OR/13/015 Appendix 6 - Petrographic descriptions of Falkirk building stones

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Gillespie, M R, Everett, P A, Albornoz-Parra, L J, and Tracey, E A. 2013. A survey of building stone and roofing slate in Falkirk town centre. Nottingham, UK, British geological Survey. (OR/13/015).

This appendix contains petrographic descriptions for nineteen samples of stone collected from Falkirk; see section 5.3 and Table 7 for sample details. Each description is presented on a Petrographic Description Form designed for sandstone. Two photographs of the stone as it appears in thin section (viewed using an optical microscope) accompany the descriptions of the twelve samples for which a thin section was prepared. A page of notes at the end of this appendix describes each property recorded on the forms.

BGS sample number: ED11047
PRIMARY_KE: 963250
Building stone: Buff sandstone 4
Sample dimensions: 130 x 40 x 35 mm
Building address: 30 Newmarket Street

HAND SPECIMEN OBSERVATIONS

Stone type1 (general classification): sandstone
Stone colour2 – fresh stone: light buff
Stone colour2 – weathered stone: orangeish buff
Stone colour2 – exterior surface: greyish buff
Stone cohesion3 – fresh stone: strongly cohesive
Stone cohesion3 – weathered stone: strongly cohesive
Primary sedimentary structure4: uniform (some orientated grains)
Distinctive features5: speckled
Reaction to 10% HCl6: moderate
Water absorption (permeability)7: high

THIN SECTION OBSERVATIONS
Stone constituents:8 Granular (detrital) constituents Intergranular constituents
Quartz 57% Silica (overgrowth) 2%
Feldspar 3% Feldspar (overgrowth) <1%
Rock fragments 2% Carbonate 3%
Mica 1% Iron/manganese oxide 1%
Opaque material <1% Clay 10%
Other <<1% Hydrocarbon 0%
Intragranular pores 2% Intergranular pores 18%
Stone type1
(detailed classification):
subfeldspathic-arenite
Grain-size:9 fine-sand-grade to medium-sand-grade
Grain sorting:10 moderately well sorted
Grain roundness:11 sub-angular to sub-rounded
Cement distribution:12 silica cement continuous; carbonate cement isolated
Supergene changes:13 moderate dissolution of feldspar


BGS sample number: ED11048-1
PRIMARY_KE: 963306
Building stone: Buff sandstone 1
Sample dimensions: 110 x 80 x 30 mm
Building address: Burgh Buildings

HAND SPECIMEN OBSERVATIONS

Stone type1 (general classification): sandstone
Stone colour2 – fresh stone: buff
Stone colour2 – weathered stone: buff
Stone colour2 – exterior surface: buff to greenish buff
Stone cohesion3 – fresh stone: strongly cohesive
Stone cohesion3 – weathered stone: moderately cohesive
Primary sedimentary structure4: parallel lamination
Distinctive features5: faintly speckled; carbonaceous matter
Reaction to 10% HCl6: none
Water absorption (permeability)7: high

THIN SECTION OBSERVATIONS
Stone constituents:8 Granular (detrital) constituents Intergranular constituents
Quartz 45% Silica (overgrowth) 4%
Feldspar 3% Feldspar (overgrowth) 0%
Rock fragments 1% Carbonate 7%
Mica <1% Iron/manganese oxide 7%
Opaque material <1% Clay 12%
Other <1% Hydrocarbon 0%
Intragranular pores 3% Intergranular pores 17%
Stone type1
(detailed classification):
subfeldspathic-arenite
Grain-size:9 fine-sand-grade to medium-sand-grade
Grain sorting:10 moderately well sorted
Grain roundness:11 sub-angular to well rounded
Cement distribution:12 silica cement continuous; carbonate cement isolated
Supergene changes:13 moderate dissolution of feldspar



Comments

  1. The sample is a chip from rock faced walling at low level on the front elevation.
  2. The exterior surface is weathered to a depth of c. 2-3 mm.
  3. The laminae are concentrations of small sand grains, iron oxide minerals and mica flakes.
  4. Speckles are orange-brown, probably an altered iron-rich mineral.
  5. Detrital grains are commonly well rounded, but enclosed by thick overgrowths of silica cement.
  6. Rounded zircon grains are present in accessory proportion.
  7. Most of the pores are partially infilled with clay minerals.
  8. The carbonate mineral is iron-rich (ferroan dolomite, ankerite or siderite).
Figure A6.2    Two images of sample ED11048-1 as it appears in thin section. White grains are mainly quartz. Black areas are mainly iron oxide minerals. Greyish, mottled patches are carbonate mineral. Mottled bluish patches are clay mineral. Pore spaces appear clear blue. The field of view is 3.3 mm wide.