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

Comments

  1. The sample consists of a chip from a window dressing.
  2. The exterior surface is weathered to a depth of less than 1 mm.
  3. The speckles are mainly black and evenly scattered. Some are pale orangeish brown.
  4. The exterior surface is weathered to a maximum depth of 4 mm.
  5. Tourmaline and rounded grains of zircon appear as accessory minerals.
  6. The stone contains rare, scattered grains of coarse-sand grade.
  7. Iron oxide rims on carbonate crystals suggest the carbonate mineral is ferroan (ferroan dolomite, ankerite or siderite).
Figure A6.1    Two images of sample ED11047, as it appears in thin section. White grains are mainly quartz. Black areas are mainly iron oxide minerals. Dark mottled patches are carbonate mineral. Pore spaces appear blue. The field of view is 3.3 mm wide.


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.


BGS sample number: ED11048-2
PRIMARY_KE: 963306
Building stone: Buff sandstone 1
Sample dimensions: 90 x 65 x 25 mm
Building address: Burgh Buildings

HAND SPECIMEN OBSERVATIONS

Stone type1 (general classification): sandstone
Stone colour2 – fresh stone: very light buff
Stone colour2 – weathered stone: orangeish buff
Stone colour2 – exterior surface: green
Stone cohesion3 – fresh stone: moderately cohesive
Stone cohesion3 – weathered stone: moderately cohesive
Primary sedimentary structure4: uniform
Distinctive features5: faintly speckled
Reaction to 10% HCl6: strong
Water absorption (permeability)7: very high

THIN SECTION OBSERVATIONS
A thin section was not prepared.

Comments

  1. The sample consists of a chip from rock-faced walling on the side elevation, at ground level.
  2. The exterior surface is weathered to a depth of c.3 mm and is strong orangeish buff.
  3. The faint speckles are of weak orange-brown colour due to iron oxide remobilization.
  4. Strong reaction to 10% HCl suggests that calcite is present as a carbonate mineral cement.

There are no thin section images of this sample.


BGS sample number: ED11049-1
PRIMARY_KE: 961318
Building stone: Buff sandstone 1
Sample dimensions: 80 x 55 x 30 mm
Building address: The Steeple

HAND SPECIMEN OBSERVATIONS

Stone type1 (general classification): sandstone
Stone colour2 – fresh stone: light buff to buff
Stone colour2 – weathered stone: light buff to buff
Stone colour2 – exterior surface: very light buff
Stone cohesion3 – fresh stone: strongly cohesive
Stone cohesion3 – weathered stone: very friable
Primary sedimentary structure4: parallel lamination
Distinctive features5: speckled
Reaction to 10% HCl6: moderate
Water absorption (permeability)7: moderate to high

THIN SECTION OBSERVATIONS
Stone constituents:8 Granular (detrital) constituents Intergranular constituents
Quartz 62% Silica (overgrowth) 3%
Feldspar 2% Feldspar (overgrowth) 0%
Rock fragments 0% Carbonate 3%
Mica 1% Iron/manganese oxide 6%
Opaque material 0% Clay 10%
Other 0% Hydrocarbon 0%
Intragranular pores 1% Intergranular pores 12%
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 discontinuous; carbonate cement isolated
Supergene changes:13 none

Comments

  1. The sample consists of a small chip from the internal walling of The Steeple, at third floor level.
  2. Faint laminae are formed by accumulations of iron oxide minerals and mica flakes.
  3. Orange-brown speckles up to 1.5 mm across are composed mainly of iron oxide.
  4. The carbonate mineral is iron-bearing (ferroan dolomite, ankerite or siderite)
  5. A substantial proportion of the space between detrital grains is infilled by clay.
  6. The stone shows parallel lamination, with hints of possible cross-lamination or ‘wispy bedding’.
Figure A6.3    Two images of sample ED11049-1, as it appears in thin section. White grains are mainly quartz. Black areas are mainly iron oxide minerals. Pore spaces appear blue. The field of view is 3.3 mm wide.


BGS sample number: ED11049-2
PRIMARY_KE: 961318
Building stone: Buff sandstone 1
Sample dimensions: 190 x 35 x 25 mm
Building address: The Steeple

HAND SPECIMEN OBSERVATIONS

Stone type1 (general classification): sandstone
Stone colour2 – fresh stone: not applicable
Stone colour2 – weathered stone: light buff to buff
Stone colour2 – exterior surface: very light buff
Stone cohesion3 – fresh stone: not applicable
Stone cohesion3 – weathered stone: very friable
Primary sedimentary structure4: parallel lamination
Distinctive features5: speckled
Reaction to 10% HCl6: no
Water absorption (permeability)7: very high

THIN SECTION OBSERVATIONS
A thin section was not prepared.


Comments

  1. The sample consists of a small chip from the internal walling of The Steeple, at third floor level.
  2. The laminae consist of concentrations of iron oxide minerals and mica flakes.
  3. The high permeability and friable character reflect the weathered condition of the stone. A former mineral cement composed of carbonate minerals may have dissolved due to weathering.
  4. The exterior surface has a lime mortar wash and is therefore paler than the weathered stone.
  5. Orange-brown speckles up to 1.5 mm across are probably weathered spots of iron oxide.

There are no thin section images of this sample.