Agricola's Classification

Worm's classification

During Revival, all fields of culture have known an accelerated development, and mineralogy among them.
The important development of alchemic studies clearly needed chemical products extracted from minerals, and, as a consequence, the knowledge, although imperfect, associated with them. Improving living conditions, related to the developing trade exchanges, demanded greater quantities of raw materials, and led to the reopening of old mine pits and the discovery of new deposits.
But another motive notably contributes to the development of mineralogic knowledge, the need to classify the minerals to arrange the items of the museums' collections, once they had been collected for the sake of curiosity.

The habit to collect, always present in man, as the natural objets deposited in prehistoric tombs demonstrate, finds a revival in the Wunderkammer, which follow the parenthesis of the Middle Ages during which only the churches and monasteries reserved "curosities" for the peers of science, culture and art.

These "marvels' rooms", ranging from simple "cabinets"  to true encyclopedic exhibitions, can be considered as the ancestors of today's museums of Natural History. As an example of cabinet, one can mention that of blind philosopher Jan Amos Komensky, whose name was latinized as Comenius (1592-1670) who gives a curious definition of a museum in his pedagogical work Orbis sensalium pietus. Among the numerous "encyclopedic museums", one can mention those of pharmacist Ferrante Imperato in Naples (1550-1631), physician Ludovico Settala in Milan (1552-1631), or pharmacist Francesco Calzolari in Verona (1522-1609), naturalist Ole Worm in Copenhagen (1588-1654), Jesuit scholar Athanasius Kircher in Rome (1602-1680).

In these Wunderkammer, curiosity gradually gives way to natural finds picked up not only for their strange aspect, but as a very partial at first, but becoming more complete, representation of Nature, and therefore it has been said that, paradoxically, the end of encyclopedic collections corresponded to the publication of the Encyclopédie.

From the middle of the XVIth century to the middle of the XVIIIth century, for about 200 years, the encyclopedic museum  not only had a function of aesthetic pleasure, of surprise and wonder for idle noblemen, but also constituted, with its attempt to represent Nature at a smaller scale, the starting point of the naturalistic systematic classification. For mineralogy in particular, the mining experience of Agricola has played a fundamental role in  clarifying the fancy genetic hypotheses of the Middle Ages, but  it is on the samples picked for collections that Stenone  was able to verify on a series of crystals of quartz and haematite, that non mutatis angulis, thus giving its scientific aspect to Biringuccio's on-field observation of  precisely orthogonal parallelepided crystals of Pyrite.

The need to arrange natural materials leads to attempts of classification, maybe very imperfect to our eyes, but representing the first steps ever towards today's systematic mineralogy.

Obviously, the external aspect of minerals and rocks suggested the first classification criteria, and thus Avicenne, as soon as the XIIth century, distinguished Stones, Metals, Sulphur and Salts.

It it impossible to recount the innumerable classification schemes proposed between the XVIth and the XVIIIth century. One can mention as examples Agricola's proposal in his 1546 treatise De natura fossilium, and the scheme adopted by Ole Worm in his Copenhagen museum and published in 1655, both based on the external aspect of materials.
 

Not very different is Worm's classification one century later. It is an attempt at separating the homogenous, even only by the eye, like Mineralia from Lapides, but still keeping apart the Metalla, the aspect of which is too different.

 
Worm's classification

In the XVIth century, two personalities can be mentioned among those who brought the first scientific contributions to mineralogy.

Vannoccio Biringuccio (or Biringucci) (Siena 1480 - Rome 1540) was not a man of great culture, but a technician who, in his book De La Pirotechnia, puts to paper his knowledge as a mine director and an expert metallurgist, even if he sometimes wanders into alchemic divagations on the nature of materials. This work coud have been subtitled "not why, but how", given the numerous accurate informations it provides. Two of these deserve to be mentioned.

Reference to quantitative analysis is constant (a long-armed balance is even represented in the figures), and the most interesting is the observation that, with calcination, the weight of lead increases by 8-10%. For the explanation, one will have to wait until Lavoisier, but it's the first time that this quantitative fact is noted.

Another observation concerns Pyrite which is "in cubic shape like dice orthogonal parallelepided all exactly square or square based prism. It's only a qualitative observation, but it precedes by one century Stenone's non mutatis angulis.

Georg Bauer, latinized as Georgius Agricola (Glachau 1494 - Chemnitz 1555) never met Biringuccio, his elder by several years, but he was influenced by his Pirotechnica  from which he drew figures and arguments, even if more thoroughly treated and in a beautiful Latin style, in the famous treatise De Re Metallica .

The thoroughness of this treatise, as compared to the former,  but also another important work, De Natura Fossilium,  justify why Agricola is called "the father of mineralogy", insofar as the first treatise is a perfect synthesis of the knowledge accumulated in the preceding centuries on metallurgy and the treatment of minerals, and the second one, as was stressed already, a compendium of all the knowledge of the time about minerals and rocks, based not on erudition but on mineral practice in the Saxon deposits, even if not a direct one.
The XVIIth century sees a few first-rank figures who sign important, and even sometimes fundamental advances in the history of mineralogy.

The spectacular outer aspect of the crystals presented in the Wunderkammers and collections stupefied the visitors and kindled curiosity among scholars. Some of them, such as the great astronomer Johannes Kepler (1571-1630) tried to discover their metaphysical significance by associating the polyhedra with Aristotle's elements, opposing the great Greek philosopher. The latter had denied any correspondence between his four elements and Plato's solids, because it was impossible to fill up space completely with them, which contradicted the theory of impossibility of void in nature (except with the cube). Kepler, instead, held that the cube, with its obviously square faces giving an impression of solidity, represented earth, that the tetrahedron, with its few sides and acute angles and its slender shape was the symbol of fire, that the octahedron, well balanced, was a figure of air, and that the icosahedron, with its many sides and obtuse angles, stood for water, and even the quintessence, the celestial bodies, found their representation in the pentagonododecahedron, with a number of sides equal to that of Zodiac. But a simple snowfall which caught him by surprise in the street on a New Year's day, led this great scientist back to the direct observation of hexagonal crystals, and to the formulation of an hypothesis about the constancy of the hexagon notwithstanding the great variety of shapes taken by snowflakes.

Reusing an idea already put forward in 1650 by the Italian physican Girolamo Cardano (1501-1576), and possibly suggested to both of them by the heaps of cannonballs arranged near the pieces of ordnance in the castles of the time, Kepler supposed the crystals were constituted by a compact packing of little spheres.
This hypothesis will be reused in 1665 by an Englishman called Hooke and later, in 1690, by Christian Huygens, but with ellipsoids this time, to explain the double refraction of calcite.

After 1619, year of publication of Kepler's Harmonices mundi, 50 years pass and see the apparition of De solido intra solidum naturaliter contento written in Florence by Niccolo Stenone (Niels Strensen, 1638-1686). In the brief "prodrome" of a treatise which will never be written, and in particular in the legends of the figures at the end of the text, under the drawings of sections of crystals of quartz and haematite of the most diverse forms, three simple, three little word appear, non mutatis angulis, which represent the birth of the science of crystallography. Crystals may have different aspects, but the angular values don't change : this is already the law of constant dihedral angles, which will need to be verified on a vast number of crystalline species and will be formalized only a century later by Romé de l'Iole.  It's not unnecessary to recall that Stenone's observations were made on crystals belonging to his collection, later acquired for the collections of the Great Duke of Tuscany, and today kept in Florence's Museum of Mineralogy.

The next year, 1670, another Dane, Erasmus Bartholin (1625-1698) publishes in Copenhagen (Hafnia), a volume headed Experimenta cristalli islandici which reports the fundamental discovery of double refraction in the spath of Iceland, the limpid and highly transparent variety of calcite typical of this island. This observation, which proved of great importance both for the theories on the nature of light and the optical behaviour of crystals, lead its author to a series of determinations related on the one hand to discovering the nature of the materials, and on the other hand to measuring the angles of the crystal.

When rubbed with a cloth, the crystals, like amber, attract light bodies ; with acid they produce a strong effervescence ; when heated strongly, they form quicklime. Angular mesures returned 101° and 79° on the rhombic sides, and 103°40' for the dihedral angle of the rhombohedron. These experimental data were mingled with the pride that this extraordinary phenomenon of double refraction was fonud on Danish material (although coming from a "colony"), and not on crystals of much higher value such as precious stones, or at least diamond.

Not until 1728, posthumously, is the work of the Dutch scholar Christian Huygens (1629-1695) published. It exposes the wave theory of light, explaining, in particular, the double refraction and easy cleaving of calcite, under the assumption that it is consituted by the juxtaposition of tiny ellipsoids with an axis ratio of 1:8.

Another personality of importance is Robert Boyle (1627-1691), seventh son of the Earl of Cork, in Ireland. Among his multiple fields of interests - just think of the law on gases which bears his name - one remembers his theories on the colors and transparency of gems in relation with their genesis embedded in his work Medicina Hydrostatica in 1690, and also his systematic measurements of the minerals' densities: If we pose that water = 1, quartz gives 2 2/3 (instead of 2.65), haematite 5.7 (instead of 5.26), magnesioferrite 4.6 (instead of 4.56), and jet 1.22 (instead of 1.33).

Mineralogy owes a debt to the Dutch Anton Leewenhoek (1632-1723) not only, like the other natural sciences, for the invention  of the microscope, but also for a series of observations made on materials in which a crystalline form could be recognized, in particular chalk. In these crystals he measured the angles between planes at 112° and 68°, making the hypothesis that they were made of infinitesimal tablets, but above all, through a series of ingenious procedures aimed at eliminating external influences, he managed, for the first time, to verify the liberation of water by crystals of chalk in a proportion of 1:5 of the weight when heated (theoretical value : 20.43%).

The researches of the Italian physician Domenico Guglielmini (1655-1710) must be attributed to the same century, even though their results were published in 1705 in the work  De salibus and later, posthumously, in the Opera omnia of 1719. On a series of salts ranging from rock salt to aluminium nitrate and vitriol, he verifies the constancy of the forms typical of each crystalline species, and above all the constancy of their angles "nevertheless, always stable is the inclination of planes and angles". Also important is his theory about the constitution of the different salts as juxtapositions of tiny cleavage polyhedra, reused one century later by Hauy.

The first half of the XVIIIth century sees no substantial change as compared to the previous century. Linné himself (Carl von Linné, latinized as Linnaeus, 1707-1778), founding father of the botanical and zoological systematic classifications, does not bring any valid contribution to the mineralogical one, and in his attempt to unify all natural products he even speaks of procreation and vegetative growth of the minerals, an hypothesis already rejected in the previous century by Stenone. To his credit, one must mention the importance he gave to the crystalline form, with precise drawings and wooden models.
It is necessary to wait until the second half of the century, with the first chemical determinations and systematic goniometric measurements, to shift from a purely descriptive mineralogy to a quantitative science which reaches the present day with constant progress.