8.6.3: Metamorphosed Limestones and Dolostones (Marbles)

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8.52.jpg — Figure 8.52: Marble from near Tate, Georgia, 7.6 cm across

Geologists generally call metamorphosed carbonate rocks marbles, although this term is used in different ways by building contractors and others. The metamorphism of limestone or dolostone composed only of carbonate minerals produces few mineralogical changes. A general increase in grain size may take place – similar to what happens when sandstone turns into quartzite, but no diagnostic minerals can form because of the limited chemical composition and the high stabilities of both calcite and dolomite. The photo in Figure 8.52 shows a marble that contains only course crystals of white calcite. Figure 8.36, earlier in this chapter, showed a marble consisting only of blue calcite.

However, most limestones contain some quartz and other minerals besides carbonates. In these rocks, a series of interesting Ca-silicates, Ca-Mg-silicates, and Ca-Al-silicates form as metamorphism progresses. The table below lists the most important of these minerals, roughly in order of their appearance in response to increasing metamorphic grade.

Minerals Common in Metacarbonates
low grade	→	high grade
calcite CaCO₃	talc Mg₃Si₄O₁₀(OH)₂	grossular (garnet) Ca₃Al₂Si₃O₁₂
dolomite CaMg(CO₃)₂	tremolite Ca₂Mg₅Si₈O₂₂(OH)₂	periclase MgO
quartz SiO₂	forsterite (Mg,Fe)₂SiO₄	wollastonite CaSiO₃
phlogopite (Mg-rich biotite) K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂	diopside CaMgSi₂O₆	monticellite CaMgSiO₄

If quartz is present, the metamorphic reactions in marbles are often decarbonation reactions that involve the breakdown of carbonates to release CO₂. If a pluton intrudes a limestone or dolostone, contact metamorphism may cause CO₂ to flow out of the carbonate and combine with H₂O that comes from the pluton. The CO₂-H₂O fluid can have profound effects on the carbonate nearby, and fluid composition controls the formation of many minerals. Fluids may also cause significant metasomatism and a significant change in rock chemistry.

8.53.png — Figure 8.53: Phlogopite in marble from Orange County, New York

8.54.png — Figure 8.54: Tremolite and graphite in marble from Franklin, New Jersey

8.55.png — Figure 8.55: Forsterite marble

8.56.png — Figure 8.56: Diopside marble from the Adirondack Mountains, New York

Phlogopite is typically one of the first minerals to form during carbonate metamorphism. The first photo in the block above (Figure 8.53) shows large, somewhat hexagonal, flakes of phlogopite with calcite behind. The second photo (Figure 8.54) shows gray blades of tremolite in a marble that also contains small (hard to see) specs of graphite. The bottom left photo (Figure 8.55) show a marble that contains green forsterite (olivine). The last photo (Figure 8.56) shows a marble that contains green diopside. These four photos are in order of increasing metamorphic grade. The diopside marble is the highest grade of the four.