Coal in Halong Bay EarthCache

Ha Long is well known as a tourist hotspot due to the UNESCO site of Halong Bay. However, up to 2012 its economy was driven by coal mining, and the coal mines near Cam Pha were some of the largest in Southeast Asia.

Coal Mining in Vietnam has been carried out for over a century, using open pit mining methods. By 2021, 35% of Vietnams coal was mined in open pits, and this is planned to reduce to 10% by 2030. Nui Beo, the largest open-cast coal mine in the northern province of Quang Ninh opened in 1987, and was closed in 2012 following the province's target to switch to more environmentally-friendly coal mining. They extracted their first tonnes of coal from the underground mines in 2017.  Nui Beo included two pits some 125 metres deep, and may need some 30 million cubic metres of soil to fill the pits and reinstate the local landform.

Coal is a black or brownish black sedimentary rock that is formed when dead plant matter decays in an environment that prevents biodegradation and oxidation, and is subsequently buried, compressed and heated. This process, known as coalification, concentrates the carbon from the plant matter, and drives off water, methane and carbon dioxide.

The grade of coal produced during coalification will depend on the pressure and temperature reached as the dead vegetation is buried, with the most important factor being the temperature that is reached. The pressure and time are less important factors.

There are four major types (or “ranks”) of coal:

• Anthracite: The highest rank of coal. It is a hard, brittle, and black lustrous coal, often referred to as hard coal, containing a high percentage of fixed carbon and a low percentage of volatile matter.
• Bituminous: Bituminous coal is a middle rank coal between subbituminous and anthracite. Bituminous coal usually has a high heating (Btu) value and is commonly used in electricity generation and steel making. Bituminous coal is blocky and appears shiny and smooth when you first see it, but look closer and you might see it has thin, alternating, shiny and dull layers. 
• Subbituminous: Subbituminous coal is black in color and is mainly dull (not shiny). Subbituminous coal has low-to-moderate heating values and is mainly used in electricity generation.
• Lignite: Lignite coal, aka brown coal, is the lowest grade coal with the least concentration of carbon. Lignite has a low heating value and a high moisture content and is mainly used in electricity generation.

To achieve anthracite grade, it requires a temperature of 180 to 245 degrees Celsius, yet to produce subbituminous coal, a temperature of only 35 to 80 degrees is required.

Typical properties of the different ranks of coal are as follows:

Anthracite

• Dense, black and shiny
• 92 to 95 per cent carbon.
• Negligibly small proportion of moisture.
• Very little volatile matter (less than 10 per cent).

Bituminous Coal

• Dense, compact, and is usually of black colour.
• 80 to 92 per cent carbon.
• Low moisture content (10 to 20 per cent)
• Low volatile content (10 to 20 per cent)

Subbituminous Coal

• Dense, compact, and is usually of black colour. Does not have traces of original vegetable material
• 70 to 80 per cent carbon.
• Moderate Moisture content (20 to 45 per cent)
• Moderate volatile content (20 to 45 per cent).

Lignite

• Dark to black brown. Can identify remains of original vegetable material
• 60 to 70 per cent carbon.
• Moisture content is high (35 to 75 per cent).
• High volatile content (over 30 percent)

A maceral is the component of coal that is organic in origin, which means it has been derived from organic matter. The term 'maceral' in reference to coal is analogous to the use of the term 'mineral' in reference to other rocks. Depending on the type of organic matter that Examples of macerals are inertinite, vitrinite, and liptinite.

Vitrinite is one of the primary components of coals, typically comprising 50 to 90 percent. Vitrinite has a shiny appearance resembling glass (vitreous). It is derived from the cell-wall material or woody tissue of the plants from which coal was formed. Chemically, it is composed of polymers, cellulose and lignin.

Vitrinite forms diagenetically by the thermal alteration of lignin and cellulose in plant cell walls. It is therefore also common in other sedimentary rocks that are rich in organic matter, such as shales and marls with a terrigenous origin, or some terrigenous content. Conversely, carbonates, evaporites and well-sorted sandstones have very low vitrinite contents. Vitrinite is absent in pre-Silurian rocks because land plants had not yet evolved

Inertinite is oxidized organic material or fossilized charcoal. The presence of inertinite is significant in the geological record, as it signifies that wildfires occurred at the time that the host sediment was deposited. It is also an indication of oxidation due to atmospheric exposure or fungal decomposition during deposition. Inertinite is a common maceral in most types of coal.

liptinite is the finely-ground and macerated remains found in coal deposits. Liptinites were originally formed by spores, pollen, dinoflagellate cysts, leaf cuticles, and plant resins and waxes.

The physical properties of the coals classify them into two major categories: humic and sapropelic. Humic coals are banded and made up mainly of microscopic plant debris. They consist of 4 lithotypes, which are megascopically recognized bands in coal – Vitrain, Clarain, Durain and Fusain.

Sapropelic coals are non-banded, composed mainly of microscopic plant debris, spores, pollens, algae and most commonly show conchoidal fracture. Sapropelic coals are further divided into cannel coal and boghead coal. They typically have a high hydrogen content

The Carboniferous period, part of the late Paleozoic era, takes its name from the large underground coal deposits that date to it. 90% of known coal deposits were formed during the 60 million years that the Carboniferous period lasted for. The majority of these deposits are found in parts of Europe, North America, and Asia, areas that were lush, tropically located regions during the Carboniferous period. These were part of a huge landmass known as Laurasia, or Laurussia, which was separate from the larger, cooler supercontinent of Gondwana to the south. To the east, parts of Asia, including China, were surrounded by warm oceans. While Gondwana became progressively colder as it began another poleward migration, the tropical landmasses remained wet and humid.

Why such a large proportion of coal formed during the Carboniferous period is subject to debate. Certainly there was a profusion of plant species that evolved, particulalry with woody bark, that comprised organic matter that could be preserved. There was also appropriate climatic and terrestrial conditions within the Laurasia continent. Another factor that might be important is the lack of bacteria, fungi and microbes with an ability to ingest and breakdown the lignin and cellulose, even if the plant matter had been buried in an anaerobic environment. The later evolution of such organisms may be part of the reason that more recent deposits of coal are not found.

At the GZ you can find samples of coal mined locally, together with some minecars that may be used during the mining operation.

To solve this earthcache, carry out the following tasks and send answers as a message to me.

1. 1. Observe the pieces of coal at the GZ. How many pieces are there? What is the approximate size of them.
   2. Based on the information provided, which rank of coal do you think these are. What feature or features make you decide this.
   3. Would you say these are Humic coal or Sapropelic coal? What lithotypes can you see?
   4. Look at the top of (one of) the middle pieces of coal. Briefly describe the structure within the rock, and the lustre of the surfaces. Can you identify different types of maceral? What do you think was the dominant type of organic material that was used to form this coal?
   5. Optional - You are welcome to post a picture of yourself near the GZ or in Halong Bay, but please do not include any spoilers that give away any of the answers.