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Id 68
Identification Type Mineral
Name Feldspar
Distinguishing Features good-cleavage
low-birefringence
Description 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.
Tags feldspar
silicate
twinned
transparent
translucent
External Resources Feldspar on MinDat
Imposters Quartz
Quartz and feldspar have similar low-order birefringence colors (white to gray) and regularly co-occur. They can be distinguished from each other by cleavage: feldspar has good cleavage, and quartz has no cleavage. That is, if the grain has a straight edge, it's not quartz.
Images
Unique Identification: Feldspar
Site name: Reid Lake, ON
Light type: Cross-Polarized Light
Magnification: not specified
Submitted by: Andy Breckenridge
Notes: microcline, detrital, glaciolacustrine
Image Tags: none
Unique Identification: Feldspar
Site name: Reid Lake, ON
Light type: Plane-Polarized Light
Magnification: not specified
Submitted by: Andy Breckenridge
Notes: microcline, detrital, glaciolacustrine
Image Tags: none
Unique Identification: Feldspar
Site name: Reid Lake, ON
Light type: Cross-Polarized Light
Magnification: not specified
Submitted by: Andy Breckenridge
Notes: microcline, detrital, glaciolacustrine
Image Tags: none
Unique Identification: Feldspar
Site name: Bear
Light type: Plane-Polarized Light
Magnification: 400
Submitted by: Joe Smoot
Notes: low birefringence, twinning lamellae
Image Tags: none
Unique Identification: Feldspar
Site name: Bear
Light type: Cross-Polarized Light
Magnification: 400
Submitted by: Joe Smoot
Notes: high relief needles consistent size with minor whole diatoms
Image Tags: none
Unique Identification: Feldspar
Site name: Tyrrhenian Sea, Mediterranean Sea
Light type: Plane-Polarized Light
Magnification: not specified
Submitted by: Guy Rothwell
Notes: Euhedral feldspar crystal showing oscillatory zoning, crystal surrounded by a thin envelope of glass, showing pyroclastic origin. Volcanic glass also in the field., oscillatory-twinning pyroclastic volcanic-glass
Image Tags: none
Unique Identification: Feldspar
Site name: Reid Lake, ON
Light type: Plane-Polarized Light
Magnification: not specified
Submitted by: Andy Breckenridge
Notes: microcline, detrital, glaciolacustrine
Image Tags: none