Id Identification Type Name Description Distinguishing Features Tags
104 Mineral Fecal pellet
68 Mineral Feldspar Transparent colorless to cloudy. Nonpleochroic. Low order birefringence. Tabular to prismatic. Relief varies by composition.

"Feldspar" includes several minerals or groups of minerals with solid solutions. According to Rothwell (1989, p.77) the distinguishing features of feldspars are "Subhedral prismatic to tabular form, turbidity due to incipient alteration, distinctive twinning (seen in cross-polars) ... Low birefringence."

Alkali feldspars tend to display simple Carlsbad twinning, though Carlsbad twinning can be found in all feldspars (Rothwell, 1989, p.74). Grid (cross-hatched polysynthetic) twinning in two directions is a diagnostic characteristic of microcline, and lamellar (multiple) twinning occurs mainly in plagioclase (Rothwell, 1989, p.74). The width of lamellae in plagioclase relates to composition, with more narrow lamellae in oligoclase and wider lamellae in albite and anorthite (Rothwell, 1989, p.75).

More quantitative distinction between plagioclase feldspar minerals of individual mineral grains can be achieved using the Michel-Levy method, which is based on the maximum extinction angles of twin lamellae (Rothwell, 1989, p.71-73). Refer to Rothwell (1989) p.71-73 for a step-by-step description of the Michel-Levy method.

Some identifiable feldspars are listed separately in TMI: albite, microcline, and orthoclase.
124 Contaminant Fiber contaminant
145 Contaminant Floral Foam contaminant

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