Compare three grain sizes
Geological complex of Cabeceiras de Basto · regional study
Granite varies in grain size too
In A. Almeida’s study, granite from the Cabeceiras de Basto complex does not have a single grain size. The classification uses three average ranges: 0.5–1 millimetre for fine grain, 2–4 millimetres for medium grain and 5–7 millimetres for coarse grain. In the last group, the author also mentions rare larger crystals measuring 8–10 millimetres. This distinguishes two observations: the usual grain size and the presence of a few crystals that stand out. The description does not turn every visible point in a photograph into a measurement. Comparing sizes in an image requires a scale; magnification on a screen can make small grains look large. The study identifies three genetically independent series despite their similar chemical-composition ranges. Appearance is a starting point for investigation, not the entire explanation of origin. In a wall or façade, grains spark curiosity, but do not alone prove where the stone came from.
Vamos experimentar?
- In a millimetric ruler, 1 mm, 3 mm and 6 mm are examples within the three ranges described.
- Draws three small fields with points of different sizes, identifying them as a scaleless scheme.
- Adds some larger crystals to the third field. It distinguishes the average dimension of the presence of exceptions.
Exercise on paper. It does not require collecting rocks, scratching walls or removing fragments.
Open a história e os seus recursos →Sources of this proposal
My findings
An idea, a drawing or a question to go on.