Black sand is sand that is black in color. It seems to be very simple. But what is behind this concept? How is this type of sand formed? What is it made of?
There is no single and easy answer to these questions because there are number of different dark sand grains that can form black sand and hence there are several different ways how black sands can be formed.
The realm of black sands can be broadly divided into two parts, both of them having subdivisions. Most widespread type of black sand is composed of volcanic minerals and lava fragments. Such sands are especially common on the coasts of volcanic islands (Hawaii, Canary Islands, Aleutians, etc.) Such sand of black sand beaches are black because many volcanic minerals and rocks are dark. Common rock types of volcanic islands are basalt (black when fresh), andesite (usually dark gray) and volcanic glass (often black in color).
Coarse-grained black beach sand composed mainly of andesitic volcanic rocks from Kamari, Santorini Island, Greece. The width of the view is 10 mm.
The minerals that give black color to these rocks are predominantly pyroxenes (mostly augite), amphiboles (mostly hornblende) and iron oxides (mostly magnetite). Such sands are heavier than ‘normal’ light-colored sands and become very hot on a sunny day. Black volcanic sands may contain many non-black grains like green olivine crystals, reddish (usually because of weathering) volcanic rocks, light-colored quartz (when the source area is continental) and carbonate biogenic grains (coral sand). Most volcanic minerals are not very stable. They decompose pretty rapidly. These sands are said to be compositionally immature (mature sands are composed of quartz and other minerals very resistant to weathering). They also contain unusally high content of lithic (rock) fragments which have not broken up yet to form a sand composed of individual mineral grains.
Fine-grained volcanic beach sand from Martinique. Green prismatic mineral is augite. Black is magnetite. The width of the view is 5 mm.
Another type of black sand occurs mostly in the continental settings. It is heavy mineral sand. Heavy minerals are minerals which have a specific gravity above 2.9. There are almost all colors present among the heavy minerals but compared to usually light-colored quartzose sand, they seem to be dark. Heavy mineral sands are usually composed of minerals that are relatively resistant to weathering. Such minerals are tourmaline, magnetite, garnet, rutile, ilmenite, zircon, epidote, staurolite, etc. Heavy minerals are in most cases disseminated among the light-colored (and usually much larger) quartz grains but in certain conditions they tend to accumulate.
You probably have seen dark stripes on the sandy beach which may even be mistaken for an oil pollution. Those streaks are composed of tiny gems that were carried high on the beach by the big waves but they successfully managed to avoid flowing back with the receding waves because of their above average density. Such sand samples composed largely of these mineral grains are the most beautiful and especially valued by the sand collectors who use to study their samples microscopically. The most common heavy minerals forming black sands are perhaps magnetite, garnet and epidote. They are widespread enough in the rocks and resist weathering moderately well. They are more resistant than typical minerals of volcanic black sands (olivine, pyroxene, hornblende) but not as resistant as rutile, tourmaline, and zircon. But these minerals never make up the bulk of a rocks composition and therefore are rarely very concentrated in the composition of a sand.
