BETA

Weymouth

Submitted by Ed FitzGerald on Tue, 2014-11-25 15:11
Trade Name Alias: 
Stone Type (Commercial): 
Description: 

Light-gray to pinkish-gray, fine- to medium-grained granite.

Availability: 
Available
Geology
Stone Type (Lithologic): 
Chronostratigraphy: 
Rock Unit: 
Grain Shape: 
Angular
Grain Size: 
Coarse
Petrographic Description: 

Specimen: 2014-079-1
This sample is a plutonic rock dominated by anhedral feldspar (~60 percent) and a lesser amount of quartz (~40 percent). The crystalline structure is coarse. Maximum crystal dimension is 2.4 mm (feldspar) and minimum is <.1 mm (quartz). Modal grain size is around 1.2 to .6 mm. Quartz and feldspars have similar grain size distributions. Many quartz grains present wave extinction, but there is only minimal evidence of stress induced nucleation. Feldspar twinning patterns include tartan (microcline), Carlsbad, and albite patterns. Grain boundaries range from smooth curving to substantially consertal sutured, there are a few instances of straight edged equant grains. Complex compositional textures appear to be lacking. Accessory minerals include biotite, pyroxenes, opaques, and muscovite. Together these minerals constitute 5 to 10 percent of slide area. Feldspars display significant degrees of weathering in the form of sericitization, epidotization, as well as the accumulation of clay minerals, imparting a dusty appearance to grains. Accessory minerals are also noticeably weathered with the formation of chlorite and other alteration products. There are no macro scale signs of weathering, such as cracks.
MP 9/4/2014

Specimen: 2014-079-2
This sample is a plutonic rock dominated by anhedral feldspar (~60 percent) and a lesser amount of quartz (~40 percent). The crystalline structure is coarse. Maximum crystal dimension is 3.6 mm (feldspar phenocryst) and minimum is <.1 mm (quartz). Modal grain size is around .2 to .5 mm. Quartz has a slightly smaller average grain size. Many quartz grains present wave extinction, but there is only minimal evidence of stress induced nucleation. Feldspar twinning patterns include tartan (microcline), Carlsbad, and albite patterns. Grain boundaries range from smooth curving to substantially consertal sutured, there are a few instances of straight edged equant grains. Complex compositional textures are rare with only a few instances of micrographic and zoned grains. Accessory minerals include muscovite, biotite, opaques, and highly altered pryroxenes/amphiboles. Together these minerals constitute less than 5 percent of slide area. Feldspars display significant degrees of weathering in the form of sericitization, epidotization, as well as the accumulation of clay minerals, imparting a dusty appearance to grains. Accessory minerals are also noticeably weathered with the formation of chlorite, epidote and other alteration products. Chlorite presents some anomalous violet birefringence. There are no macro scale signs of weathering, such as cracks.
MP 9/4/2014

Specimen: 2014-079-3
This sample is a plutonic rock dominated by anhedral feldspar (~60 percent) and a lesser amount of quartz (~40 percent). The crystalline structure is coarse. Maximum crystal dimension is 3.2 mm (feldspar phenocryst) and minimum is <.1 mm (quartz). Modal grain size is around .15 to .4 mm. Quartz has a slightly smaller average grain size. Many quartz grains present wave extinction, but there is only minimal evidence of stress induced nucleation. Feldspar twinning patterns include tartan (microcline), Carlsbad and albite patterns. Many feldspar grains have a near lath like habit. Grain boundaries range from smooth curving to substantially consertal sutured. Complex compositional textures are rare with only a few instances of micrographic/graphic and zoned grains. Accessory minerals include muscovite, biotite, opaques, and highly altered pryroxenes/amphiboles. Together these minerals constitute 5 to 10 percent of slide area. Feldspars display significant degrees of weathering in the form of sericitization, epidotization, as well as the accumulation of clay minerals, imparting a dusty appearance to grains. Accessory minerals are also noticeably weathered with the formation of chlorite, epidote and other alteration products. Chlorite presents some anomalous blue and violet birefringence. There are no macro scale signs of weathering, such as cracks.
MP 9/4/2014

Specimen: 2014-079-4
This sample is a plutonic rock dominated by anhedral feldspar and a lesser amount of quartz. The overall texture is aplitic, but there are several notably larger phenocryst of feldspar. Maximum crystal dimension is 6 mm (feldspar phenocryst) and minimum is <.1 mm (quartz). Modal grain size (excluding phenocryst) is around .2 to .6 mm. Many quartz grains present wave extinction, but there is only minimal evidence of stress induced nucleation. Feldspar twinning patterns include tartan (microcline) and polysynthetic. Grain boundary morphologies include straight-equant (phenocrysts) smooth scurving, and substantially consertal sutured. Granophyric intergrowth of feldspar and quartz are very common Zoning in feldspars is also prevalent, especially among larger phenocrysts. Accessory minerals constitute 5 to 10 percent of slide area and include highly colored amphiboles, pyroxenes, muscovite, biotite, and opaques. Feldspars display significant degrees of weathering in the form of clay mineral development and lesser amounts of sericitization and epidotization. Many feldspars are now near opaque, owing to the accumulation of clay minerals. Accessory minerals are also noticeably weathered with the formation of chlorite, epidote and other alteration products. There are no macro scale signs of weathering, such as cracks. Crystal size, and intergrowth textures suggest this sample may have formed under hypabyssal conditions.
MP 9/5/2014

Images

Thin Sections

Click thumbnail for larger image.
Specimen ID Non-polarized Cross-polarized
2014-079-1 Specimen shown in non-polarized light.download image Specimen shown in cross-polarized light.download image
2014-079-2 Specimen shown in non-polarized light.download image Specimen shown in cross-polarized light.download image
2014-079-3 Specimen shown in non-polarized light.download image Specimen shown in cross-polarized light.download image
2014-079-4 No image available.

Quarry

NOTE: Quarry locations are approximate.

Plymouth Quarry
Whiting St. Rte 53
Hingham , Massachusetts
United States
42° 11' 30.5844" N, 70° 54' 57.816" W
Massachusetts US

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