2.2 Examine the Rocks

The variety of minerals that can be produced by the metamorphism of shales makes these rocks good indicators of changes in metamorphic conditions across a metamorphic belt. Therefore, learning to recognize common metamorphosed shale minerals in the field and under the petrographic microscope is an important task for new petrologists. You can see galleries of the Dutchess County samples in outcrop, in hand sample, and in thin section by clicking the appropriate button:

   Hand Sample Gallery    Thin Section Gallery

Look at the Dutchess County metamorphosed shale samples and identify the minerals present. Most of the minerals you will find are in the following list (in alphabetical order). Some of the distinctive properties of the minerals are given, as well as links to additional information.

• biotite (Bt): black mica in hand sample, pale to dark brown in thin section, low second order retardation
• chlorite (Chl): pale green mica-like mineral in hand sample, colorless to pale green in thin section, low to anomalous first order retardation
• chloritoid (Ctd): black rod or plates in hand sample, blue-green in thin section, low first order retardation, polysynthetic twinning
• garnet (Grt): equant, reddish crystals in hand sample, pink in thin section, isometric
• ilmenite (Ilm): the elongate opaque grains in these rocks are mostly ilmenite
• kyanite (Ky): white to pale blue prisms in hand sample, colorless with 2 cleavages in thin section first order retardation
• muscovite (Ms): silvery mica in hand sample, colorless in thin section, second order retardation
• plagioclase (Pl): white with cleavage in hand sample, colorless in thin section, first order retardation and may have polysynthetic twinning
• quartz (Qz): grey in hand sample, colorless in thin section, first order retardation with no cleavage
• sillimanite (Sil): white prismatic or fibrous in hand sample, colorless in thin section, high first order retardation
• staurolite (Sil): brown to black prisms in hand sample, yellow in thin section, first order retardation

To get more information about these and other minerals click this button:

Mineral Names and Info



When you have examined the samples and are ready to name the mineral assemblage for each rock, click here:

Name the Assemblages