white to gray. Because the dominant mineral is calcite, limestone reacts with a strong effervescence when tested using a dilute acid such as hydrochloric acid or vinegar. Groundwater is slightly acidic and, hence, slowly dissolves limestone. This process has produced the caves of Texas.
Variations of limestone include marl, a muddy limestone, and chalk, a limestone composed of microscopic calcareous algae and tiny shells of marine organisms. Limestone is common in the Hill Country and the Trans-Pecos of Texas. Limestone in Texas is quarried for cement, aggregate, and building stone. High-calcium-content limestone is used in water purification and sewage treatment.
The limestone sample in the Texas Rock Kit was collected in Travis County and is Cretaceous in age.
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