What is mineral paragenesis?
Mineral paragenesis is the order in which a specimen’s minerals formed. Three rules read it; 12 iron phosphate species sheets show how far their wording goes.
Mineral paragenesis is the order in which the minerals of a rock, vein or specimen formed: in Craig and Vaughan’s words, the order of formation of associated minerals in time succession. Three rules read it: what crosscuts is younger, what is replaced is older, crystals overgrown by others came first. On 12 phosphate species sheets, triphylite comes first.
In short
- The definition. Craig and Vaughan, Ore Microscopy and Ore Petrography, chapter 8 (Mineralogical Society of America open-access edition) defines paragenesis as the order of formation of associated minerals in time succession. They note that the word has also been used, particularly in European literature, for characteristic ore mineral assemblages, and use it only for sequence. The Dictionary.com entry for paragenesis gives both senses.
- The three reading rules. From the same chapter: a veinlet or feature that crosscuts another is younger, except where the older phase has been replaced or both were remobilized; a mineral being replaced predates the one replacing it; and euhedral crystals mixed with or overgrown by other minerals were the first formed, except that some minerals, pyrite and arsenopyrite among them, grow good crystals at any stage.
- The worked reading. Twelve Handbook sheets for pegmatite iron and manganese phosphates were read by script. Triphylite is “the most abundant primary phosphate”. Nine others print alteration, replacement, oxidation or leaching of primary phosphates, four of them naming triphylite. Four print “late”: ferrisicklerite, huréaulite, strengite and laueite. The order the words give: triphylite first, its alteration products after.
- What the formulas add. On the IMA formulas, triphylite LiFe2+(PO4) has lithium, no ferric iron and no hydrogen. Heterosite has no lithium and is all ferric; barbosalite and rockbridgeite add hydroxyl; phosphosiderite, beraunite and strengite reach 4 hydrogens per phosphorus; laueite 9. The chemistry agrees with the wording: oxidation and hydration of the primary phosphate.
- Documented failure: company is not order, and “late” is not a clock. Association lines say who was found together, not who came first. “Late” ferrisicklerite has no water in its formula, while beraunite, with 4 hydrogens per phosphorus, is not called late. The sequence on a specimen comes from its textures, as how do mineral specimens form says: the earliest on the wall, the latest on top.
| Mineral | Formula | Occurrence wording (Handbook, abridged) | Wording class | Li per P | Ferric share of the iron | H per P |
|---|---|---|---|---|---|---|
| Triphylite | LiFe2+(PO4) | The most abundant primary phosphate in complex zoned granite pegmatites | Primary | 1 | 0% | 0 |
| Ferrisicklerite | Li(Fe3+,Mn2+)PO4 (Handbook; no IMA row) | Formed by late hydrothermal alteration or weathering of triphylite-lithiophilite | Alteration; late; names triphylite | — | — | 0 |
| Heterosite | Fe3+(PO4) | Secondary, in the oxidized zone of complex granite pegmatites, replacing primary phosphate minerals, principally triphylite | Replacing; names triphylite | 0 | 100% | 0 |
| Barbosalite | Fe2+Fe3+2(PO4)2(OH)2 | Formed by oxidation and hydration of primary phosphates | Alteration | 0 | 67% | 1.00 |
| Rockbridgeite | (Fe2+0.5Fe3+0.5)2Fe3+3(PO4)3(OH)5 | In “limonite” iron ore deposits; an alteration product of primary phosphates in complex granite pegmatites | Alteration | 0 | 80% | 1.67 |
| Frondelite | (Mn2+0.5Fe3+0.5)2Fe3+3(PO4)3(OH)5 | An alteration product of primary phosphates in complex granite pegmatites | Alteration | 0 | 100% | 1.67 |
| Huréaulite | Mn2+5(PO3OH)2(PO4)2(H2O)4 | A late-stage secondary mineral formed by alteration of primary phosphates | Alteration; late | 0 | no iron | 2.50 |
| Phosphosiderite | Fe3+(PO4)(H2O)2 | Typically an alteration product of triphylite in zoned complex granite pegmatites | Alteration; names triphylite | 0 | 100% | 4.00 |
| Beraunite | Fe3+6(PO4)4O(OH)4(H2O)4·2H2O | Bog iron ores; a cement in clays and sands; an alteration product of triphylite in granite pegmatites | Alteration; names triphylite | 0 | 100% | 4.00 |
| Strengite | Fe3+(PO4)(H2O)2 | A late secondary mineral in complex granite pegmatites; also in “limonite” iron ores and gossans | Late | 0 | 100% | 4.00 |
| Stewartite | Mn2+Fe3+2(PO4)2(OH)2·8H2O (Handbook) | A rare oxidation product formed by the leaching of primary phosphates | Oxidation, leaching | 0 | 100% | 9.00 |
| Laueite | Mn2+Fe3+2(PO4)2(OH)2(H2O)6·2H2O | A common late-stage hydrothermal mineral in oxidized triphylite-bearing complex granite pegmatites | Late; names triphylite | 0 | 100% | 9.00 |
What does paragenesis mean? Order, and sometimes company
Paragenesis is the order in which the minerals of a deposit or specimen formed. Craig and Vaughan’s Ore Microscopy and Ore Petrography (2nd edition, made open-access by the Mineralogical Society of America) opens its chapter 8 by naming two objectives of ore microscopy: the determination of the order of formation of associated minerals in time succession, or paragenesis, and the conditions under which they formed. A paragenetic sequence is that order written out.
The other sense. The same chapter notes that “paragenesis” has also been used, particularly in European literature, for characteristic ore mineral assemblages, and that the book uses it only for the sequential formation of minerals. The Dictionary.com entry for paragenesis shows the split: its American entry gives the order in which the minerals in a rock or vein crystallized, and the origin of minerals in contact so as to affect one another’s formation; its Collins British entry gives a characteristic association of minerals in a particular rock or ore deposit. When a label or a paper says “paragenesis”, check which is meant.
Why a collector cares. A specimen with several species is a record of more than one event, and reading the order is part of how to identify a mineral specimen. The background, that display crystals grow into open space, is on how do mineral specimens form.
How do you read paragenesis on a specimen? Three rules
Crosscutting, replacement and overgrowth are the three rules a hand lens can apply. All three are from Craig and Vaughan, chapter 8, written for polished sections, and they carry their own exceptions.
1. Crosscutting. A veinlet or other feature that crosscuts another is younger than what it cuts, except when the older phase has been replaced, or when both features result from metamorphic remobilization. 2. Replacement. The mineral being replaced predates the one replacing it; replacement usually proceeds inward from crystal boundaries or along fractures, the replacing phase with convex boundaries and the replaced phase with concave ones, sometimes left as residual islands. Pyrite’s cubic shape is, in their words, probably the most readily recognized of replaced minerals: cubes of chalcopyrite, covellite or goethite are usually regarded as pseudomorphs after pyrite; see what is a pseudomorph. 3. Overgrowth. For many minerals a euhedral crystal shows growth into open space, and euhedra mixed with or overgrown by other minerals were the first formed.
The exceptions, in their words. Some minerals, pyrite and arsenopyrite among them, form well-developed crystals regardless of their position in the sequence, and many minerals may have more than one paragenetic position, with different generations of different habit. A good pyrite cube is therefore not, by itself, evidence that pyrite came first. The live how do mineral specimens form page puts the same idea simply: the earliest sit on the wall, the latest on top.
A worked reading: the iron phosphates of a pegmatite
Read together, the Handbook sheets for twelve pegmatite phosphates put triphylite first and its alteration products after it. Method: the Occurrence field of each sheet was extracted and searched by script for “primary phosphate”, for alteration, replacing, oxidation or leaching, and for “late”. Triphylite, LiFe2+(PO4), is “the most abundant primary phosphate in complex zoned granite pegmatites”. Nine of the other eleven print alteration, replacement, oxidation or leaching of primary phosphates, and four of those name triphylite: ferrisicklerite, heterosite, phosphosiderite and beraunite. Four sheets print “late”: ferrisicklerite, huréaulite, strengite and laueite. That is a sequence in words: a primary lithium iron phosphate, then minerals made from it.
The formulas agree. Worked per phosphorus from the IMA formulas, triphylite carries one lithium, no ferric iron and no hydrogen. Heterosite has no lithium and all its iron ferric. Barbosalite (67% ferric, 1 H per P) and rockbridgeite (80% ferric, 1.67 H per P) add hydroxyl; phosphosiderite, beraunite and strengite are all ferric with 4 H per P; laueite and stewartite reach 9. The barbosalite sheet names the process outright: “formed by oxidation and hydration of primary phosphates”.
What it predicts on a specimen. If the sheets are right, triphylite should be the core and the others should sit on it, replace it from its edges or fill its fractures, not the reverse; a fibrous green crust of rockbridgeite should overlie, not underlie, the primary phosphate. That is a test for the three rules on a real piece, not an observation this page claims for any specimen. The dufrénite page shows why an iron phosphate crust needs identifying before it can be placed.
What association lists cannot tell you
Documented failure: an Association line names company, not order. The rockbridgeite sheet lists triphylite as an associate, and the triphylite sheet does not list rockbridgeite; the strengite sheet lists rockbridgeite, and the rockbridgeite sheet does not list strengite. Neither listing says which came first. Order comes from the Occurrence wording and, on a specimen, from its textures.
Documented failure: “late” is not a water clock. On the twelve sheets the word does not follow the chemistry. Ferrisicklerite is formed by “late” alteration, yet its formula has no hydrogen at all; beraunite and phosphosiderite, at 4 H per P, are not called late; huréaulite, called late-stage, has 2.5. The hydration trend is real across the set, but it ranks no individual pair, and Craig and Vaughan’s warning that one mineral can occupy more than one paragenetic position applies: the strengite sheet puts strengite in pegmatites and also in “limonite” iron ores and gossans.
So read the specimen. The sheets give a hypothesis; the specimen’s crosscutting, replacement and overgrowth test it. More guides are in the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.