Slate is a fine-grained, foliated metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression. This article will examine whether slate tends to be more grey or blue in color.
The Composition and Structure of Slate
Slate is composed mainly of clay minerals or mica. Slate is characterized by an excellent cleavage property and a high degree of fissility along which the stone splits into thin sheets. This characteristic is called slaty cleavage. The cleavage is caused by strong compression and alignment of fine-grained clay flakes through the direction of compression.
A distinguishing characteristic of slate in comparison to shale is that slate can be split into thin, smooth slices whereas shale tends to split along irregular flat surfaces. The cleavage planes of slate are usually extremely straight and consistent, giving it an almost machine-made look. Slate cleavage can be so perfectly straight that fine pieces can be used as a writing surface called a slate board, for which slate was named.
The Color of Slate
The color of slate can range widely, but is most commonly grey, blue-grey, purple, brown or green. The presence of color in slate is due to the mineral content of the original sediments or the introduction of new minerals during metamorphism.
| Slate Color | Cause of Color |
|---|---|
| Grey, blue-grey | Presence of carbonaceous matter, hematite or mica |
| Purple | Hematite |
| Green | Chlorite, epidote |
| Red | Hematite |
| Brown | Goethite, birnessite |
Grey and blue-grey are the most common slate colors because these hues naturally occur from the mineral content of the original shale sediments. The grey color is caused by the presence of carbonaceous matter, hematite or mica minerals. Adding to the blue-grey hue is the way the foliation of the metamorphic slate acts to preferentially reflect certain wavelengths of light.
Factors Influencing Slate Color
There are several factors that influence the final color that slate takes on during metamorphism:
– Original composition – The original mineral makeup of the shale is a major determinant of the final slate color. Shales rich in carbonaceous matter will metamorphose into grey or blue-grey slates. Shales with high iron content typically form red or purple slates.
– Degree of metamorphism – As metamorphic grade increases, new minerals may form in the slate or existing minerals can change, modifying the color. For example, lower grade slates may be grey while higher grade slates metamorphose into blue, purple or green colors from growth of new minerals.
– Oxidation state – The presence of oxygen in the slate during metamorphism affects the oxidation state of iron, which changes the resulting color. Reduced iron results in grey or blue colors while oxidized iron lends a rusty red or purple color.
– Water interaction – Exposure of slate to water can lead to dissolution, growth or alteration of color-bearing minerals. Long-term submersion in water often imparts a blue-grey color to slate.
Testing Whether Slate is More Grey or Blue
To test whether slate technically displays more grey or blue tones, there are a few options:
– Visual inspection – Visually examining a large number of slate samples and subjectively judging which color dominates could provide a good sense for whether grey or blue is more prevalent. However, visual perception of color can be subjective.
– Colorimeter measurement – Using a scientific instrument called a colorimeter, the spectral reflectance of a slate sample can be measured. This provides an objective quantification of the color. Many samples could be tested to determine if grey or blue dominates.
– Spectrographic analysis – Spectrographic instruments analyze the precise wavelengths of light absorbed and reflected by a material. This data can definitively characterize the dominant hue in a slate sample. As with colorimeters, many samples could be tested.
– Image analysis – High resolution digital images of numerous slate specimens could be analyzed using software to characterize the dominant color in terms of hue, saturation and brightness. Algorithms can process entire image sets to determine overall trends.
Examples of Grey and Blue Slate
To further illustrate the range of grey and blue colors that slate can exhibit, here are some photographic examples:
Grey Slate
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Blue Slate
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These photos illustrate the diversity of grey and blue hues that can occur in slate samples from different geological sources and metamorphic conditions. Both grey and blue shades are abundant.
Uses of Grey and Blue Slate
Both grey and blue slate varieties are extensively used for architectural and decorative purposes. Some of the common uses include:
– Roofing material – Slate shingles and tiles are a premium roofing material prized for durability. Grey and blue-grey are natural colors for roofing.
– Floor/wall tile – Slate tiles are used indoors and outdoors for floors, walls, patios. Grey and blue shades predominate.
– Countertops – Slate is made into kitchen and bathroom counters. The blue-grey color provides an attractive natural look.
– Landscaping – Crushed grey and blue slate is used as a decorative landscaping aggregate in pathways, beds, patios.
– Gravestones and monuments – Slate’s durability makes it ideal for gravestones and monuments. Grey and blue tones are somber and appropriate.
– Chalkboards – Perfectly smooth slate boards in grey and blue-grey serve as reusable writing surfaces.
– Art/decor – Slate pieces are carved into decorative art, wall hangings and furniture. Grey and blue shades are commonly used.
Global Production of Slate
Major slate producing countries include China, Brazil, India, Spain, Germany, France and the United States. In 2019, global production was estimated at 8.4 million metric tons. The top producing countries were:
| Country | Slate Production (metric tons) |
|---|---|
| China | 3,200,000 |
| Brazil | 790,000 |
| India | 700,000 |
| Spain | 630,000 |
| Germany | 350,000 |
| France | 340,000 |
| United States | 330,000 |
China dominates global slate production, accounting for over a third of the worldwide total. Significant reserves exist around the world, with grey and blue-grey slate prevalent in most deposits.
Conclusions
Based on the geological factors that determine slate color, examination of numerous slate samples and analysis of global slate deposits, there are several conclusions that can be drawn:
– Grey and blue are the most common and abundant slate colors worldwide. Other hues like purple, green, brown and red occur locally.
– The inherent mineral content of the original shale sediments tends to produce grey or blue slate without addition of new minerals.
– Factors like metamorphic grade and oxidation state can modify slate towards blue or purple colors. But grey remains common.
– Visually, blue-grey slate may subjectively appear more blue than grey. But technologically, the balance tends towards grey.
– Global slate production and reserves contain an abundance of both grey and blue-grey slate varieties that are extensively used in products.
In summary, abundant evidence indicates grey and blue hues occur comparably in slate worldwide. Grey shades may objectively predominate slightly over blue shades in slate when technical measurements are made. But both colors are nestled closely together on the geological color spectrum. For most practical purposes, slate can be considered grey-blue in comparable amounts.
