126 Subsurface Geologic Methods 



present in quantities that exceed one percent of the rock. If the distin- 

 guishing properties of these minerals are learned, the mineral composition 

 of most subsurface samples can be determined by megascopic or binocular 

 examination. 



Ordinarily the grains in coarse clastic rocks (sands and siltstones) 

 will be composed of^one or more of the following: quartz, detrital chert, 

 feldspar, mica (usually muscovite) , calcite, dolomite, glauconite, and 

 collophane. Fragments of sedimentary, igneous, and metamorphic rocks 

 also are important constituents of some clastic sediments. The crystalline 

 sedimentary rocks may contain calcite, dolomite, anhydrite, gypsum, 

 halite, and chert. The chief cements in clastic and crystalline sedimentary 

 rocks are quartz, chert, calcite, and dolomite. Less frequently the cements 

 may be pyrite, hematite, limonite, and opal. The dominant constituents 

 of some mudstones and shales are the kaolin, illite, and montmorillonite 

 (bentonite) groups of clay minerals and other fine-grained, platy minerals 

 like sericite and chlorite. 



Probably the best method for a beginner to learn these minerals is 

 to obtain and study a series of samples in which they are present, prefer- 

 ably a series that has been worked previously by a good mineral man. The 

 writer knows of no table that is designed primarily for identification of 

 the common minerals as they are observed in cuttings. Observations ordi- 

 narily may be made on some or all of the following: 



Color — Not very diagnostic because some minerals may occur in 

 several colors, and color may be given by a small amount of cement or 

 matrix. The green color of glauconite, however, is distinctive. 



Hardness — Not exceptionally useful because of the small size of 

 the cuttings. Hardness may be determined with a probe under the binocu- 

 lar microscope. 



Grain size — Very useful. 



Grain shape — Useful. 



Reaction in acid — Very useful, particularly under the binocular 

 microscope. Eff"ervescence in cold dilute hydrochloric acid separates 

 calcite from other carbonates. Effervescence of powder in cold dilute 

 hydrochloric acid or of fragments in hot dilute hydrochloric acid separ- 

 ates other carbonates. Gypsum and anhydrite are slowly soluble in hot 

 dilute hydrochloric acid but do not "effervesce. Etching for about 30 

 seconds in cold dilute hydrochloric acid often brings out structures and 

 textures. Etching gives dolomite a flat-gray appearance that is distinctive 

 and often makes the rhombic structure visible. 



Sparkle — Usually indicates (a) quartz cement, (6) dolomite, or (c) 

 mica. Mica reflections will be only from the bedding plane. 



Drilling appearance — Usually more important in indicating perme- 

 ability or in differentiating beds or formations than in identifying indi- 

 vidual minerals. 



Shaly parting — Useful. 



