The present glossary of terms is meant to provide some general explanations for the more technical, sand-related terms used on the Sand Atlas website. Please consider the descriptions here as being rather general and not really scientific (I am not a geologist nor mineralogist and don’t pretend to be one). The list will be updated regularly so please check this page more often. Some entries may contain photos and links to some example pages for more information. If you’d like to help me add some new terms or correct some possible mistakes, please feel free to contact me at info [at]



Rock-forming mineral with a deep red color (slightly purple) belonging to the garnet group having the chemical formula: Fe3Al2(SiO4)3. The name almandine (also known as carbuncle) comes from Alabanda, a region in Asia where the minerals were firstly found in ancient times. The term carbuncle is derived from Latin and means “live coal” or burning charcoal. Almandine occurs frequently in metamorphic rocks like mica schist (see also this post for more details).


Arenophile is the term used to describe a person who is interested in collecting sand samples as hobby. The criteria for collecting sand can be various: colors, location, mineralogy (composition) etc. Although in general only sand is collected, sometimes other samples from other materials are collected too such as crushed rocks, mud, soils, etc. Very often people like to swap sand with other collectors and use to gather in online communities such as Sand Forum International.



A biogenic substance is generally defined as a substance resulted from life processes. The term ‘biogenic sand’ refers to sand made of skeletal remains of plants and animals. Most sands of biogenic origin contain skeletal rests of corals, algae and different small marine animals such as mollusks (snails, shells), foraminifera, sponge spicules, worm tubes etc. The biogenic sands can be distinguish from mineral sands by their high content in calcium carbonate (CaCo3).



Corals are marine organisms living in compact colonies secreting calcium carbonate to form a hard skeleton. The coral colonies are spread over the oceans mostly surrounding atolls and islands. Skeleton rests can form nice sands much appreciated by sand collectors.

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Green colored form of muscovite (common mica) with high chromium content (1-5%) and chemical formula: K(Al,Cr)3Si3O10(OH)2.



Group of minerals used since the old times as gemstone also renowned for displaying the greatest variety of color than any other mineral, occurring in any color (except blue). Most common color is purple red.


Iron silicate from the larger mica group whose name derives from the Greek glaucos (meaning “gleaming” or “silvery” and referring to its blue-green color, which can vary from bluish green to olive green). The chemical formula of glauconite is K0.08R11.33R20.67[(Al0.13Si3.87O10](OH)2.


Heavy mineral sands

Sands containing good amounts of minerals such as ilmenite, zirconium, rutile, titanium, etc. and trash minerals such as magnetite and garnet. The heavy mineral sands are to be found especially on the beaches where tidal action brought to the light the heavier minerals.


Igneous (rock)

One of the three main types beside sedimentary and metamorphic rocks. The igneous rocks are formed by cooling off and solidification of magma. Further classification divide igneous rocks in intrusive (magma solidifies under crust surface) and extrusive (magma solidifies above crust surface).





Maerl sands

Deposits formed by the shells of some specific red algae living mostly off the Brittany and Ireland coasts, primary composed of calcium and magnesium carbonate, but containing also high amounts of magnesium, iron and other elements.


Mineral known also as mica, first use was in caves painting, nowadays it is used in various industrial applications.


Group of silicate minerals characterized by almost perfect cleavage yielding remarkably-thin laminæ (sheets) which are often highly elastic. Common mica is also known under the name muscovite.

Mud volcanoes

Volcano-like formations having similar characteristics as the “real volcanoes” just that magma is made out of water and clay from the underground layers. This is deposited in the form of mud at the top of the volcanoes, with the eruption of gases coming out like some lazy bubbles.





Phyllite is a foliated metamorphic rock found in the surroundings of stones of the pre-Cambrian epoch. In Tharandt Open-Air Geological Museum we found phyllite in Mohorn-Grund (north of Tharandter Forest). Due to the high content in mica, phyllite has a nice silvery aspect but is very fragile and breaks easily just under the pressure of fingers.


Pitchstone is a volcanic rock with dull, glassy aspect and very resistant to erosion. In comparison with obsidian (a volcanic rock with similar characteristics), pitchstone contains about 8% water in its structure. The main color of pitchstone is black but, in case of rock formations from Tharandt, Germany, the stone contains small red patches. Their presence in the main block is not yet clearly explained, some theories suggesting that they are foreign rock particles (most probably hematite – a red iron ore mineral) assimilated by the molten lava on its way to the surface.


Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The color of porphyry is usually reddish due to the presence of hematite.




Measure describing the sharpness of a grain’s corners and edges, regardless of shape. A perfect rounded particle would have the roundness = 1, all the others being included in different roundness classes.



Shale rock (in German: Schiefer) is the name used for a sedimentary rock made of… mud (actually a mix of clay and some other minerals like quartz and calcite). The fine-graded rock can be broken into parallel layers and it is often used for covering the roofs of houses in mountain areas.


Measure of the degree to which the shape of a particle approaches that of a sphere. A perfect spherical grain would have the sphericity factor = 1 but this is rather uncommon in nature as most sand grains have sphericity numbers around 0.7.


Semi-precious mineral whose name reflects its high sodium content (chemical formula: Na8[Cl2|Al6Si6O24]), used mainly as ornamental stone.