THE MOHS HARDNESS SCALE

This scale (from1 to 10) provides a qualitative indication of the scratch resistance of common minerals versus a “standard” of known minerals. Using this “scratch test,” one can quickly establish the hardness of an “unknown” mineral relative to a series of common minerals whose hardness is already known. For example, if some sample is scratched by fluorite but not by quartz, the hardness of the sample on the Mohs Scale would fall between 4 and 6.

The scale was created in 1822 by the German mineralogist Friedrich Mohs. According to Wikipedia, this scale has been used to even evaluate the hardness of smartphone screens. Most modern ones use “Gorilla Glass,” which scratches at level six.

The Mohs Scale is ordinal: corundum [includes sapphire and ruby] (9) is twice as hard as topaz (8), but diamond (10) is four times as hard as corundum.

Some useful comparisons:

Your fingernail scratches talc, gypsum, and graphite.

Quartz scratches glass.

Diamond scratches all other minerals.

Artificial compounds (simulants), like cubic zirconium has a hardness of 9, and moissanite (see one of my previous posts) has a hardness of 9.5 to near 10.

A streak plate (unglazed porcelain) has a hardness of approximately 7.0.

Scratch kits for determining Mohns Hardness are available for a modest sum (but diamond is not included!).

The following 10 minerals, given in an increasing order of hardness, are used commonly as “standards” in the Mohs Scale:

                                                     talc

                                                 gypsum

                                                   calcite

                                                fluorite

                                                 apatite

feldspar

                                                  quartz

                                                   topaz

                                                 corundum

                                                 diamond

 The largest known ammonite

Parapuzosia seppenradensis (Landois, 1895) is the largest known planar coiled (coiled like a rope) ammonite. It is from Germany and is of Late Cretaceous (early Campanian) age, and it is 1.8 m (5.9 feet) in diameter. Its shell is incomplete because the living chamber is missing. The largest specimen of this species was found in Germany. Many museums throughout the world have replicas of this desmoceratid ammonite species on display, including the Los Angeles County Museum of Natural History. Ammonites are extinct cephalopods that died out at the end of the Mesozoic Era (65 million years ago). Living cephalopods include the chambered Nautilus, squids, octopods, and a few other groups. 

GROSSULAR GARNET

Garnet is not a single neosilicate mineral; rather it is comprised of several groups of closely related silicate minerals. The generalized chemical formula for the entire garnet group is Ca₃Al₂Si₃O₁₂, but there can be much substitution of other elements (e.g., iron, magnesium, manganese, chromium] for the calcium [Ca] and the aluminum [Al]. As a result, there is a huge range of colors in each of the groups, and identification as to the specific group of garnet can be difficult. Some crystals of garnet are gemstones because of their beautiful color and/or rarity. Garnets can form in both metamorphic and, to a lesser extent, in igneous rocks.

This post concerns two examples of grossular [also called grossularite] garnet. 

The pure form of grossular garnet is colorless, but if iron substitution takes place, the color can be white to brownish red to green or black (no color). A South African variety of grossular garnet with considerable iron substitution is called “South African Jade” or “Transvaal Jade.” It is, however, not jadeite or nephrite (see my recent post concerning these two minerals). 

The first three images show different crystals of a cinnamon-brown grossular garnet. Their provenance is probably Mexico. They are all 3 cm in height. The first image shows the rhombic dodecahedron crystal habit found in all garnets. The next two are interior views: one showing a cross-section of a large black core (iron rich), surrounded by an outer thick lighter color zone (much less iron). The next view shows fine internal layering of a partial specimen. The dark color of the core and the layering within the outer (lighter) core might be the result of compositional zonation related to temperature change during the complicated formation of the crystal.

The last image, shown below, is a hand specimen (8 cm maximum dimension) of quartz-rich rock containing abundant garnets. The provenance is unknown to the writer. There are two kinds of garnet in this rock. One is possibly a brownish-red grossular garnet. Its crystals are abundant, large in size (up to 1 cm), and brownish red, with a few having black cores. The other kind is green grossular? garnet. Its crystals are mostly small-sized and can contain some tiny “dots” of black inclusions. A few of the green grossular? crystals are large (up to 1 cm).